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| WEPMS001 | Application of Induction Module for Energy Perturbations in the University of Maryland Electron Ring | 2322 |
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Funding: Work supported by the U. S. Department of Energy grant numbers: DE-FG02-94ER40855 and DE-FG02-92ER54178, ONR and Joint Technology Office The University of Maryland Electron Ring (UMER) is a scaled storage ring using low-energy electrons to inexpensively model beams with high-space-charge. With the ability to inject such beams comes the problem of longitudinal end erosion of both the head and tail. It is important therefore to apply suitably designed longitudinal focusing forces to confine the beam and prevent it from its normal expansion. This paper presents the design and prototyping of an induction cell for this purpose. Successful operation of the induction cell would push the achievable number of turns and also enable us to perform studies of the longitudinal physics of such highly space-charge dominated beams. The pulsed voltage requirements for such a system on UMER would require ear-fields that switch 3kV in about 8ns or so for the most intense flat-top rectangular beam injected into the ring. This places a considerable challenge on the electronics used to deliver ideal waveforms with a compact module. Alternate waveforms are also being explored for other various injected beam shapes into UMER. |
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| WEPMS002 | Polyhedral Cavity Structure for Linac Colliders | 2325 |
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Funding: DOE grant #DE-FG02-06ER41405 A polyhedral superconducting cavity is being developed for possible use in linac colliders. In side view it has the contour of a Tesla-type multi-cell string. The surfaces of the cavity are formed by bonding flat foils to solid copper wedge-shaped segments, so that the end view is a polyhedron of such segments. Several features of this structure make it interesting for linac colliders: the cavity segments are totally open for cleaning, polishing, and inspection until the final assembly step; narrow slot gaps at the boundaries between segments strongly suppress all deflecting modes without penalty to the accelerating mode; the solid copper substrate accommodates cooling channels and eliminates the need for an immersion cryostat; and the open geometry makes it possible to utilize advanced superconductors (e.g. multi-layer Nb/Nb3Sn, YBCO, MgB2) on the cavity surface, opening the possibility of higher gradients. |
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| WEPMS003 | Design of Half-Reentrant SRF Cavities for Heavy Ion Linacs | 2328 |
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Funding: DOE #DE-FG02-06ER41411
The Spallation Neutron Source (Oak Ridge), the proposed 8 GeV Proton Driver (Fermilab), and the proposed Rare Isotope Accelerator use multicell elliptical SRF cavities to provide much of the accelerating voltage. This makes the elliptical cavity segment the most expensive part of the linac. A new type of accelerating structure called a half-reentrant elliptical cavity can potentially improve upon existing elliptical designs by reducing the cryogenic load by as much as 30% for the same accelerating voltage. Alternatively, with the same peak surface magnetic field as traditional elliptical cavities, it is anticipated that half-reentrant designs could operate at up to 25% higher accelerating gradient. With a half-reentrant shape, liquids can drain easily during chemical etching and high pressure rinsing, which allows standard multicell processing techniques to be used. A half-reentrant cavity for β = v/c = 1, suitable for the proposed ILC, has been designed and fabricated, with RF tests in progress*. In this paper, we present electromagnetic designs for three half-reentrant cell shapes suitable for an ion or proton linac (β = 0.47, 0.61 and 0.81, f = 805 or 1300 MHz).
* M. Meidlinger et al., in Proc. XXIII Int. Linac Conf., Knoxville, TN, Aug 2006 |
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| WEPMS004 | Deflecting Cavity for Beam Diagnostics in ERL Injector | 2331 |
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Funding: Work is supported by the National Science Foundation grant PHY 0131508. A 1300 MHz deflecting cavity will be used for beam slice emittance measurements, and to study the temporal response of negative electron affinity photocathodes in the ERL injector currently under construction at Cornell University. A single-cell TM110-mode cavity was designed to deflect the beam vertically. The paper describes the cavity shape optimization procedure, its mechanical design and performance at low RF power. |
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| WEPMS005 | Temperature Mapping Results on the High-Field Q-Slope of 1500 MHz Single Cell Superconducting Radiofrequency Cavities Baked In-situ at 400 C. | 2334 |
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Funding: NSF The heat treatment of a niobium cavity between 100 C - 120 C for 48 hours substantially improves cavity performance, presumably by healing the nature of the oxide-metal interface, although the nature of the healing is not yet understood. The heat treatment at higher temperatures is found to deteriorate the performance. Our tests on 1500 MHz single cell cavities are always equipped with a temperature mapping system consisting of 700 thermometers. The effect of heat treatment at various temperatures has been studied in detail using the temperature mapping system. In this contribution we report on several interesting findings from studies of a 400 C heat treatment. |
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| WEPMS006 | High Gradient Studies for ILC with Single Cell Re-entrant Shape and Elliptical Shape Cavities made of Fine-grain and Large-grain Niobium | 2337 |
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Funding: Work supported by DOE
Based on the encouraging results of the first 1300 MHz 70 mm aperture single cell re-entrant cavities*, we continue the high gradient studies for ILC with new re-entrant cavities made of fine-grain as well as large-grain niobium. These new cavities have smaller aperture of 60 mm, providing a further reduced Hpk/Eacc or a further improved ultimate gradient. Four 1300 MHz 60 mm aperture re-entrant cavities are made, two out of fine grain niobium and the other two out of large-grain niobium. In addition, two elliptical shape 1500 MHz cavities are also made out of large-grain niobium. We present the testing results of these cavities.
* R. L. Geng et al., PAC2005, p.653. |
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| WEPMS007 | Manufacture and Performance of Superconducting RF Cavities for Cornell ERL Injector | 2340 |
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Funding: Work supported by NSF Six 1300 MHz superconducting niobium 2-cell cavities are manufactured for the prototype of Cornell ERL injector to boost the energy of a high current, low emittance beam produced by a DC gun. Designed for high current beam acceleration, these cavities have new characteristics as compared to previously developed low-current cavities such as those for TTF. Precision manufacture is emphasized for a better straightness of the cavity axis so as to avoid unwanted emittance dilution. We present the manufacturing, processing and vertical test performance of these cavities. We also present the impact of new cavity characteristics to the cavity performance as learnt from vertical tests. Solutions for improving cavity performance are discussed. |
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| WEPMS009 | Results on 9-cell ILC and 9-cell Re-Entrant Cavities | 2343 |
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Funding: DOE We have recently upgraded our chemical treatment, high pressure rinsing systems and low temperature RF testing system to prepare and test 9-cell cavities for ILC. After removal of 120 um by BCP we reached 26 MV/m accelerating field limited by the high-field Q-slope. There was no quench and no field emission, showing that our facilities are well qualified. We have also extended our vertical electropolishing system to 9-cell cavities. Previously we have successfully used vertical electropolishing for one-cell cavities of the re-entrant shape to reach 47 MV/m accelerating. Test results on 9-cell electropolished cavities will be presented. AES has manufactured the first 9-cell cavity with re-entrant cell shapes. The surface magnetic field is 10% lower than for the standard TESLA-shape cavity. Half-cells were electropolished 100 um before welding. We will present results on the first tests of the 9-cell re-entrant cavity. |
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| WEPMS010 | Surface Studies of Contaminants Generated During Electropolishing | 2346 |
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Funding: NSF Electropolishing is now the preferred method for chemical treatment of niobium cavity surfaces. It provides a very smooth surface and after baking accelerating fields between 35 - 40 MV/m. However the reproducibility of performance needs to be improved substantially. Some of the leading causes are related to contaminant residues after electropolishing, these include sulphur particles, niobium pentoxide particles and traces of aluminum from reaction between the aluminum cathode and the acid electrolyte. We have carried out studies to enhance the deposition of such particles so that we can isolate and study the residues. We will present analysis of these studies using optical microscopy, SEM, and Auger. In at attempt to dissolve these contaminants, we have also conducted studies on the effectiveness of various rinsing agents, such as degreasing agents, dilute HF, hydrogen peroxide. |
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| WEPMS011 | Comparative Surface Studies on Fine-grain and Single Crystal Niobium Using XPS, AES, EBSD and Profilometry | 2349 |
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Funding: Supported by NSF As the surface magnetic field in niobium cavities approaches the theoretical critical field, rf losses begin to grow sensitive to increasingly subtle features of the material and the surface. A striking example is the familiar occurrence of the high-field Q-slope, where rf losses increase exponentially with field above an onset field. A surprising feature of the high-field Q slope is its positive response to mild baking at 120 C. But the Q-slope returns after the first 20 nm of the niobium metal surface is converted to loss-less pentoxide via anodization, a key feature. The latter result suggests that the cause of the fast growing losses resides in the first 20 nm of the rf surface. Although there are several propositions, the exact mechanism for the high-field Q-slope is not yet fully understood and demands further research. We are conducting surface analytic studies with XPS, SIMS, and Auger to shed light on the mechanism of the high-field Q-slope. We are comparing the behavior of fine-grain samples with single crystal samples, BCP treatments with EP treatments and properties before and after 120 C bake. We also study the effect of baking at temperatures up to 400 C. |
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| WEPMS012 | Low Loss and High Gradient SC Cavities with Different Wall Slope Angles | 2352 |
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Funding: NSF The introduction of reentrant shape for superconducting cavities has made it possible to achieve record high gradients. In this paper it is shown that lowest losses in the cavities are also achievable employing the reentrant shape. Influence of the cavity wall slope angle on the extreme gradient and losses is analyzed. |
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| WEPMS013 | High Power Tests of First Input Couplers for Cornell ERL Injector Cavities | 2355 |
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Funding: Work is supported by the National Science Foundation grant PHY 0131508 First RF power couplers for the ERL injector, currently under construction at Cornell University, have been fabricated. The couplers were assembled in pairs in the liquid nitrogen cryostat, built for their tests. A 15 kW CW IOT transmitter was available for coupler tests. A resonant ring was used for additional increase of the power. The couplers were successfully tested up to the goal power level of 50 kW CW. However, the first pair of couplers showed excessive temperature rise in some points. Therefore, minor changes in the design have been done to improve cooling. |
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| WEPMS014 | Vacuum Insulator Studies for the Dielectric Wall Accelerator | 2358 |
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Funding: This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. As part of our ongoing development of the Dielectric Wall Accelerator, we are studying the performance of multilayer high-gradient insulators. These vacuum insulating structures are composed of thin, alternating layers of metal and dielectric, and have been shown to withstand higher gradients than conventional vacuum insulator materials. This paper describes these structures and presents some of our recent results. |
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| WEPMS015 | An Improved SF6 System for the FXR Induction Linac Blumlein Switches | 2361 |
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Funding: This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory, under Contract W-7405-Eng-48.
