AES, Medford, NY
BNL, Upton, Long Island, New York
Funding: AES is funded under DOE SBIR contract #DE-FG02-06ER84450. BNL work is performed under DOE contract #DE-AC02-98CH10886.
The use of an RF electron gun with a magnetized cathode in place of a DC gun for ILC may reduce the requirements for emittance damping rings. Maintaining adequate lifetime of the necessary cathode material requires vacuum levels in the 10-11 torr range. While vacuum levels around the 10-9 torr range are common in a normal conducting RF gun, the cryogenic pumping of the cavity walls of a superconducting RF (SRF) gun may maintain vacuum in the range needed for GaAs cathode longevity. Advanced Energy Systems, Inc. is collaborating with Brookhaven National Laboratory to investigate the generation of polarized electron beams using a SRF photocathode gun. The team is developing an experiment to study the quantum lifetime of a GaAs cathode in a SRF cavity and investigate long term cavity performance while integrated with a cesiated GaAs cathode*. In addition to the experimental investigation, a design is being developed that is compatible with the production of high aspect ratio polarized electron beams. The mechanical and physics aspects of this design will be discussed.
*J. Kewisch, et. al., Presentation at PAC09.
BNL, Upton, Long Island, New York
A linac-ring eRHIC design requires a high-intensity CW source of polarized electrons. An SRF gun is viable option that can deliver the required beam. Numerical simulations presented elsewhere have shown that ion bombardment can occur in an RF gun, possibly limiting lifetime of a NEA GaAs cathode. In this paper, we analytically solve the equations of motion of ions in an RF gun using the ponderomotive potential of the RF field. We apply the method to the BNL 1/2-cell SRF photogun and demonstrate that a significant portion of ions produced in the gun can reach the cathode if no special precautions are taken. Also, the paper discusses possible mitigation techniques that can reduce the rate of ion bombardment.
DESY, Hamburg
JLAB, Newport News, Virginia
The Andrzej Soltan Institute for Nuclear Studies, Centre Swierk, Swierk/Otwock
In this contribution, we report progress on the development of a hybrid lead/niobium superconducting RF (SRF) photoinjector. The goal of this effort is to build a Nb injector with the superconducting cathode made of lead, which demonstrated in the past superior quantum efficiency (QE) compared to Nb Three prototype hybrid devices, consisting of an all-niobium cavity with an arc-deposited spot of lead in the cathode region, have been constructed and tested. We present the cold test results of these cavities with and without lead.
Cornell University, Ithaca, New York
CLASSE, Ithaca, New York
Funding: This work is supported by the National Science Foundation.
Cornell University has developed and fabricated a SRF injector cryomodule for the acceleration of the high current (100 mA) beam in the Cornell ERL injector prototype. The injector cryomodule is based on superconducting rf technology with five 2-cell rf cavities operated in cw mode. To support the acceleration of a low energy, ultra low emittance, high current beam, the beam tubes on one side of the cavities have been enlarged to propagate Higher-Order-Mode power from the cavities to broadband rf absorbers located at 80 K between the cavities. The axial symmetry of these absorbers, together with two symmetrically placed input couplers per cavity, avoids transverse on-axis fields, which would cause emittance growth. Each cavity is surrounded by a LHe vessel and equipped with a frequency tuner including fast piezo-driven fine tuners for fast frequency control. The cryomodule provides the support and precise alignment for the cavity string, the 80 K cooling of the ferrite loads, and the 2 K LHe cryogenic system for the high cw heat load of the cavities. In this paper results of the commissioning phase of this injector cryomodule will be reported.
ANL, Argonne
Several high power proton and ion linac projects based on superconducting accelerating technology are currently under study and drive an important worldwide R&D effort on superconducting cavities, especially for low and medium energy linacs. Multi-cell elliptical cavities, single or multi-spoke cavities, half-wave and quarter-wave superconducting cavities have been developed at many laboratories and institutions and continue to extend the state-of-the-art for this class of cavities. This talk reviews recent developments and results for SC cavity performance along with a brief overview of associated components such as mechanical slow tuning systems, fast tuners and rf power couplers.
