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| MO1002 | Commissioning and Initial Operating Experience with the SNS 1-GeV Linac | linac, target, emittance, beam-transport | 1 | ||
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The Spallation Neutron Source accelerator complex consists of a 2.5 MeV H- front-end injector system, a 186 MeV normal-conducting linear accelerator, a 1 GeV superconducting linear accelerator, an accumulator ring and associated beam transport lines. The SNS linac was commissioned in five discrete runs, starting in 2002 and completed in 2005. The remainder of the accelerator complex was commissioned in early 2006. With the completed commissioning of the SNS accelerator, the linac has begun initial low-power operations. In the course of beam commissioning, most beam performance parameters and beam intensity goals were achieved at low duty factor. A number of beam dynamics measurements have been performed, including emittance evolution and sensitivity to mismatch of the input beam. The beam commissioning results, achieved beam performance and initial operating experience of the SNS linac will be presented.
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| MOP045 | Performance of SNS Front End and Warm Linac | linac, beam-losses, emittance, focusing | 145 | ||
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The Spallation Neutron Source accelerator systems will deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of an H- injector, capable of producing one-ms-long pulses at 60Hz repetition rate with 38 mA peak current, a 1 GeV linear accelerator, an accumulator ring and associated transport lines. The 2.5MeV beam from the Front End is accelerated to 86 MeV in the Drift Tube Linac, then to 185 MeV in a Coupled-Cavity Linac and finally to 1 GeV in the Superconducting Linac. With the completion of beam commissioning, the accelerator complex began operation in June 2006. Injector and warm linac performance results will be presented including transverse emittance evolution along the linac, longitudinal bunch profile measurements at the beginning and end of the linac, and the results of a beam loss study.
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| MOP057 | A Fault Recovery System for the SNS Superconducting Cavity Linac | linac, klystron, beam-loading, proton | 174 | ||
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One of the advantages for the change of the Spallation Neutron Source (SNS) linac from copper to superconducting cavities, was the possibility of fault tolerance. Namely, the ability to rapidly recover from a cavity failure, retune the downstream cavities with minimal user disruption. While this is straightforward for electron machines, where beta is constant, it is more involved for the case of proton machines, where the beta changes appreciably throughout the Superconducting Linac (SCL). For SNS when the SCL is first turned on, each cavity’s RF amplitude and phase (relative to the beam) are determined with a beam based technique. Using this information a model calculated map of arrival time and phase setpoint for each cavity is constructed. In the case of cavity failure(s) the change in arrival time at downstream cavities can be calculated and the RF phases adjusted accordingly. Typical phase adjustments are in the 100 – 1000 degree range. This system has been tested on the SNS SCL in both controlled tests and a need based instance in which more than 10 cavity amplitudes were simultaneously reduced. This scheme and results will be discussed.
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| MOP068 | Beam-Loss Measurement and Simulation of Low-Energy SNS Linac | linac, simulation, radiation, beam-losses | 202 | ||
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We have installed a number of Neutron detectors from the MEBT to the end of CCL [186 MeV]. These detectors are made in collaboration with INR. In this paper we present our implementation and simulation of the losses by inserting Faraday Cups at different energies. We also calibrated neutron detectors and their high voltage dependence. The measured losses are simulated by 3-D transport codes during SCL commissioning. We also discuss future improvements such as interpreting the loss signal in terms of beam current lost in warm part of SNS linac with accurate longitudinal loss distribution as well as plan to automate voltage dependence of the neutron detectors. We compare two different sets of Beam Loss Monitors: Ionization Chambers (detecting X-ray and gamma radiation) and Photo-Multiplier Tubes with a neutron converter (detecting neutrons). We outline such combination is better way to deal with the beam losses than relying on detectors of one type.
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| TU1004 | Development of High-Current, High-Duty-Factor H- Injectors | emittance, ion, electron, plasma | 213 | ||
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SNS, FNAL, and CERN have projects that require the production of H- beams with increased intensity and increased duty factors. The most demanding requirements are set by SNS, which plans to upgrade its power to 3 MW. This power level requires a LINAC peak current of 59 mA, which results from an RFQ input current between 67 and 95mA when injecting with rms-emittances between 0.20 and 0.35 Pi-mm-mrad, respectively. Predicted downstream losses exclude the use of higher emittance beams. Ion source lifetime and reliability requirements are also stringent to meet the 99.5% availability goal for the injector of a user facility with 95% availability. LEBT options are currently being studied to optimally match the ion source output into the RFQ with a minimal distortion of the beam emittance. Several ion source and LEBT options under consideration will be discussed.