The now-mature FXR (Flash X-Ray) radiographic facility at Lawrence Livermore National Laboratory will be briefly described with emphasis on its pulsed power system. The heart of each accelerating cell's pulse-forming Blumlein is it's sulfur hexafluoride-based triggered closing switch. FXR's recent upgrade to a recirculating SF6 gas reclamation system will be described and the resulting accelerator performance and reliability improvements documented. This was accompanied by a detailed switch breakdown study on FXR's Test Stand* and the recent analysis of the resulting statistics will be shown.
* W. DeHope, D. Goerz, R. Kihara, M. Ong, G. Vogtlin, J. Zentler, "An Induction Linac Test Stand", 21st Particle Accelerator Conference, Knoxville, TN, May 20, 2005 |
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| WEPMS016 | Modeling the Pulse Line Ion Accelerator (PLIA): An Algorithm for Quasi-Static Field Solution | 2364 |
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Funding: Work performed under auspices of U. S. DoE by the Univ. of CA, LLNL & LBNL under Contract Nos. W-7405-Eng-48 and DE-AC02-05CH11231
The Pulse-Line Ion Accelerator* (PLIA) is a helical distributed transmission line. A rising pulse applied to the upstream end appears as a moving spatial voltage ramp, on which an ion pulse can be accelerated. This is a promising approach to acceleration and longitudinal compression of an ion beam at high line charge density. In most of the studies carried out to date, using both a simple code for longitudinal beam dynamics and the Warp PIC code, a circuit model for the wave behavior was employed; in Warp, the helix I and V are source terms in elliptic equations for E and B. However, it appears possible to obtain improved fidelity using a "sheath helix" model in the quasi-static limit. Here we describe an algorithmic approach that may be used to effect such a solution.
*R. J. Briggs, PRST-AB 9, 060401 (2006). |
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| WEPMS017 | High-Power Coupler Component Test Stand Status and Results | 2367 |
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Funding: This work was performed under the auspices of the U. S. DOE by the University of California, LLNL under Contract No. W-7405-Eng-48. SLAC Work supported under Contract No. W-7405-Eng-48. Fundamental power couplers for superconducting accelerator applications like the ILC are complicated RF transmission line assemblies due to their having to simultaneously accommodate demanding RF power, cryogenic, and cleanliness constraints. When these couplers are RF conditioned, the observed response is an aggregate of all the parts of the coupler and the specific features that dominate the conditioning response are unknown. To better understand and characterize RF conditioning phenomena toward improving performance and reducing conditioning time, a high-power coupler component test stand has been built at SLAC. Operating at 1.3 GHz, this test stand was designed to measure the conditioning behavior of select components of the TTFIII coupler independently, including outer-conductor bellows, diameter changes, copper plating and surface preparations, and cold window geometries and coatings. A description of the test stand, the measurement approach, and a summary of the results obtained are presented. |
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| WEPMS018 | Superconducting Materials Testing with a High-Q Copper RF Cavity | 2370 |
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| Magnesium diboride (MgB2) has a transition temperature (Tc) of ~40 K, i.e., about 4 times higher than niobium (Nb) that has been used for recent accelerators. The studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (Rs) than Nb and much less power dependence compared to high-Tc materials such as YBCO up to ~400 Oe. The tests to check the RF critical magnetic field, an important parameter to determine the feasibility for accelerator application, are underway. We are planning to test different thickness films and with different coating methods. This paper describes the results obtained so far. One of the objectives is to verify Gurevich's theory of getting higher critical field than Nb by adding a very thin layer (less than penetration depth) to Nb. In addition, some CW tests on power dependence up to higher magnetic fields are planned and some results will be shown if available at the time of conference. | ||
| WEPMS020 | Commissioning the DARHT-II Scaled Accelerator | 2373 |
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| The DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse when completed this summer. After exiting the accelerator, the long pulse will be sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to bremsstrahlung for radiography. In order to provide early tests of the kicker, septum, transport, and multi-pulse converter target we assembled a short accelerator from the first available refurbished cells, which are now capable of operating of operating at over 200 kV. This scaled accelerator was operated at ~ 8 Mev and ~1 kA, which provides a beam with approximately the same nu/gamma as the final 17-MeV, 2-kA beam, and therefore the same beam dynamics in the downstream transport. In this presentation we will show the results of beam measurements made during the commissioning of this scaled accelerator. | ||
| WEPMS021 | RF-loss Measurements in an Open Coaxial Resonator for Characterization of Copper Plating | 2376 |
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Funding: Work supported by the Office of Naval Research and the High-Energy Laser Joint Technology Office. An experiment has been conducted to measure small differences in cavity Q caused by various cavity surface treatments. A requirement of the experiment was that it show little sensitivity to the reassembly with various test pieces. We chose a coaxial half-wave resonator, with an outer conductor extending significantly beyond the length of the inner conductor. The outer conductor acts as a cut-off tube, eliminating the need for electric termination and thus any RF-contacts that can influence the Q-measurements. The experiment is aimed at qualifying the performance of cyanide-copper plated GlidCop in comparison with that of a machined GlidCop surface. To maximize the sensitivity of the measurement we use a fixed outer conductor made of annealed OFE copper and only replace the inner conductor, which is mounted on a low-loss Teflon pedestal located in the low electric field region. The Q-values of machined GlidCop and cyanide-copper plated GlidCop inner conductors are measured against the reference Q of the annealed OFE co-axial cavity. This simple configuration allows a statistically significant number of repetitions of measurements and should provide accurate comparative measurements. |
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| WEPMS022 | Gain Scheduled Neural Network Tuned PI Feedback Control System for the LANSCE Accelerator | 2379 |
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| The current LANSCE LLRF system is an analog proportional/integral (PI) feedback control system that achieves amplitude and phase errors within 1% and 1 degree. The feedback system receives the cavity amplitude and phase, and the crosstalk between the amplitude and phase is significant. In this note, we propose an In-phase (I) and Quadrature (Q) based feedback control system which easily decouples the crosstalk of I and Q channels. For LANSCE-R, the current RF amplifier chain has to be preserved, so the controller output I/Q is transferred back to amplitude/phase values which drive the RF amplifier chain. The resultant feedback system reduces transient peaks of the RF and hence reduces the degradation of the RF amplifier chain. Self-tuning is performed every clock cycle. This feature of the feedback controller can reduce the beam loading transient drastically. The proposed control system is implemented with the Altera Stratix II FPGA. The proposed control system will first be tested on the low power test-stand to determine the robustness of the algorithm and will then be tested on a LANSCE Drift Tube Linac (DTL) tank. | ||
| WEPMS023 | Progress on New High Power RF System for LANSCE DTL | 2382 |
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Funding: Work supported by the United States Department of Energy, National Nuclear Security Agency, under contract DE-AC52-06NA25396 A new 200 MHz RF system is being developed for the LANSCE proton drift tube linac (DTL). A planned upgrade will replace parts of the DTL RF system with new generation components. When installed for the LANSCE-R project, the new system will reduce the total number of electron power tubes from twenty-four to seven in the DTL plant. The 3.4 MW final power amplifier will use a Thales TH628 Diacrode. This state-of-the-art device eliminates the large anode modulator of the present triode system, and will be driven by a new tetrode intermediate power amplifier. In this mode of operation, this intermediate stage will provide 150 kW of peak power. The first DTL tank requires up to 400 kW of RF power, which will be provided by the same tetrode driver amplifier. A prototype system is being constructed to test components, using some of the infrastructure from previous RF projects. High voltage DC power became available through innovative re-engineering of an installed system. A summary of the design and construction of the intermediate power amplifier will be presented and test results will be summarized. |
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| WEPMS024 | Upgrades to the DAHRT Second Axix Induction Cells | 2385 |
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| The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. The second axis, DARHT II, features a 3-MeV injector and a 15-MeV, 2-kA, 1.6-microsecond accelerator consisting of 74 induction cells and drivers. Major induction cell components include high flux swing magnetic material (Metglas 2605SC) and a MycalexTM insulator. The cell drivers are pulse forming networks (PFNs). The DARHT II accelerator cells have undergone a series of test and modeling efforts to fully understand their operational parameters. Physical changes in the cell oil region, the cell vacuum region, and the cell drivers, together with different operational and maintenance procedures, have been implemented in the prototype. A series of prototype acceptance tests have demonstrated that the required cell lifetime is met at the increased performance levels. Shortcomings of the original design are summarized and improvements to the design, their resultant enhancement in performance, and various test results are discussed. | ||
| WEPMS025 | LANSCE-R Low Level RF Control System | 2388 |
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| The Los Alamos Neutron Science Center proton accelerator is scheduled for refurbishment. A new low level RF(LLRF) system is part of the refurbishment plan since the existing LLRF system is analog-based and requires significant setup and maintenance time. Both field and resonance control aspects of the current system do not have the flexibility to meet future performance requirements. The LANSCE accelerator provides both H+ and H- beams and due to the various user requirements there are a number of different beam pulse types varying in timing and current. In order to meet user needs, LANSCE must simultaneously transport both H+ and H- in the accelerator. These requirements have motivated the development of a new LLRF system based on software defined radio technology. The new system will include field control using feedback and adaptive feed forward techniques, an upgraded resonance controller with frequency agility to improve startup and fault recovery times and a high power amplifier pre-compensation controller for improved cavity fill times and amplifier efficiency. Among the challenges with implementing the new system are interfacing with existing subsystems of the accelerator. | ||