JLAB, Newport News, Virginia
A major portion of the ILC R&D effort is focused on increasing the sustainable gradients in the baseline TESLA-shape SC cavities. This is a world-wide effort with major contributions from DESY (in parallel with their XFEL program), JLAB, FNAL and KEK. During the past year, the work in the US and Japan has ramped up considerably, and PAC09 is an opportune time to review the contributions from the groups in these regions, as well as at DESY.
ANL, Argonne
CEA, Gif-sur-Yvette
JLAB, Newport News, Virginia
Fermilab, Batavia
NHMFL, Tallahassee, Florida
IIT, Chicago, Illinois
Funding: DOE/OHEP
We have begun using Atomic Layer Deposition (ALD) to synthesize a variety of surface coatings on coupons and cavities as part of an effort to produce rf structures with significantly better performance and yield than those obtained from bulk niobium, The ALD process offers the possibility of conformally coating complex cavity shapes with precise layered structures with tightly constrained morphology and chemical properties. Our program looks both at the metallurgy and superconducting properties of these coatings, and also their performance in working structures. Initial results include: 1) evidence from point contact tunneling showing magnetic oxides can be a significant limitation to high gradient operation, 2) experimental results showing the production sharp niobium/oxide interfaces from a high temperature bake of ALD coated Al2O3 on niobium surfaces, 3) results from ALD coated structures.
BNL, Upton, Long Island, New York
This paper will present the preliminary results of the testing of the 703 MHz SRF cryomodule designed for use in the ampere class ERL under construction at Brookhaven National Laboratory. The preliminary VTA cavity testing, carried out at Jefferson Laboratory, demonstrated cavity performance of 20 MV/m with a Qo of 1x1010, results we expect to reproduce in the horizontal configuration. This test of the entire string assembly will allow us to evaluate all of the additional cryomodule components not previously tested in the VTA and will prepare us for our next milestone test which will be delivery of electrons from our injector through the cryomodule to the beam dump. This will also be the first demonstration of an accelerating cavity designed for use in an ampere class ERL, a key development which holds great promise for future machines.
BNL, Upton, Long Island, New York
Funding: Work supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.
A 56 MHz Superconducting RF cavity is developed for the luminosity enhancement of the Relativistic Heavy Ion Collider (RHIC). The 56 MHz SRF cavity enables to adiabatically rebucket the beam from the 28 MHz accelerating cavities, which with shorter bunch lengths will enhance the luminosity significantly. The 56 MHz SRF cavity fundamental mode must be damped during injection and acceleration by a fundamental mode damper (FD), which is physically withdrawn at store for operation. The cavity frequency changes from the withdrawing motion but is kept below the beam frequency at store by a judicious axial placement of the FD. Physics studies by numerical simulations, tests of the FD in the prototype cavity, and the challenging engineering issues are here addressed. In addition, higher-order mode (HOM) dampers are necessary for the stable operation of the 56 MHz SRF cavity. The HOM’s are identified and the external Q factors are obtained from tests of the prototype cavity and are compared to simulations with the CST MWS program. The HOM damper blocks the fundamental mode by a 5 element high pass filter. The HOM stability criteria of the cavity are satisfied with four HOM dampers.
CLASSE, Ithaca, New York
Funding: Work Supported by the U.S. Department of Energy
Cornell University’s superconducting niobium nine-elliptical-cell cavity testing and repair program is one contributor to the collaborative effort on critical SRF R&D for the ILC. The Cornell University program benefits from several unique features which provide for the rapid testing and, if necessary, repair of ILC nine-cell cavities: a continuous vertical electropolish procedure, superfluid helium second sound defect location, and tumble polishing. First, we report on the cavity 2K RF performance and the effect of micro-EP preceding the cavity test. Single-cell results at KEK have shown that micro-EP as a final surface treatment reduces the spread in gradients, but micro-EP has not yet been tried with multi-cell cavities. Secondly, we report on the highly efficient method of detecting defects using a few He-II second sound wave detectors and powering several modes of the 1.3GHz TM010 passband.