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| TUP002 | High-Dynamic-Range Current Measurements in the Medium-Energy Beta Transport Line at the Spallation Neutron Source | Spallation-Neutron-Source, linac, extraction, rfq | 244 | ||
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It is desired to measure the effectiveness of the LEBT (low energy beta transport) chopper system. Since this chopper is required to chop the H- beam to a 1% level, it is required therefore to accurately measure the beam during the chop. A system is developed with a high dynamic range that can both accurately measure the beam to tune the chopper system as well as provide an input to the MPS (machine protection system) to stop the beam in the event of a chopper system failure. A system description, beam based calibration, and beam measurements are included.
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| TUP003 | Spallation Neutron Source Linac Beam Position and Phase Monitor System | linac, pick-up, controls, instrumentation | 247 | ||
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The SNS linac currently has 6x beam position monitors which allow the measurement of both beam position and phase from a single pickup. The signals from the pickup lobes are down converted from either 402.5MHz or 805 MHz to 50-MHz IF signals for processing. The IF signals are synchronously sampled at 40 MHz to generate I and Q signals from which the beam position and phase are calculated. Each BPM sampling reference frequency is locked to a phase-stable 2.5 MHz signal distributed along the linac. The system is continuously calibrated by generating and measuring rf bursts in the processor that travel to the BPM pickup, reflect off of the shorted BPM lobes and return to the processor for re-measurement. The electronics are built in a PCI card format and controlled vith LabVIEW. Details of the system design and performance are presented.
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| TUP032 | Comparison of SNS Superconducting Cavity Calibration Methods | acceleration, beam-loading, controls, pick-up | 315 | ||
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Three different methods have been used to calibrate the SNS superconducting cavity RF field amplitude. Two are beam based and the other strictly RF based. One beam based method uses time-of-flight signature matching (phase scan method), and the other uses the beam-cavity interaction itself (drifting beam method). Both of these methods can be used to precisely calibrate the pickup probe of a SC cavity and determine the synchronous phase. The initial comparisons of the beam based techniques at SNS did not achieve the desired precision of 1% due to the influence of calibration errors, noise and coherent interfaces in the system. To date the beam-based SC cavity pickup probe calibrations agree within approximately 4%, comparable to the conventional RF calibrations.
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| TUP053 | Initial Tests of an Elemental Cs-System for the SNS Ion Source | ion, ion-source, plasma, injection | 364 | ||
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The ion source employed in the Spallation Neutron Source* (SNS) is an RF-driven, Cs-enhanced, multi-cusp H- source. To date, the source has been successfully utilized in the commissioning of the SNS accelerator producing 10–40 mA. Presently, Cs is dispensed within the source using Cs2CrO4 cartridges located in an air heated/cooled cylindrical collar surrounding the outlet aperture. The temperature of the collar is elevated to release Cs into the source. Typically, this process can only be repeated 2-3 times before the Cs is depleted and the source needs to be replaced. In addition, the dispensers are subject to poisoning by the residual gases in the source leading to beam decay. This is especially problematic at high duty-factor. This report describes the design of an elemental Cs system incorporating an external reservoir based on the proven Fermilab system. Source performance is characterized and compared for both the original and the elemental Cs systems.
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| TUP054 | A Proposed Helicon Driver for the SNS Ion Source | plasma, ion, ion-source, extraction | 367 | ||
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The H- ion source employed in the Spallation Neutron Source* (SNS) is an RF-driven, multi-cusp source, which utilizes a helical antenna to inductively couple power into the source plasma. To date, the source has been successfully utilized in the commissioning of the SNS accelerator producing 10–40 mA of H- with duty-factors of ~0.1%. Ultimately, the SNS facility will require beam duty-factors of 6% and ~60 mA of H- injecting the linac. This may require currents of up to ~100 mA from the source depending on the ion source emittance. To date, the SNS source has only delivered sustained currents of ~33 mA at full duty factor. Therefore, we are developing plasma generators capable of achieving much higher plasma densities. Plasmas generated through helicon-wave coupling can develop densities up to 100 times greater than those produced by conventional inductive coupling. This report presents an initial design and discusses considerations for a source which combines the forward portion of the SNS source with a helicon system. The helicon system consists largely of components retrofitted from the proven hydrogen VASIMR system employed in space propulsion.