| WEPMS026 | Improved Tuning Methods for Converter-Modulators | 2391 |
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Funding: This work is supported by the office of Basic Energy Sciences and the Office of Science of the Department of Energy The converter-modulator is a resonant power conditioning configuration that is optimized for a particular load impedance or parameter space. Although traction motor IGBT's are typically used for hard-switching application in the 1 kHz regime, the present use of high-power (10 - 15 MW) converter-modulators have used a 20 kHz resonant switching topology. This presents design challenges to maintain efficient and reliable switching characteristics for the IGBT's. Improved tuning methods and circuit topological changes now offer a significant reduction in IGBT switching losses as compared to those used with the Spallation Neutron Source (SNS) design (perhaps by 10). These circuit and topology changes should also permit Pulse Width Modulation (PWM) of the modulator output voltage to provide a regulated voltage without anomalous IGBT switching characteristics. This paper will review the results of this investigation based on models developed from the SNS converter-modulator operational data. |
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| WEPMS027 | The Klystron RF Systems for the Indiana University LENS Accelerator | 2394 |
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Funding: This work supported by the Indiana University Cyclotron Facility. This paper describes the Klystron RF systems for the Indiana University Low Energy Neutron Source (LENS) accelerator 425 MHz Radio Frequency Quadrupole (RFQ) and Drift Tube Linac (DTL) systems. Of interest in the power conditioning system is the design of the totem-pole grid-catch modulator for the mod-anode klystrons. This topology provides a fast rise and fall and closed loop regulation for the klystron mod-anode to cathode voltage, which minimizes RF amplitude and phase droop while maximizing efficiency. Another advantage is that short pulse high rep-rate operation is viable within the average power capabilities of the klystron. The 425 MHz, 1.25 MW klystron amplifier chain will also be detailed. Of final interest, is the digital low level RF system. This provides vector control of the cavity field using direct conversion, non-I/Q sampling architecture, at a sampling rate of 132 MHz with a 12-bit ADC. Four input and two output channels are integrated into a 6U VME module, with all DSP functions performed in Xilinx Spartan-3 field-programmable gate arrays. The design and implementation of these systems, coupled with LENS operational results, will be presented. |
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| WEPMS028 | Converter-Modulator Design and Operations for the ILC L-band Test Stand | 2397 |
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Funding: This work supported by Stanford Linear Accelerator Center, Oak Ridge National Laboratory, and the Department of Energy. To facilitate a rapid response to the International Linear Collider (ILC) L-Band development program at SLAC, a spare converter-modulator was shipped from Los Alamos. This modulator was to be a spare for the Spallation Neutron Source (SNS) accelerator at ORNL. The ILC application requires a 33% higher peak output power (15 MW) and output current (130 Amp). This presents significant design challenges to modify the existing hardware and yet maintain switching parameters and thermal cycling within the semiconductor component ratings. To minimize IGBT commutation and free-wheeling diode currents, a different set of optimizations, as compared to the SNS design, were used to tune the resonant switching networks. Additional complexities arose as nanocrystalline cores with different performance characteristics (as compared to SNS), were used to fabricate the resonant "boost" transformers. This paper will describe the electrical design, system modifications, modeling efforts, and resulting electrical performance as implemented for the ILC L-band test stand. |
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| WEPMS029 | LANSCE RF System Refurbishment | 2400 |
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| The Los Alamos Neutron Science Center (LANSCE) is in the planning phase of a refurbishment project that will sustain reliable facility operations well into the next decade. The LANSCE accelerator was constructed in the late 1960s and early 1970s and is a national user facility that provides pulsed protons and spallation neutrons for defense and civilian research and applications. The refurbishment will focus on systems that are approaching 'end of life' and systems where modern upgrades hold the promise for significant operating cost savings. The current baseline consists of replacing all the 201 MHz RF systems, upgrading a substantial fraction of the 805 MHz RF systems to high efficiency klystrons, replacing the high voltage systems, and replacing the low level RF cavity field control systems. System designs will be presented. The performance improvements will be described and the preliminary cost and schedule estimates will be discussed. | ||
| WEPMS030 | Design and Initial Testing of Omniguide Traveling-wave Tube Structures | 2403 |
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Funding: This work was funded in part by the LDRD Director's Postdoctoral Fellowship, Los Alamos National Laboratory. We propose to use the photonic band gap (PBG) structures for the construction of a traveling-wave tube (TWT) at W-band. Interest in millimeter-waves has increased in recent years due to applications in environmental monitoring and remote sensing. The development of wide-band mm-wave TWT amplifiers is underway at Los Alamos National Laboratory. A TWT would present a wide bandwidth source for remote mm-wave spectroscopy. PBG TWT structures have great potential for very large bandwidth and linear dispersion. In addition, being cheap to fabricate, the PBG structures enhance the commercial transferability of the W-band TWT technology. We employ an omniguide which is a one-dimensional version of the PBG structure representing a periodic system of concentric dielectric tubes as a slow-wave structure. A silica omniguide was designed to support a TM01-like mode with a phase velocity matching the one of a 120keV electron beam. The structure was fabricated, cold-tested and installed at our laboratory for the hot test. |
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| WEPMS031 | Development of a Temperature Mapping System for 1.3-GHz 9-Cell SRF Cavities | 2406 |
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| The mapping of outer wall temperature during the vertical test of a superconducting radio-frequency (SRF) cavity has been one of the most useful tools to detect bad spots of the cavity. However, few systems except a rotating-arm type one have been developed so far for 9-cell cavities. Since it will be an essential tool to identify the failure of the cavities, we started to develop a fix-type temperature mapping system that will enable us to map the temperature of 9-cell cavities in a much shorter time than rotating-arm type. This paper describes the design, status of the development and hopefully some results of the mapping of actual cavities. | ||
| WEPMS032 | Pre-conceptual Design of Automated Systems for SRF Cavity Assembly and Optical Inspection | 2409 |
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| The International Linear Collider (ILC) will require ~20,000 Superconducting Radio- Frequency (SRF) cavities. Improving the yield of high-gradient (>35 MV/m) cavities is currently one of the most critical issues for the ILC. The LANL has been tasked to analyze the failure and feedback the results to the industry and academia. We have started an effort to develop a pre-conceptual design of an automated system to optically inspect the inner surface that showed heating with a thermometry system. We have also started a pre-conceptual design of an automated system for assembling and sealing the flanges after high-pressure rinsing in the clean room. This could reduce the chance of particle contamination due to touch labor, leading to a field emission free cavities and higher yield of high-gradient cavities. | ||
| WEPMS033 | LANSCE 201 MHz and 805 MHz RF System Experience | 2412 |
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Funding: Work supported by the United States Department of Energy, National Nuclear Security Agency, under contract DE-AC52-06NA25396 The LANSCE RF system consists of four RF stations at 201 MHz and 44 klystrons at 805 MHz. In the LANSCE accelerator, the beam source is injected into the RF system at 0.75 MeV. The beam is then accelerated to 100 MeV in four drift tube linac (DTL) tanks, driven at 201.25 MHz. Each 201 MHz RF system consists of a train of amplifiers, including a solid state amplifier, a tetrode, and then at triode. After the DTL, the beam is accelerated from 100 MeV to 800 MeV in the forty-four coupled cavity linac (CCL) tanks at 805 MHz. The machine operates with a normal RF pulse width of 835 microseconds at a repetition rate up to 120 Hz, and sometimes operates with a pulse width up to 1.2 microseconds for single pulses. This RF system has been operating for about 37 years. This paper summarizes the recent operational experience. The reliability of the 805 MHz and 201 MHz RF systems is discussed, and a summary the lifetime data of the 805 MHz klystrons and 201 MHz triodes is presented. |
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| WEPMS034 | Mitigation of Electric Breakdown in an RF Photoinjector by Removal of Tuning Rods in High-Field Regions | 2415 |
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Funding: United States Department of Energy The pi-mode resonant frequency of the 1.6 cell SLAC/BNL/UCLA style RF photoinjector electron gun is conventionally tuned using cylindrical copper tuning pieces that extend into the full-cell cavity through holes in the side of the gun. This design begins to fail in many versions of this popular gun design at higher voltage levels, when the cavity undergoes electric breakdown in the vicinity of the tuners. In order to remove the tuners from the region of high electric field, mitigating this problem, the full cell geometry must be changed significantly. We report on a method of accomplishing this, in which we use a mechanical device of custom design to stretch the cavity structure of an existing photoinjector in order to tune the resonant frequency up by over 2 MHz. We present results of testing the modified photoinjector in an RF test bed with both copper and magnesium cathodes, succeeding in putting approximately 8 - 10 MW of RF power into the gun. This is an improvement over the 4 MW routinely achieved in a similar gun using conventional tuning methods installed at the UCLA Neptune laboratory. |
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| WEPMS035 | Measurement of the UCLA/URLS/INFN Hybrid Gun | 2418 |