CLASSE, Ithaca, New York
Funding: National Science Foundation
Diagnosing field-limiting behavior in multi-cell superconducting cavities can be difficult due to the lack of direct local measurements of cavity surface properties. The results of multiple vertical tests on several 5-cell vertically electropolished 1.3GHz superconducting cavities with measurements of cavity surface properties are presented. A combination of oscillating superleak transducer and resistive thermometry data for various accelerating passband modes are used to infer the field-limiting mechanism for several cells of each multi-cell cavity.
Fermilab, Batavia
DESY, Hamburg
A series of four automatic tuning machines for 9-cell TESLA-type cavities are being developed and fabricated through a collaboration between DESY, FNAL and KEK. These machines are intended to support high-throughput cavity fabrication for construction of large SRF-based accelerator projects. Two of these machines will be delivered to cavity vendors for the tuning of XFEL cavities. The control system for these machines must support a high level of automation adequate for industrial use by non-expert operators. This paper describes the control system hardware and software designs, and shows preliminary results obtained with a tuning machine prototype.
Fermilab, Batavia
Combined temperature mapping and optical inspection of SRF cavities has revealed defects near the edge of the heat affected zone (HAZ) of numerous welds. We have re-created similar defects on welded coupons that were electropolished in the lab and characterized by a variety of tools. Several features that may have bearing on the origin of these defects are discussed, such as their location relative to grain boundaries, the strain state of the niobium prior to welding and etching, the electropolishing parameters, and so forth. Since coupons are useful for distribution to academic researchers, we also describe other detailed characterizations.
Fermilab, Batavia
Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-07CH11359.
Fermilab is in a collaboration with DESY to provide a cryomodule containing 4-3.9 GHz superconducting RF cavities to be placed in TTF/FLASH. The purpose of this 'Third Harmonic' module is to linearize the nonlinear beam energy-time profile produced by the 1.3 GHz accelerating gradient. The completed module has now been shipped to DESY and is awaiting cold, powered testing and installation into FLASH later this year. We report on experience with fabricating, testing, assembling, and shipping the module and its components with a focus on cavity test results.
KEK, Ibaraki
KAKEN Inc., Mito
Sokendai, Ibaraki
In order to achieve higher and more stable performance of super conducting radio-frequency (SRF) cavities, extensive effort in development and application has been done for surface treatment and conditioning methods. Those methods have been evaluated with vertical tests showing lots of remarkable results in cavity performance. However it cannot be well understood yet how surface treatment or conditioning contributed to the results and which step of process in the treatment or conditioning affected the results. In this article, we describe our try to understand those questions focusing on the surface analyses with scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) for electro-polishing (EP) processed niobium sample coupons.
KEK, Ibaraki
A cryomodule including three 2-cell sc cavities was designed for the ERL injector Linac, which is operated at the beam energy of 10 MeV and the beam current of 100 mA. A prototype 2-cell cavity with two input coupler ports and four HOM couplers was fabricated. The double input couplers is to reduce the power per coupler and to keep a symmetric filed configuration around the coupler port. Required rf power in the input couplers is about 200 kW in the cw operation. First vertical test of the 2-cell cavity will be carried out in March.
ANL, Argonne
Funding: Work supported by U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
ERL staging is a novel concept that provides a practical path to upgrading an existing synchrotron light source while minimizing disruption to the users and managing the technical risk. In the very first stage, the accelerator operating parameters are comparable to CEBAF without recirculation. Therefore, initially, energy recovery is not required and the injector is more modest. Consequently, the technical risk is significantly reduced relative to the full ERL. Using the APS as an example, the first stage is based on a full-energy, 7-GeV superconducting radiofrequency (srf) linac and an electron source that is almost off-the-shelf. The linac would initially deliver a low average current beam (<200 uA), but with a geometric emittance that is much smaller than the storage ring, the x-ray brightness can exceed the APS. Furthermore, the spatial coherence fraction would be about 100 times higher and the pulse length up to 100 times smaller than the APS. Valuable srf operating experience is attained at an early stage while allowing critical energy recovery issues to be studied. Energy recovery is commissioned in stage 2. The optics design and performance at each stage will be presented.