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| TUP055 | A Plasma Gun Driver for the SNS Ion Source | plasma, ion, gun, ion-source | 370 | ||
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The ion source developed for the Spallation Neutron Source (SNS) is an RF-driven, multi-cusp source designed to produce ~ 40 mA of H- with a normalized rms emittance of less than 0.2 π mm mrad. To date, the source has been successfully utilized in the commissioning of the SNS accelerator producing 10–40 mA of H- with duty-factors of ~0.1%. Recently, we found the H- yield from the source could be increased dramatically with the introduction of streaming plasma particles injected into the primary RF plasma from a hemispherical glow discharge chamber located in the rear of the source. In some cases, a 50% increase in the H- beam current was observed. The system also eliminated the need for other plasma ignition systems like a secondary low-power RF generator. This report details the design of the plasma gun as well as the parametric dependence of H- current on source operating conditions. Comparisons are made with and without the gun energized. Finally, an off-line test stand was employed to characterize the plasma current emitted directly from the gun as well as perform lifetime characterization.
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SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U. S. Department of Energy. |
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| TUP056 | The Development of a High-Power, H- Ion Source for the SNS-Based on an External Antenna | plasma, ion, ion-source, gun | 373 | ||
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The ion source developed for the Spallation Neutron Source* (SNS) is a radio frequency, multi-cusp H- source, which utilizes an internal antenna immersed within the source plasma. To date, the source has been utilized successfully in commissioning of the SNS accelerator delivering 10 - 40 mA with duty-factors of ~0.1% for periods of several weeks. Ultimately, the SNS facility will require beam currents of ~60 mA at 6% duty-factor. Tests have shown that the internal antenna is susceptible to failure at this duty-factor. Currently, two ion sources are being developed which feature ceramic plasma chambers surrounded by an external antenna. The first is a low-power, test version which employs a high-inductance external antenna and produces considerably higher H- beam currents than the original SNS source when both are operated without Cs. The second is a high-power version which features a Faraday shield with an integrated magnetic confinement structure and is designed to operate at full duty factor. The performance of this source should also greatly exceed that of the present SNS source. Details of the design and the measured performance of each source are discussed.
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| TUP061 | The HERA RF-Driven Multicusp H- Ion Source | plasma, electron, vacuum, coupling | 388 | ||
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The HERA RF-Volume Source is the only source that delivered routinely a H- current of 40 mA without Cs. This current has been improved to 60 mA. For HERA a pulse length of less than 200 μsec is necessary. It was possible to demonstrate a pulse length of 3 msec with the HERA source at DESY in a cooperation with SNS, FNAL and CERN. RF H- sources are now in permanent use for accelerators like HERA or SNS. The reliability of these sources becomes very important. Special techniques for a reliable external RF coupling to the plasma, ignition, filter field, collar transition for extraction and electron dumping have been developed at DESY. The physics of the extraction plasma region was the subject of very detailed investigations with special sets of collars, cones and Langmuir probes.
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| TUP069 | Design of 132MeV DTL for CSNS | focusing, simulation, linac, vacuum | 412 | ||
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A description is given to the drift-tube-linac (DTL) of the Beijing Spallation Neutron Source (BSNS). The DTL accelerate a 3MeV, 30mA H- beam from the RFQ to the LRBT. Tank body and drift tube configuration design using the SUPERFISH code has enabled efficient optimization of the effective shunt impedance and avoided high surface field. Accelerating cells design and particle tracking were made by using PARMILA code. Special emphasis is given to the transverse focusing system design, which was compared with two usual schemes, constant phase focusing and equipartitioning focusing. Details of beam dynamics analysis will be presented in this paper.
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| TUP070 | Comparison of Phase Scan vs Acceptance Scan for the SNS DTL | linac, simulation, Spallation-Neutron-Source, target | 415 | ||
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There are two widely used techniques for setting the rf set-point of the Drift Tube Linac (DTL). The Phase Scan and the Acceptance Scan techniques were applied to the SNS DTL and were benchmarked against each other. Commissioning data indicate that both techniques produce quite consistent results and the model used is quite accurate. Both of the models are based on multiparticle tracking with space charge effects.
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| TUP071 | Beam-Loading Effects on Phase Scan for the Superconducting Cavities | linac, simulation, beam-loading, impedance | 418 | ||
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When the beam is passing through superconducting cavities, it excites beam induced field in cavities. A systematic study was performed to study the beam loading effects by the nonrelativistic beam for β = 0.81 superconducting cavities of the SNS linac. The analysis indicates that the induced field level is quite close to the estimation and its effect on the phase scan is consistent with the model.