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Funding: This work performed under the auspices of the U. S. Department of Energy under contract numbers DE-FG-98ER45693 and DE-FG03-92ER40693. The hybrid photoinjector is a high current, low emittance photoinjector/accelerator and is under design and collaboration at Roma University La Sapienza, INFN - Laboratori Nazionali di Frascati and the UCLA Particle Beam Physics Lab. The hybrid standing wave-traveling wave photoinjector uses a coupling cell to divide power between a high-field 1.6 cell standing wave photoinjector, for electron emission and collection, and a low power traveling wave accelerator, for acceleration to desired energies at low emittances. Simulation results show promising beam properties of less than 4 mm-mrad emittance, energy spreads of 1.5%, and currents as high as 1.2 kA at energies of 21 MeV. We report on the progress of RF design and results of cold test RF measurements at the UCLA Pegasus Laboratory, including methods for measurements and difficulties arising in the transition from simulation to physical measurements. |
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| WEPMS036 | LCLS LLRF Upgrades to the SLAC Linac | 2421 |
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Funding: DOE The Linac Coherent Light Source at SLAC will be the brightest X-ray laser in the world when it comes on line. In order to achieve the brightness a 100fS length electron bunch is passed through an undulator. To creat the 100fS bunch, a 10pS electron bunch, created from a photo cathode in an RF gun, is run off crest on the RF to set up a position to energy correlation. The bunch is then compressed chicanes. The stability of the RF system is critical in setting up the position to energy correlation. Specifications derived from simulations require the RF system to be stable to below 100fS in several critical injector stations and the last kilometer of linac. The SLAC linac RF system is being upgraded to meet these requirements. |
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| WEPMS037 | RF Distribution Optimization in the Main Linacs of the ILC | 2424 |
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Funding: Work supported by the U. S. Department of Energy under contract DE-AC02-76SF00515. The nominal design gradient for the ILC is 31.5 MV/m, but the L-band superconducting cavities built to date have demonstrated a range in sustainable gradient extending below this goal, limited by Q-dropoff and quenching. An economically feasible cavity acceptance rate will include in the linacs a certain percentage of sub-performing cavities. We examine how, with a customizable RF distribution scheme, one can most efficiently distribute power from one klystron amongst 24 nine-cell cavities. The nominal cavity fills to the design gradient at the time the beam arrives, after which the beamloading voltage exactly cancels any further rise, yielding constant gradient during the bunch train. Along with adjustable RF power, we assume adjustable cavity coupling, or loaded quality factor, so that the gradient can be leveled in non-nominal cavities, to avoid quench-inducing overshoots. We explore these and related issues for the ILC linac high-power RF. |
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| WEPMS038 | RF Design of Normal Conducting Deflecting Structures for the Advanced Photon Source | 2427 |
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| Use of normal conducting deflecting structures for production of short x-ray pulses is now being implemented at the Advanced Photon Source (APS). The structures have to produce up to 6 MV maximum deflection per structure at a 1kHz repetition rate. At the same time, the nominal beam quality must be maintained throughout the APS ring. Following these requirements, we proposed 2815 MHz standing wave deflecting structure with heavy wakefield damping. In this paper, we discuss the design considerations and present our current results. | ||
| WEPMS039 | High Power Tests of Normal Conducting Single-Cell Structures | 2430 |
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Funding: This work was supported by the U. S. Department of Energy contract DE-AC02-76SF00515.
We report results of the first high power tests of single-cell traveling-wave and standing-wave accelerating structures. These tests are part of an experimental and theoretical study of RF breakdown in normal conducting structures at 11.4 GHz*. The goal of this study is to determine the gradient potential of normal conducting, RF powered particle beam accelerators. The test setup consists of reusable mode converters and short test structures powered by SLAC?s XL-4 klystron. This setup was created for economic testing of different cell geometries, cell materials and preparation techniques with short turn-around time. The mode launchers and structures were manufactured at SLAC and KEK and tested in the klystron test laboratory at SLAC.
* V. A. Dolgashev et al., "RF Breakdown In Normal Conducting Single-Cell Structures," SLAC-PUB-11707, Particle Accelerator Conference (PAC 05), Knoxville, Tennessee, 16-20 May 2005, pp. 595- 599. |
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| WEPMS040 | Active RF Pulse Compression Using Electrically Controlled Semiconductor Switches | 2433 |
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| In this paper, we will present our recent results on the research of the ultrafast high power RF switches based on silicon. We have developed a switch module at X-band which can use a silicon window as the switch, and scaled it to 30GHz for the CLIC application. The switching is realized by generation of carriers in the bulk silicon. The carriers can be generated electrically or/and optically. The electrically controlled switches use PIN diodes to inject carrier. We have built the PIN diode switches at X-band, with <300ns switching time. The optically controlled switches use powerful laser to excite carriers. By combining the laser excitation and electrical carrier generation, significant reduction in the required power of both the laser and the electrical driver is expected. High power test is under going. | ||
| WEPMS041 | Multipacting Simulations of TTF-III Coupler Components | 2436 |
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Funding: This work was supported by US DOE contract No. DE-AC02-76SF00515. This work was performed under the auspices of the US DOE by the University of California, LLNL under Contract No. W-7405-Eng-48. The TTF-III coupler adopted for the ILC baseline cavity design has shown a tendency to have long initial high power processing times. A possible cause for the long processing times is believed to be multipacting in various regions of the coupler. To understand performance limitations during high power processing, SLAC has built a flexible high-power coupler test stand. The plan is to test individual sections of the coupler, which includes the cold and warm coaxes, the cold and warm bellows, and the cold window, using the test stand to identify problematic regions. To provide insights for the high power test, detailed numerical simulations of multipacting for these sections will be performed using the 3D multipacting code Track3P. The simulation results will be compared with measurement data. |
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| WEPMS042 | Optimization of the Low-Loss SRF Cavity for the ILC | 2439 |
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Funding: Work supported by DOE contract DE-AC02-76SF00515. The Low-Loss shape cavity design has been proposed as a possible alternative to the baseline TESLA cavity design for the ILC. The advantages of this design over the TESLA cavity are its lower cryogenic loss, and higher achievable gradient due to lower surface fields. High gradient prototypes for such designs have been tested at KEK (ICHIRO) and JLab (LL). However, issues related to HOM damping and multipacting (MP) still need to be addressed. Preliminary numerical studies of the prototype cavities have shown unacceptable damping for some higher-order dipole modes if the typical TESLA HOM couplers are directly adapted to the design. The resulting wakefield will dilute the beam emittance thus reduces the machine luminosity. Furthermore, high gradient tests on a 9-cell prototype at KEK have experienced MP barriers although a single LL cell had achieved a high gradient. From simulations, MP activities are found to occur in the end-groups of the cavity. In this paper, we will present the optimization results of the end-groups for the Low-Loss shape for effective HOM damping and alleviation of multipacting. Comparisons of simulation results with measurements will also be presented. |
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| WEPMS043 | An RF Waveguide Distribution System for the ILC Test Accelerator at NML | 2442 |
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Funding: Work supported by the U. S. Department of Energy under contract DE-AC02-76SF00515. An ILC R&D facility is being constructed in the NML building at Fermilab which, in addition to an injector and beam dump with spectrometer, will contain up to three cryomodules worth of ILC-type superconducting 9-cell cavities, 24 in all. This linac will be powered by a single klystron. As part of SLAC?s contribution to this project, we will provide a distribution network in WR650 waveguide to the various cavity couplers. In addition to commercial waveguide components and circulators and loads developed for TESLA, this sytem will include adjustable tap-offs, and customized hybrids. In one configuration, the circulators will be removed to test pair-wise cancellation of cavity reflections through hybrids. The system will be pressurized with nitrogen to 3 bar absolute to avoid the need for SF6 at windows or circulator. The full distribution for the first cryomodule will be delivered and installed later this year. We describe the design of the system and completed RF testing. |
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| WEPMS044 | High Power Switch for the SMTF Modulator | 2445 |
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Funding: Work supported by the Department of Energy under contract No. DE-AC03-76SF00515. A compact, water cooled, high power switch for the Superconducting Module Test Facility (SMTF) long-pulse klystron modulator has been designed and implemented at the Fermi National Accelerator Laboratory (FNAL). This solid-state switch is composed of six series devices, each having a rating of 4.5 kV at 2000 Adc. Latest generation, press-pack IGBT modules are utilized to reduce the physical size and complexity of the switch assembly. The new switch and its associated controller provide a high degree of redundancy and fail-safe operation, which meets the modulator requirements. This paper describes the general switch assembly, IGBT protection and control schemes, and test results. |
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| WEPMS045 | Power Modulators for FERMI Linac's Klystrons. | 2448 |
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The conventional line type modulators used for ELETTRA will have to be replaced for FERMI due to the increase in the pulse repetition frequency (PRF) from 10 to 50 Hz. The requirements for the FERMI modulator are as follows. The klystron used is a Thales TH2132 with a microperviance of 1.9-2.1 uA/V**(3/2). The peak voltage from the modulator is 320 kV, and the current is 350 A. The pulse width is 4.5 us, with a PRF of 50 Hz. Flat top should be better than ?0.5 % of the peak voltage. Prototypes for an upgraded line type modulator and a solid state induction type modulator[1] are in fabrication. The solid state design uses eight induction cells, each cell driven by two parallel Insulated Gate Bipolar Transistors (IGBT). Each IGBT will power a METGLAS 2605CO core with 4 kV and 3 kA for up to 5 us. A single turn is passed through the aperture of each of the cells, inductively adding the pulse voltages. The output from the modulator is then fed to a conventional pulse transformer to reach the 320 kV requirement. This paper presents the system design of both modulator types as well as details of the IGBT drivers, control electronics, IGBT and klystron protection and test data.