Fermilab, Batavia
Funding: Work supported by the Fermi Research Alliance, LLC under contract No. DE-AC02-07CH11359 with the U.S. Dept. of Energy.
Project-X is a leading candidate of the next major accelerator construction project at Fermilab. The mission need of Project-X is to establish an intensity frontier for particle physics research, or more precisely, to build a multi-MW proton source for neutrino and other particle studies. Coupled with an upgraded Main Injector (MI) and Recycler, an 8 GeV superconducting RF (SRF) H linac meets this need. However, a more cost effective approach would be a hybrid design, namely, a combination of a 2 GeV SRF linac and an 8 GeV rapid cycling synchrotron (RCS) in lieu of an 8 GeV SRF linac. This alternative design also meets the mission need but at a lower cost since a synchrotron is cheaper than a SRF linac. It retains the ability to use a 2 GeV SRF linac for ILC technology development. It reuses the existing Debuncher enclosure and Booster RF. The transport line of 2 GeV H particles is shorter than the present 8 GeV design. The requirement of a cryogenic beam screen can be eliminated. The efficiency of stripping foil is higher and injection loss (kJ) will be lower.
*W. Chou, “A Simple Transition-Free Lattice of an 8 GeV Proton Synchrotron,” this conference.
ORNL, Oak Ridge, Tennessee
Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy
The Spallation Neutron Source (SNS) is a second generation pulsed-neutron source and designed to provide a 1-GeV, 1.44-MW proton beam to a mercury target for neutron production. Since the initial commissioning of accelerator complex in 2006, the SNS has begun neutron production operation and beam power ramp-up has been in progress toward the design goal. Since the design beam power is almost an order of magnitude higher compared to existing neutron facilities, all subsystems of the SNS were designed and developed for substantial improvements compared to existing accelerators and some subsystems are first of a kind. Many performance and reliability aspects were unknown and unpredictable, for which it takes time to understand the systems as a whole and/or needs additional performance improvements. A power ramp-up plan has been revised based on the operation experiences and understandings of limits and limiting conditions through extensive studies with an emphasis on machine availability. In this paper the operational experiences of SNS Superconducting Linac (SCL), the power ramp-up status and plans will be presented including related subsystem issues.
Bilbao, Faculty of Science and Technology, Bilbao
ESS Bilbao, Bilbao
Elytt Energy, Madrid
Funding: ESS-Bilbao Consortium
A baseline design for the proton linear accelerator as proposed by the European Spallation Source-Bilbao bid to host the installation (ESS-B) is here described. The new machine concept incorporates advances which have been registered within high power accelerators during the last decade. The design of such a new accelerator layout heavily relies upon low-beta superconducting spoke resonators which are already under development.
NSRRC, Hsinchu
National Tsing Hua University, Hsinchu
The Taiwan Light Source (TLS), a third generation accelerator, has been operated successfully since 1993. It has been upgraded to increase the beam energy from 1.3 GeV to 1.5 GeV and the consequent capability of full energy injection afterwards. While the beam current has been increased from 200 mA to 300 mA after replacement of RF cavities with superconducting one. The vacuum pressure tends to decrease continuously after installations of 3 undulators and 4 wigglers as well as the new front ends. The accumulated beam dose increased faster up to > 14500 Ah after the routine operational top-up mode since 2006 with average pressure has been maintained below 0.13 nPa/mA. The beam life time of 6 hours at 300 mA has been kept with a limitation of Touschek life time at a stable beam with variation of photon flux < 0.05%. However, the photon absorbers of front ends have been replaced with new ones for subjecting the higher irradiation power after upgrading. The good dynamic pressure reflects the effective pumping performance. The experiences of components failures will be summarized in this paper.