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| TUP073 | Simulations of RF Errors in the SNS Superconducting Linac | linac, simulation, emittance, beam-losses | 423 | ||
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Minimizing beam emittance growth in the SNS superconducting linac due to RF errors, either correlated or uncorrelated, is essential since it can lead to beam loss in the linac and in the downstream ring. From multi-particle simulation studies of both matched and mismatched linac lattices, for the design peak beam current of 38 mA, as well as a typical commissioning beam current of 20 mA, we conclude that the linac may tolerate much higher non-correlated RF errors, especially in the second half of the superconducting linac, where errors in synchronous phase up to 10 degrees and that of cavity field amplitude up to 10% is acceptable. However, tolerance to correlated RF errors in the linac is within only 0.5 degree and 0.5 %, from simulations using a simple longitudinal linac model. Beam parameter measurement results acquired during linac beam commissioning confirmed the simulations.
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| TUP076 | First TRACK Simulations of the SNS Linac | linac, simulation, lattice, focusing | 432 | ||
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In an effort to benchmark the code TRACK* against the recent commissionnig data from the SNS linac, we started updating the code TRACK to support SNS-type elements like DTL's and CCL's. 2D electric field tables were computed using SUPERFISH and 3D magnetic fields from PMQ's were calculated using EMS-Studio. A special DTL routine was implemented and successfully tested. The first results of TRACK simulations using a realistic beam will be presented. A comparison with the code PARMILA will also be presented and discussed.
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* "TRACK: The New Beam Dynamics code", V. N. Aseev et al, in Proceedings |
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| TUP083 | Development of High-Power RF Vector Modulator Employing TEM Ferrite Phase Shifters | controls, impedance, linac, vacuum | 451 | ||
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Construction and installation of cavity RF power distribution system in a high power superconducting RF accelerator can have cost savings if a fan-out configuration that feeds many cavities with a single high power klystron is realized. The configuration however requires independent control of RF amplitudes and phases to the cavities to perform properly. A prototype high power RF vector modulator for the control is built and tested. The vector modulator employs a quadrature hybrid and two fast ferrite phase shifters in square coaxial TEM transmission lines. The square coaxial format can provide the power handling capability and thermal stability. RF properties of the design and result of high power system testing of the design are presented.
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| TUP084 | Drifting Beam Application for SNS Superconducting Cavity Setting | controls, linac, beam-loading, Spallation-Neutron-Source | 454 | ||
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A software application for tuning superconducting linac cavity has been developed and tested at the Spallation Neutron Source (SNS). The application is based on the drifting beam method and the XAL online model. The drifting beam method and the application were proved to be consistent with other cavity tuning method during the SNS commissioning runs. Detail algorithm and data acquisition for the application will be presented.
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| THP004 | Digital Low-Level RF Control Using Non-IQ Sampling | controls, feedback, linac, rfq | 568 | ||
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The success of digital feedback with synchronous IQ sampling for cavity field control in recent accelerator projects make this LLRF control scheme a popular choice. This short-period synchronous sampling does not, however, average out well-known defects in modern ADC and DAC hardware. That limits the achievable control precision for digital IQ LLRF controllers, while demands for precision are increasing for future accelerators such as International Linear Collider. For this reason, a collaborative effort is developing a digital LLRF control evaluation platform to experiment using coherent sampling with much longer synchronous periods, on the order of the cavity closed-loop bandwidth. This exercise will develop and test the hardware and software needed to meet greater future RF control challenges.
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| THP005 | Digital Control of Cavity Fields in the Spallation Neutron Source Superconducting Linac | controls, linac, feedback, beam-loading | 571 | ||
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Control of the pulsed RF cavity fields in the Spallation Neutron Source (SNS) superconducting Linac uses both the real-time feedback regulation and the pulse-to-pulse adaptive feed-forward compensation. This control combination is required to deal with the typical issues associated with superconducting cavities, such as the Lorentz force detuning, mechanical resonance modes, and cavity filling. The all-digital implementation of this system provides the capabilities and flexibility necessary for achieving the required performance, and to accommodate the needs of various control schemes. The low-latency design of the digital hardware has successfully produced a wide control bandwidth, and the developed adaptive feed forward algorithms have proved to be essential for the controlled cavity filling, the suppression of the cavity mechanical resonances, and the beam loading compensation. As of this time, all 96 LLRF systems throughout the Linac have been commissioned and are in operation.