1. "NLC Hybrdi Solid State Induction Modulator" R. L. Cassel, etal, Lubeck, Germany, Linac 2004. |
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| WEPMS047 | Selecting RF Amplifiers for Impedance Controlled LLRF Systems - Nonlinear Effects and System Implications | 2451 |
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Funding: Work supported by the U. S. Department of Energy under contract #DE-AC02-76SF00515 Several high-current accelerators use feedback techniques in the accelerating RF systems to control the impedances seen by the circulating beam. These Direct and Comb Loop architectures put the high power klystron and LLRF signal processing components inside feedback loops, and the ultimate behavior of the systems depends on the individual sub-component properties. Imperfections and non-idealities in the signal processing leads to reduced effectiveness in the impedance controlled loops. In the PEP-II LLRF systems non-linear effects have been shown to reduce the achievable beam currents, increase low-mode longitudinal growth rates and reduce the margins and stability of the LLRF control loops. We present measurements of the driver amplifiers used in the PEP-II systems, and present measurement techniques needed to quantify the small-signal gain, linearity, transient response and image frequency generation of these amplifiers. Results are presented from measurements of 5 different types of amplifiers, and the trade-offs in selecting between them highlighted. |
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| WEPMS048 | Modelling Imperfection Effects on Dipole Modes in TESLA Cavity | 2454 |
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Funding: Work supported by DOE contract DE-AC02-76SF00515 The actual cell shape of the TESLA cavities differ from the ideal due to fabrication errors, the addition of stiffening rings and the frequency tuning process. Cavity imperfection shift the dipole mode frequencies and alter the Qext's from those computed for the idea cavity. A Qext increase could be problematic if its value exceeds the limit required for ILC beam stability. To study these effects, a cavity imperfection model was established using a mesh distortion method. The eigensolver Omega3P was then used to find the critical dimensions that contribute to the Qext spread and frequency shift by comparing predictions to TESLA cavity measurement data. Using the imperfection parameters obtained from these studies, artificial imperfection models were generated and the resulting wakefields were used as input to the beam tracking code Lucretia to study the effect on beam emittance. In this paper, we present the results of these studies and suggest tolerances for the cavity dimensions. |
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| WEPMS050 | HOM and LOM Coupler Optimizations for the ILC Crab Cavity | 2457 |
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Funding: Work supported by DOE contract DE-AC02-76SF00515 The FNAL 9-cell 3.9GHz deflecting cavity designed for the CKM experiment was chosen as the baseline design for the ILC BDS crab cavity. Effective damping is required for the lower-order TM01 modes (LOM), the same-order TM11 modes (SOM) as well as the HOM modes to minimize the beam loading and beam centroid steering due to wakefields. Simulation results of the original CKM design using the eigensolver Omega3P showed that both the notch filters of the HOM/LOM couplers are very sensitive to the notch gap, and the damping of the unwanted modes is suboptimal for the ILC. To meet the ILC requirements, the couplers were redesigned to improve the damping and tuning sensitivity. With the new design, the damping of the LOM/SOM/HOM modes is significantly improved, the sensitivity of the notch filter for the HOM coupler is reduced by one order of magnitude and appears mechanically feasible, and the LOM coupler is simplified by aligning it on the same plane as the SOM coupler and by eliminating the notch filter. In this paper, we will present the coupler optimization and tolerance studies for the crab cavity. |
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| WEPMS051 | One Channel, Multi-Mode Active Pulse Compressor | 2460 |
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Funding: Research sponsored in part by US DoE. Basic studies of factors that limit RF fields in warm accelerator structures require experiments at RF power that can be produced using pulse compression. This approach is being implemented to compress output pulses from the Yale/Omega-P 34-GHz magnicon to produce ~100-200 MW, 100 ns pulses. A new approach for passive pulse compression is a SLED-II type circuit operating with axisymmetrical modes of the TE0n type that requires only a single channel instead of the usual double channel scheme. This allows avoidance of a 3-dB coupler and need for simultaneous fine tuning of two channels. A 30 GHz passive prototype was tested at low power level in order to demonstrate key principles. The prototype showed a power gain 3,8 at a compression ratio 6:1 for an efficiency 63%. An active version of the one-channel pulse compressor is also suggested. It is attractive due to a possibility to achieve higher power gain. The mentioned active version naturally requires an electrically controlled coupler. In particular, as active elements of the coupler we suggest to use gas filled discharge tubes or ferroelectrics which have well recommended itself at 11.4 GHz experiments. |
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| WEPMS053 | Yale Ka-Band Facility For High-Gradient Accelerator R&D: Status Report | 2463 |
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Funding: Research sponsored by US DoE Development of a future multi-TeV warm collider demands new technological solutions and new accelerator structure materials. The Ka-Band test facility being put into operation at Yale University that centers on the Yale/Omega-P 34-GHz magnicon allows users to carry out high gradient experiments on RF breakdown, pulse fatigue, tests of new high power pulse manipulation systems, and RF components. The magnicon is now conditioned for a pulse width up to 1 μs, at an output power level high enough for basic studies of electric and magnetic RF field limits at surfaces of conductors and dielectrics. The high-power waveguide transmission system for the facility is assembled and ready for tests. It includes RF windows, phase shifters, 13 mm diameter TE 11 waveguides, mode converters, etc. Recently the assembled system has undergone conditioning in preparation for carrying out first "user" experiments. |
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| WEPMS054 | 45 MW, K-Band Second-Harmonic Multiplier for Testing High-Gradient Accelerator Structures | 2466 |
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Funding: Research supported by the Department of Energy, Division of High Energy Physics A relatively simple and inexpensive two-cavity 45 MW, 22.8 GHz second-harmonic multiplier is considered as an RF source for High-Gradient experiments. The design is to be based on use of an existing SLAC electron gun, such as the XL-4 gun. RF drive power would be supplied from a 50 MW SLAC klystron and modulator, and a second modulator would be used to power the gun in the multiplier. An important feature of the harmonic multiplier is TE 01 circular waveguide for output RF power extraction. |
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| WEPMS055 | SQUID-based Nondestructive Testing Instrument of Dished Niobium Sheets for SRF Cavities | 2469 |
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Funding: Acknowledgment: This work is supported by DOE grant DE-FG02-05ER84241 Currently available technology can only inspect flat sheets and allow the elimination of defective flat sheets before the expensive forming and machining of the SRF cavity half-cells, but it does not eliminate the problem of remaining or uncovered surface impurities after partial chemical etching of the half-cells, nor does it detect any defects that may have been added during the fabrication of the half-cells. AMAC has developed a SQUID scanning system based on eddy current technique that allows the scanning of curved Nb samples that are welded to make superconducting RF cavity half-cells. AMAC SQUID scanning system successfully located the defects (Ta macro particles about 100 mm diameter) in a flat Nb sample (top side) and was able to also locate the defects in a cylindrical surface sample (top side). It is more significant that the system successfully located the defects on the backside of the flat sample and curved sample or 3-mm from the top surface. The 3-D SQUID-based Nondestructive instrument will be further optimized and improved in making SRF cavities and allow inspection and detection during cavity manufacturing for achieving highest accelarating fields. |
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| WEPMS056 | High Current, Large Aperture, Low HOM, Single Crystal Nb 2.85GHz Superconducting Cavity | 2472 |
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Funding: Footnotes: The project was funded by the US Department of Energy under contract DE-FG02-05ER84346 There is an increasing demand for High beam Current, high Radio-Frequency (RF) power S-band cavities in existing and new accelerator projects to produce a very brilliant, broadband, teraherz coherent synchrotron radiation source (CRS). To achieve this goal, the RF cavities must be upgraded to a gap voltage of 1.5 MV in the limited space available in the machine with a high gradient superconducting cavity. At the present time there are no cavities and accessories designed to support the high beam currents of up to 100 mA and at the same time provide a high gap voltage at such a high S-band frequency. AMAC proposed a High Current, Large Aperture, Low HOM, Single Crystal Nb 2.85GHz Superconducting Cavity with high RF Power Coupler and HOM absorber device. Comprehensive simulation and optimization to determine the SRF cavity parameters to meet the requirements, provided two alternate designs for the RF input couplers, performed a detailed Higher Order Modes (HOM) analysis, and proposed an HOM absorber concept to dampen the modes exited in the cavity due to the high beam current and high bunch intensity. |