KAERI, Daejon
Funding: This work is supported by the Ministry of Education, Science and Technology of Korea.
A superconducting radio frequency (SRF) cavity with a geometrical beta of 0.42 has been designed to accelerate a proton beam after 100 MeV at 700 MHz for an extended project of Proton Engineering Frontier Project (PEFP). In order to confirm the RF and mechanical properties of the cavity, and to produce documentation for a procurement and quality control for an industrial manufacture of the cavities, two prototype copper cavities have been produced, tuned and tested. In this paper, the copper cavity’s production, tuning and testing are introduced. The testing results show that the low-beta cavity and its tuning system can work as we design.
LANL, Los Alamos, New Mexico
Funding: DTRA
A temperature mapping system with 4608 100-ohm Allen-Bradley resistors has been built and tested at LANL. With this temperature mapping system we were able to locate lossy regions in the 1.3 GHz 9-cell SRF cavity due to field emission and direct heating. The results of the temperature mapping have been correlated with the inside surface inspection of the cavity and will be shown together with Q-E curves. A brief description of the mapping system and improvements that have been made in the recent months will also be mentioned in the paper.
LANL, Los Alamos, New Mexico
JLAB, Newport News, Virginia
Funding: DTRA
805 MHz elliptical SRF cavities have been used for SNS as the first application for protons. At LANL, an R&D started to explore a capability of getting high-gradient cavities (40-50 MV/m) at this frequency for the future applications such as proton and muon based interrogation testing facility added to the LANSCE accelerator and a power upgrade of the LANSCE accelerator for the fission and fusion material test station. Optimized cell designs for “standard”, “low-loss” and “re-entrant” shapes, cavity test results for “standard” single-cell cavities with temperature mapping as well as surface inspection results will be presented.
NSCL, East Lansing, Michigan
A medium-velocity half wave resonator has been designed and prototyped at the National Superconducting Cyclotron Laboratory for use in a heavy ion linac. The cavity is designed to provide 3.7 MV of accelerating voltage at an optimum beta = v/c = 0.53, with peak surface electric and magnetic fields of 32.5 MV/m and 79 mT, respectively. The resonant frequency is 322 MHz. The cavity was designed to reduce sensitivity to bath pressure fluctuations while maintaining a structure that can be easily fabricated, cleaned, and tuned. Deep draw forming dies and a copper cavity prototype were fabricated to confirm tolerances and formability. A prototype tuner was built; the helium vessel and power coupler have been designed. Measurements were performed to confirm finite element predictions for the mechanical modes, bath pressure sensitivity, tuner stiffness, and tuning range.
MHI, Kobe
We report on the activities and achievements at MHI about cavity fabrication for ILC. Some new procedures of cavity fabrication for industrialization are reported.
BNL, Upton, Long Island, New York
Niowave, Inc., Lansing, Michigan
MHI, Kobe
NSLS-II is a new ultra-bright 3GeV 3rd generation synchrotron radiation light source. The performance goals require operation with a beam current of 500mA and a bunch current of at least 0.5mA. Ion clearing gaps are required to suppress ion effects on the beam. The natural bunch length of 3mm is planned to be lengthened by means of a third harmonic cavity in order increase the Touschek limited lifetime. After an extensive investigation of different cavity geometries a passive, superconducting 2-cell cavity has been selected for prototyping. The cavity is HOM damped with ferrite absorbers on the beam-pipes. The 2-cell cavity simplifies the tuner design as compared to two independent cells. Tradeoffs between the damping of the higher order modes, thermal isolation associated with the large beam tubes and overall cavity length are described. A copper prototype has been constructed and measurements of fundamental and higher order modes will be compared to calculated values.