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| THP008 | Detailed Modeling of the SNS RFQ Structure with CST Microwave Studio | rfq, simulation, dipole, quadrupole | 580 | ||
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We report detailed RF modeling on the SNS RFQ structure using CST Microwave Studio code. Due to the complexity of the RFQ structure, a three-dimensional model with large mesh ratio is required to adequately model the necessary details of the structure. Old 3-D codes are not capable of giving accurate predictions of resonant frequency and fields, or for including mode stabilizers and terminations. A physical prototype is needed to verify resonant frequency and field profile, including mode stabilizers and end terminations, which is expensive and time consuming. Taking advantage of CST Microwave Studio’s new Perfect Boundary Approximation (PBA) technique, we constructed a 3-dimensional computational model based on the as-built SNS RFQ dimensions with pi-mode stabilizers, end cutbacks and tuners and simulated it in the frequency domain using the CST Eigenvalue Solver. Simulation results accurately predicted the resonant frequency and field distributions. We are applying the simulation technique to the design of another RFQ.
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| THP029 | Development of an RFQ Input Power Coupling System | rfq, vacuum, coupling, simulation | 634 | ||
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An RF input coupler system is designed, manufactured, and tested for future upgrade of the coupling system of the RFQ in the SNS linac. The design employs two coaxial loops in vacuum side of two coaxial ceramic windows through coaxial transmission lines that are connected to a magic-T waveguide power splitter for 402.5 MHz operation. The couplers will be used with up to total 800 kW peak power at 8% duty cycle. RF properties of the system and fabricated structure along with vacuum and thermal properties are discussed. Two couplers are joined together through an evacuated bridge waveguide for high power RF processing. Result of the high power conditioning that is performed in the RF test facility of the SNS is presented.
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| THP030 | Investigation of Ferroelectrics for High-Power RF Phase Shifters in Accelerator Systems | impedance, simulation, vacuum, klystron | 637 | ||
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High power vector modulators enable independent control of RF power to each accelerating cavity, allowing a fan-out configuration to be used to power many cavities from a single high-power klystron. Previously, ferrite materials have been used in high-power phase shifters and vector modulators. It is shown that ferroelectric materials such as barium-strontium titanate (BST) can also be used in such tunable structures. Since ferroelectrics are controlled by an electric, rather than magnetic field, tuning can be faster than tuning a ferrite-loaded device. A BST-loaded coaxial structure is investigated theoretically and experimentally. Good high voltage performance is critical since DC biasing voltages of up to 80 kV can be impressed on the BST sections for tuning. It can also be seen that matching structures around the BST can improve performance over a wider range of amplitudes and phases.
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| THP076 | Prototyping of a Superconducting Elliptical Cavity for a Proton Linac | proton, linac, vacuum, radiation | 758 | ||
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A superconducting cavity has been designed for acceleration of particles travelling at 81% the speed of light (β = 0.81). Possible applications include the proposed Fermilab Proton Driver Linac. The cell shape is similar to the β = 0.81 cavity for the Spallation Neutron Source Linac, but the resonant frequency is 1.3 GHz rather than 805 MHz and the beam tube diameter matches that of the 1.3 GHz cavity for the TeSLA Test Facility. Six single-cell prototypes are being fabricated and tested. Three of these cavities are being formed from standard high purity fine grain niobium sheet. The rest are being fabricated from large grain niobium, following up on the work at Jefferson Lab to investigate the potential of large grain material for cost savings and/or improved RF performance. The fabrication of two 7-cell cavity prototypes (one fine grain, one large grain) is planned. A status report on this prototyping effort will be presented.
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| THP081 | Study on Fault Scenarios of Coaxial Type HOM Couplers in SRF Cavities | electron, coupling, radiation, vacuum | 770 | ||
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Coaxial type couplers are adopted in many superconducting radio-frequency (SRF) cavities to suppress higher order modes for beam dynamics and cryogenic loads issues. HERA (Hadron-Electron Ring Accelerator) and TTF (Tesla Test Facility) are equipped with this type coupler and showed successful performances. It is, however, under suspicion that a limitation or a fault could be initiated from this type of coupler at certain combinations between cavity operating conditions and engineering designs of the coupler. Some possible scenarios are summarized and also some observations in the SNS (Spallation Neutron Source) SRF cavities are also reported.
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