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| WEPMS057 | Innovative Modular, Multiple Power Levels, 325 MHz Spokes Cavities Power Couplers | 2475 |
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Funding: Footnotes: The project was funded by the US Department of Energy under contract DE-FG02-05ER84346 In order to increase the protons energy up to 8 GeV in a driver Linac, the particles must be accelerated through various stages and three different power levels (25kW, 100kW and 210kW) are required for the 325 MHz Fermilab Proton Driver couplers. The problem identified by the project is that no High RF power coupler for these cavities has ever been produced using US industrial capabilities. AMAC proposed a novel resolution by development of innovative modular, multiple power levels, 325 MHz spoke cavities power couplers, which to meet three type cavities with one coupler design. The simulation and concept design are presented. The results of HFSS, MAFIA, ANSYS, and Multipacting are also discussed. |
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| WEPMS059 | Performance of the First Refurbished CEBAF Cryomodule | 2478 |
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Funding: U. S. DOE Contract No. DE-AC05-06OR23177. This manuscript has been authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177. The Thomas Jefferson National Accelerator Facility has begun a cryomodule refurbishment project. The goal of this project is robust 6 GeV, 5 pass operation of the Continuous Electron Beam Accelerator Facility (CEBAF). The scope of the project includes removing, refurbishing and replacing 10 CEBAF cryomodules at a rate of three per year. Refurbishment includes reprocessing of SRF cavities to eliminate field emission and increase the nominal gradient from the original 5 MV/m to 12.5 MV/m. New "dogleg" couplers between the cavity and helium vessel flanges will intercept secondary electrons that produce arcing on the 2 K ceramic window in the Fundamental Power Coupler (FPC). Modification of the Qext of the FPC will allow higher gradient operations. Other changes include new ceramic RF windows for the air to vacuum interface of the FPC and improvements to the mechanical tuners. Any damaged or worn components will be replaced as well. Currently, the first of the refurbished cryomodules has been installed and tested both in the Cryomodule Test Facility and in place in the North Linac of CEBAF. This paper will summarize the results of these tests. |
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| WEPMS060 | A Digital Self Excited Loop for Accelerating Cavity Field Control | 2481 |
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Funding: Notice: Authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177. We have developed a digital process that emulates an analog oscillator and ultimately a self excited loop (SEL) for field control. The SEL, in its analog form, has been used for many years for accelerating cavity field control. In essence the SEL uses the cavity as a resonant circuit – much like a resonant ?tank? circuit is used to build an oscillator. An oscillating resonant circuit can be forced to oscillate at different, but close, frequencies to resonance by applying a phase shift in the feedback path. This allows the circuit to be phased locked to a master reference, which is crucial for multiple cavity accelerators. For phase and amplitude control the SEL must be forced to the master reference frequency, and feedback provided for in both dimensions. The novelty of this design is in the way digital signal processing (DSP) is structured to emulate an analog system. While the digital signal processing elements are not new, to our knowledge this is the first time that the digital SEL concept has been designed and demonstrated. This paper reports on the progress of the design and implementation of the digital SEL for field control of superconducting accelerating cavities. |
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| WEPMS062 | Development of a Superconducting Connection for Niobium Cavities | 2484 |
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Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177. Several, partially successful attempts have been made to develop a superconducting connection between adjacent niobium cavities with the capability to carry up to 30 mT of the magnetic flux. Such a connection would be particularly of great benefit to layouts of long accelerators like ILC because it would shorten the distances between structures and therefore the total length of an accelerator with the associated cost reductions. In addition the superconducting connection would be ideal for a super-structure, two multi-cell cavities connected through a half wavelength long beam pipe providing the coupling. Two welded prototypes of super-structure have been successfully tested with the beam at DESY. The chemical treatment and water rinsing was rather complicated for these prototypes. We have engaged in a program to develop such a connection based on the Nb55Ti material. Several options are pursued such as e.g.a two-cell cavity is being used to explore the reachable magnetic flux for the TESLA like connection with a squeezed niobium gasket between the flanges. In this contribution we will report about the progress of our investigations. |
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| WEPMS063 | Preliminary Results from Prototype Niobium Cavities for the JLab Ampere-Class FEL | 2487 |
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Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177, and by the office of Naval Research under contract to the Department of Energy.
In a previous paper the cavity* design for an Ampere-class cryomodule was introduced. We have since fabricated a 1500 MHz version of a single cell cavity with waveguide couplers for HOM and fundamental power, attached to one end of the cavity, a 5-cell cavity made from large grain niobium without couplers and a complete 5-cell cavity from polycrystalline niobium featuring waveguide couplers on both ends. A 750 MHz single cell cavity without endgroups has also been manufactured to get some information about obtainable Q-values, gradients and multipacting behavior at lower frequency. This contribution reports on the various tests of these cavities.
* R. A.Rimmer et al.; EPAC 2006, paper MOPCH182 |
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| WEPMS065 | CEBAF New Digital LLRF System Extended Functionality | 2490 |
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Funding: JSA/DOE Contract - DE-AC05-06OR23177
The new digital LLRF system for the CEBAF 12GeV accelerator will perform a variety of tasks, beyond field control.* In this paper we present the superconducting cavity resonance control system designed to minimize RF power during gradient ramp and to minimize RF power during steady state operation. Based on the calculated detuning angle, which represents the difference between reference and cavity resonance frequency, the cavity length will be adjusted with a mechanical tuner. The tuner has two mechanical driving devices, a stepper motor and a piezo-tuner, to yield a combination of coarse and fine control. Although LLRF piezo processing speed can achieve 10 kHz bandwidth, only 10 Hz speed is needed for 12 GeV upgrade. There will be a number of additional functions within the LLRF system; heater controls to maintain cryomodule's heat load balance, ceramic window temperature monitoring, waveguide vacuum interlocks, ARC detector interlock and quench detection. The additional functions will be divided between the digital board, incorporating an Altera FPGA and an embedded EPICS IOC. This paper will also address hardware evolution and test results performed with different SC cavities.
*RF Control Requirements for the CEBAF Energy Upgrade Cavities, C. Hovater, J. Delayen, L. Merminga, T. Powers, C. Reece, Proceedings 2000 Linear Accelerator Conference, Monterey, CA , August 2000 |
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| WEPMS068 | JLab High-Current CW Cryomodules for ERL and FEL Applications | 2493 |
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Funding: Authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177, and by The Office of Naval Research under contract to the Dept. of Energy. We describe the developments underway at JLab to develop new CW cryomodules capable of transporting up to Ampere-levels of beam currents for use in ERLs and FELs. Goals include an efficient cell shape, high packing factor for efficient real-estate gradient and very strong HOM damping to push BBU thresholds up by two or more orders of magnitude compared to existing designs. Cavity shape, HOM damping and ancillary components are optimized for this application. Designs are being developed for low-frequency (750 MHz), Ampere-class compact FELs and for high-frequency (1.5 GHz), 100 mA configurations. These designs and concepts can easily be scaled to other frequencies. We present the results of conceptual design studies, simulations and prototype measurements. These modules are being developed for the next generation ERL based high power FELs but may be useful for other applications such as high energy light sources, electron cooling, electron-ion colliders, industrial processing etc. |
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| WEPMS070 | Simulation and Measurements of a Heavily HOM-Damped Multi-cell SRF Cavity Prototype | 2496 |
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Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U. S. DOE Contract No. DE-AC05-06OR23177, and by The Office of Naval Research under contract to the Dept. of Energy.
After initial cavity shape optimization* and cryomodule development** for an Ampere-class FEL, we have simulated the whole 5-cell high-current (HC) cavity structure with six waveguide couplers for HOM damping and fundamental power coupling. The time-domain wakefield method using MAFIA is primarily used for calculation of the broadband impedance. Microwave Studio and Omega-3P are also used for the calculation of external Q (Qext) of individual HOMs. A half scale (1497MHz) single-cell model and a 5-cell copper cavity including dummy HOM waveguide loads were fabricated. Details of measurement results on these prototypes including HOM Qext spectrum, bead-pull data, data analysis technique and comparison to the simulations will be presented.
* H. Wang et. al., "Elliptical Cavity Shape Optimization for Acceleration and HOM Damping," Proc. PAC 05, Knoxville TN, USA, 2005* R. A.Rimmer et al.; EPAC 2006, paper MOPCH182 |
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| WEPMS071 | EVIDENCE FOR FOWLER-NORDHEIM BEHAVIOR IN RF BREAKDOWN | 2499 |
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Funding: Supported in part by DOE STTR grant DE-FG02-05ER86252 Microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 800 MHz RF cavity experiments are used to investigate the mechanism of RF breakdown. The images show evidence for melting and boiling in small regions of ~10 micron diameter on tungsten, molybdenum, and beryllium electrode surfaces. In these experiments, the dense hydrogen gas in the cavity prevents electrons or ions from being accelerated to high enough energy to participate in the breakdown process so that the only important variables are the fields and the metallic surfaces. The distributions of breakdown remnants on the electrode surfaces are compared to the maximum surface gradient E predicted by an ANSYS model of the cavity. The surface local density of spark remnants, presumably the probability of breakdown, shows a power law dependence on the maximum gradient, with E10 for tungsten and molybdenum and E7 for beryllium. This is reminiscent of Fowler-Nordheim behavior of electron emission from a cold cathode, which is explained by the quantum-mechanical penetration of a barrier that is characterized by the work function of the metal. |
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| WEPMS072 | Status and Performance of the Spallation Neutron Source Superconducting Linac | 2502 |
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Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy The Superconducting Linac at SNS has been operating with beam for almost two years. As the first operational pulsed superconducting linac, many of the aspect of its performance were unknown and unpredictable. A lot of experience has been gathered during the commissioning of its components, during the beam turn on and during operation at increasingly higher beam power. Some cryomodules have been cold for well over two years and have been extensively tested. The operation has been consistently conducted at 4.4 K and 10 and 15 pulses per second, with some cryomodules tested at 30 and 60 pps and some tests performed at 2 K. Careful balance between safe operational limits and the study of conditions, parameters and components that create physical limits has been achieved. This paper presents the experience and the performance of the superconducting cavities and of the associated systems with and without beam. |
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| WEPMS074 | Design and High Power Processing of RFQ Input Power Couplers | 2505 |
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Funding: This work was supported by SNS through UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy. A RF power coupling system has been developed for future upgrade of input coupling of the RFQ in the SNS linac. The design employs two coaxial loop couplers for 402.5 MHz operation. Each loop is fed through a coaxial ceramic window that is connected to an output of a magic-T waveguide hybrid through a coaxial to waveguide transition. The coaxial loop couplers are designed, manufactured, and high power processed. Two couplers will be used in parallel to power the accelerating structure with up to total 800 kW peak power at 8% duty cycle. RF and mechanical properties of the couplers are discussed. Result of high power RF conditioning that is performed in the RF test facility of the SNS is presented. |
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| WEPMS075 | Development and Testing of High Power RF Vector Modulators | 2508 |
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Funding: This work was supported by SNS through UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy. High power vector modulators can allow a fan-out RF power distribution system that can power many accelerating cavities from a single high-power klystron amplifier. The configuration enables independent control of amplitudes and phases of RF voltages at the cavities. A vector modulator employs either one or two hybrids with two fast phase shifters. Prototype high power RF vector modulators employing a hybrid and two fast ferrite phase shifters in coaxial TEM transmission lines for 402.5 MHz and 805 MHz are built and tested. RF properties of the design and result of high power testing are presented. |
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| WEPMS076 | Status of the SNS Cryomodule Test | 2511 |
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Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy The cryomodule tests are on going to have better understandings of physics as a whole and eventually to provide safe and reliable operation for neutron production. Some features are revealed to be interesting issues and need more attentions than expected, such as operating condition, collective effects between cavities, HOM coupler issues, end-group stability, cavity-coupler interactions, and vacuum/gas physics, waiting for more investigations. Up to now SNS cryomodules were mainly tested at 4.4 K, 10 pulse per second (pps) and 30 pps/60 pps tests are under progress. This paper presents the experiences and the observations during tests of cryomodules. |
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| WEPMS079 | Dynamic Fault Detection Chassis for the 1 MW High Voltage Converter Modulator System at the Spallation Neutron Source | 2514 |
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Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy. The high frequency switching megawatt-class High Voltage Converter Modulator (HVCM) developed by Los Alamos National Laboratory for the Oak Ridge National Laboratory's Spallation Neutron Source (SNS), is now in operation. One of the major problems with the modulator systems is shoot-thru conditions that can occur in an IGBTs H-bridge topology resulting in large fault currents and device failure in a few microseconds. The Dynamic Fault Detection Chassis (DFDC) is a fault monitoring system; it monitors transformer flux saturation using a window comparator and dV/dt events on the cathode voltage caused by any abnormality such as capacitor breakdown, transformer primary turns shorts, or dielectric breakdown between the transformer primary and secondary. If faults are detected, the DFDC will inhibit the IGBT gate drives and shut the system down, significantly reducing the possibility of a shoot-thru condition or other equipment damaging events. In this paper,we will present system integration considerations, performance characteristics of the DFDC, and discuss its ability to significantly reduce costly down time for the entire facility. |
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| WEPMS080 | SRF Cavity Transient Beam Loading Detection - Simulation and Measurement | 2517 |
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Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy. Beam phase measurement based on detection of transient beam loading signal in a Superconducting (SC) cavity is utilized to setup the cavity synchronous phase. It has the potential to become a fast tune-up technique for a high intensity SC electron linac, as cavity phase could be determined precisely with only a few beam pulses. The paper introduces a transient detector study in the Spallation Neutron Source (SNS) proton linac, and discusses one of the major challenges - stochastic noise in the cavity RF system, which deteriorates the precision and increases the time needed for phase measurement with this technique. We analyze the influence of RF noise to the phase measurement in a simulation study with a beam-cavity model. Beam signal measurement with the cavity Low Level RF (LLRF) system and the initial experiment of prototype detectors are briefly introduced. |
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| WEPMS081 | Simulation and Initial Test Result of the SNS Ring RF System | 2520 |
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Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy A simulation code has been developed for the study of the Spallation Neutron Source (SNS) ring RF control. The code uses the time-domain solvers to compute beam-cavity interactions, and FFT methods to simulate time responses of the linear RF system. The important ingredients of the system are considered in the simulation model, which include the beam loading, dynamic cavity detuning, circuit bandwidth, loop delay, proportional-integral (P-I) controller for feedback and adaptive feed forward, stochastic noise, with-in-turn RF parameter change, beam current fluctuation and beam bunch leakage, etc. The beam loss in the accumulation ring goes up as the beam power increases, and thus a precise control of bunching voltage phase and amplitude is required to limit beam loss. This simulation tool will help the development a correct RF control and to achieve the goal of minimizing the beam loss. |
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| WEPMS082 | PVC - An ILC RF Cryomodule Software Simulator | 2523 |
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The Penn Virtual Cavity (PVC) simulator is a object oriented RF Cavity simulator with a user friendly Linux GUI, as well as a web interface. It is a tool to help understand the effects of each component in the RF system. It can simulate an International Linear Collider (ILC) cryomodule consisting of eight 9-cell cavities, together with its associated high voltage modulator, a klystron, and RF power distribution system. The uses range from experts designing LLRF control algorithms, to beginners learning about the general RF characteristics of the SRF cavities. PVC explores effects such as Lorentz Detuning, beam loading (with bunch to bunch fluctuations), 8/9pi modes, I/Q feedback and feedforward, cavity Q-drop, amplitude and phase jitter and ripples, as well as calibration errors. The current status of the PVC and the conclusions derived from the simulations will be reported, along with comparisons to the DESY-TTF cryomodules.
http://einstein.hep.upenn.edu/~keungj/simulation.html |
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| WEPMS083 | A Low-Voltage Klystron for the ILC and ILC Testing Program | 2526 |
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Funding: Work supported by the US Department of Energy. FAR-TECH, Inc. is developing and building a 36 kV, 830 kW klystron for the International Linear Collider (ILC) testing program. A variant of the tube can also be used to supply RF energy for a 2-3 meter section of ILC. The tube design is of the multiple-beam klystron (MBK) type, using ten beams with confined flow focusing. The design optimizes small tube size and low cost. The initial prototype will use an electromagnet, but the design allows for the eventual use of a permanent magnet solenoid. An efficiency of 65% is expected. We will present the design and status of the construction of the klystron and supporting systems. |
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| WEPMS084 | A Solid State Driven, Parasitic Oscillation Suppressed, 17 GHz High Gain TW Klystron for Stable Operation with High Gradient Linac Structures | 2529 |
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Funding: Work performed under the auspices of the U. S. Department of Energy SBIR Grant No. DE-FG02-04ER83973. The gain of a high power TW relativistic klystron can be increased substantially with the use of a varying phase velocity, large beam aperture, lengthened output structure, designed for asynchronous interaction to control space charge fields and provide near-adiabatic bunch compression during the power extraction process. While this technique enables the replacement of a pulsed vacuum tube driver system with a small, inexpensive solid state RF source, lengthening the output circuit increases the number (and reduces the separation) of the longitudinal mode resonances in the TM01 operating band. Thus, the probability of exciting parasitic oscillations is increased, especially when the klystron is operated into a mismatched load or a high Q structure. The prevention of such oscillations, even when in close proximity to the operating frequency, using a technique that is unaffected by the phase or amplitude of reflected signals is described; and test results are presented of a solid state driven, 76dB gain 17GHz TW relativistic klystron, recently installed in the linac test facility at the MIT Plasma Science and Fusion Center. |
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| WEPMS085 | A 17 GHz High Gradient Linac having Stainless Steel Surfaces in the High Intensity Magnetic and Electric Field Regions of the Structure | 2532 |
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Funding: Work performed under the auspices of the U. S. Department of Energy SBIR Grant No. DE-FG02-05ER84362. To avoid surface erosion damage and to assist in understanding RF breakdown limitations imposed on high gradient linac operation, a gradient hardened structure is being fabricated having high temperature brazed and machined stainless steel surfaces located in the high E-field region of the beam apertures and in the high H-field regions of the racetrack shaped coupling cavities. The microwave design parameters and physical dimensions of this 17GHz, 2pi/3 mode, 22-cavity structure were established specifically to allow comparison of its high gradient performance to that of a similar all-copper structure tested under identical conditions, using an existing 4X power amplifying, RF recirculating dual ring system. Use of the 6X thicker skin depth material, the resulting de-Q-ing effects and the minimal reduction of beam energy (2%) associated with the strategically located lossy surfaces are discussed; fabrication techniques are described; and design parameters of the gradient hardened linac and the 17GHz power amplifying system are presented. |
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| WEPMS086 | Design of a 26 GHz Wakefield Power Extractor | 2535 |
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| High frequency, high output power, and high efficiency RF sources have compelling applications in accelerators for high energy physics. The 26 GHz RF power extractor proposed in this paper provides a practical approach for generating high power RF in this particular frequency range. The extractor is designed to couple out RF power generated from the high charge electron bunch train at the Argonne Wakefield Accelerator (AWA) facility traversing dielectric loaded or corrugated waveguides. In this paper we evaluate two different techniques for extracting the beam energy at the AWA: one is based on a completely metallic corrugated waveguide and coupler; and the other is based on a dielectric lined circular waveguide and coupler. Designs for both RF power extractors will be presented including parameter optimization, the electromagnetic modeling of structures and RF couplers, and the analysis of beam dynamics. | ||
| WEPMS087 | Conceptual Design of an L-Band Recirculating Superconducting Traveling Wave Accelerating Structure | 2538 |
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Funding: This research is supported by the US Department of Energy We describe a conceptual design for a superconducting traveling wave accelerator for the ILC. The RF feedback system plus phase shifter can redirect the accelerating wave that passed through the STWA section back to the input of the accelerating structure. In this paper, the STWA cell shape optimization, coupler cell design and rat race ring coupler in the feedback loop are presented. The STWA cell shape is similar to the LL cavity with a 60 mm disk diameter. A 9-cell STWA operates at the mode with group velocity as low as 0.0106 c. Both the ratio of peak electric field and magnetic field to the axial electric field are smaller than in the TESLA 9-cell cavity. The STWA structure has more cells per unit length than a TESLA structure but provides an accelerating gradient higher than a TESLA structure, consequently reducing the cost. The designed rat race directional coupler with four ports has ?3 dB direct coupling coefficients, 16.5 MHz bandwidth between ?30 dB isolations and 1.1 MHz bandwidth between ?30 dB reflection coefficients. Effects of the mechanical tolerances are also discussed. |
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| WEPMS088 | Challenges Encountered during the Processing of the BNL ERL 5 Cell Accelerating Cavity | 2541 |
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Funding: Work done under the auspices of the US DOE One of the key components for the Energy Recovery Linac being built by the Electron cooling group in the Collider Accelerator Department is the 5 cell accelerating cavity which is designed to accelerate 2 MeV electrons from the gun up to 15-20 MeV, allow them to make one pass through the ring and then decelerate them back down to 2 MeV prior to sending them to the dump. This cavity was designed by BNL and fabricated by AES in Medford, NY. Following fabrication it was sent to Thomas Jefferson Lab in VA for chemical processing, testing and assembly into a string assembly suitable for shipment back to BNL and integration into the ERL. The steps involved in this processing sequence will be reviewed and the deviations from processing of similar SRF cavities will be discussed. The lessons learned from this process are documented to help future projects where the scope is different from that normally encountered. |
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| WEPMS089 | Multipacting Analysis of a Quarter Wave Choke Joint used for Insertion of a Demountable Cathode into a SRF Photoinjector | 2544 |
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Funding: Work done under the auspices of the US DOE. The multipacting phenomena in accelerating structures and coaxial lines are well documented and methods of mitigating or suppressing it are understood. The multipacting that occurs in a quarter wave choke joint designed to mount a cathode insertion stalk into a superconducting RF photoinjector has been analyzed via calculations and experimental measurements and the effect of introducing multipacting suppression grooves into the structure is analyzed. Several alternative choke joint designs are analyzed and suggestions made regarding future choke joint development. Furthermore, the problems encountered in cleaning the choke joint surfaces, factors important in changes to the secondary electron yield, are discussed and evaluated. This design is being implemented on the BNL 1.3 GHz photoinjector, previously used for measurement of the quantum efficiency of bare Nb, to allow for the introduction of other cathode materials for study, and to verify the design functions properly prior to constructing our 703 MHz photoinjector with a similar choke joint design. |
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| WEPMS090 | High Average Current Low Emittance Beam Employing CW Normal Conducting Gun | 2547 |
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| CW normal conducting guns usually do not achieve very high field gradient and waste much RF power at high field gradient compared to superconducting cavities. But they have less trapped modes and wakefields compared to the superconducting cavities due to their low Q. The external bucking coil can also be applied very close to the cathode to improve the beam quality. By using a low frequency gun with a recessed cathode and a carefully designed beam line we can get a high average current and a high quality beam with acceptable RF power loss on the cavity wall. This paper shows that the CW normal conducting gun can be a backup solution for those projects which need high peak and average current, low emittance electron beams such as the Relativistic Heavy Ion Collider (RHIC) e-cooling project and Energy Recovery Linac (ERL) project. | ||
| WEPMS091 | Conceptual Design of the NSLS-II RF Systems | 2550 |
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| RF system requirements are derived from machine parameters and beam stability specifications. The conceptual design of the RF system for NSLS-II to meet these requirements is presented, consisting of 500 MHz superconducting main cavities, 1500 MHz SCRF harmonic cavities for bunch lengthening, and the RF power and cryogenic systems. | ||
| WEPMS092 | A Simplified Approach to Analyze and Model Inductive Voltage Adder | 2553 |
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Funding: Work performed under auspices of U. S. Departemnt of Energy. We have recently developed a simplified model and a set of simple formulas for inductive voltage adder design. This model reveals the relationship of output waveform parameters and hardware designs. A computer simulation has demonstrated that parameter estimation based on this approach is accurate as compared to an actual circuit. This approach can be used in early stages of project development to assist feasibility study, geometry selection in engineering design, and parameter selection of critical components. In this paper, we give the deduction of a simplified model. Among the estimation formulas we present are those for pulse rise time, system impedance, and number of stages. Examples are used to illustrate the advantage of this approach. This approach is also applicable to induction LINAC design. |
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| WEPMS093 | Gridless IOT for Accelerator Applications | 2556 |
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The klystron is the established microwave amplifier in accelerator driver applications, enjoying high power, gain and efficiency at saturation. Disadvantages are reduced efficiency in the linear regime and large size. Building on its success in the television broadcast market, the IOT provides a compact, high efficiency alternative for emerging accelerator applications. An integral component of the IOT input cavity is a control grid, which is positioned close to the cathode, not only to enhance the electric field for emission gating at the cathode surface, but also to limit the transit angle. The latter consideration constrains the operation of these devices to the lower frequency end of the microwave spectrum. Power is limited due to grid interception. Therefore, to fully exploit the benefits provided by density modulation, i.e., high efficiency and compact size, without the consequent frequency, power, and gain limitations, an emission gating method that does not rely on a closely spaced control grid is required. The solution is the Vector amplifier, a gridless IOT based on L-3's trajectory modulation technique* and an alternative compact, low cost RF source for the ILC.
* M. F. Kirshner et al., "Apparatus and method for trajectory modulation of an electron beam," U. S. Provisional Patent Application 60/838,580, August 17, 2006. Cleared by DoD/OFOISR for public release under 07-S-0493 on January 22, 2007 |