Muons, Inc, Batavia
CLASSE, Ithaca, New York
Cornell University, Ithaca, New York
Funding: Supported in part by USDOE Contract. DE-AC05-84-ER-40150
Superconducting HOM-damped (higher-order-mode-damped) RF systems are needed for present and future storage ring and linac applications. Superconducting RF (SRF) systems typically contain unwanted frequencies or higher order modes (HOM) that must be absorbed by ferrite and other lossy ceramic-like materials that are brazed to substrates mechanically attached to the drift tubes adjacent to the SRF cavity. These HOM loads must be thermally and mechanically robust and must have the required broadband microwave loss characteristics, but the ferrites and their attachments are weak under tensile stresses and thermal stresses and tend to crack. A HOM absorber with improved materials and design will be developed for high-gradient 750 MHz superconducting cavity systems. RF system designs will be numerically modeled to determine the optimum ferrite load required to meet the broadband loss specifications. Several techniques for attaching ferrites to the metal substrates will be studied, including full compression rings and nearly-stress-free ferrite assemblies. Prototype structures will be fabricated and tested for mechanical strength.
Muons, Inc, Batavia
JLAB, Newport News, Virginia
Funding: Supported in part by DOE SBIR grant DE-FG02-08ER85171 Supported in part by USDOE Contract DE-AC05-84-ER-40150
There are over 35 coupler designs for SRF cavities ranging in frequency from 325 to 1500 MHz. Two-thirds of these designs are coaxial couplers using disk or cylindrical ceramics in various combinations and configurations. While it is well known that dielectric losses go down by several orders of magnitude at cryogenic temperatures, it not well known that the thermal conductivity also goes down, and it is the ratio of thermal conductivity to loss tangent (SRF ceramic Quality Factor) and ceramic volume which will determine the heat load of any given design. We describe a novel robust co-axial SRF coupler design which uses compressed window technology. This technology will allow the use of highly thermally conductive materials for cryogenic windows. The mechanical designs will fit into standard-sized ConFlat® flanges for ease of assembly. Two windows will be used in a coaxial line. The distance between the windows is adjusted to cancel their reflections so that the same window can be used in many different applications at various frequencies.
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
CLASSE, Ithaca, New York
FZD, Dresden
STFC/DL, Daresbury, Warrington, Cheshire
LBNL, Berkeley, California
Stanford University, Stanford, California
DESY, Hamburg
STFC/DL/SRD, Daresbury, Warrington, Cheshire
The collaborative development of an optimised cavity/cryomodule solution for application on ERL facilities, has now progressed to final assembly and testing of the cavity string components and their subsequent cryomodule integration. This paper outlines the verification of the various cryomodule sub-components and details the processes utilised for final cavity string integration. The paper also describes the modifications needed to facilitate this new cryomodule installation and ultimate operation on the ALICE facility at Daresbury Laboratory.
SINAP, Shanghai
SSRF, Shanghai
RF system for SSRF (Shanghai Synchrotron Radiation Facility) Storage Ring consists of three RF stations, each of which has a klystron, one superconducting RF module and its low level RF feedback control. A 300kW klystron will feed the RF power to the superconducting cavity via a circulator and waveguides. Three CESR type 499.654MHz superconducting modules with tuning range ±150kHz are now in operation. A digitalized I/Q technology based on FPGA is adopted in its low level control. The commissioning and the performance of whole RF system will be described in details in this paper.
JLAB, Newport News, Virginia
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177
The great majority of experience in niobium SRF cavity processing at Jefferson Lab is with BCP etching. This has been used on CEBAF cavities and others totaling over 500 in number. With improved process quality control, field emission is now largely controlled and other factors limit performance. All of the prototype cavities developed for the 12 GeV upgrade, although meeting minimum requirements, have demonstrated a Q-drop in the 17 – 23 MV/m range that is not remedied by 120 C bake. Most of these cavities received >250 micron removal by BCP etch. Three of these cavities are being electropolished using the protocol under development within ILC R&D activities. The first such cavity was transformed from Q = 3 ·1010 at 17 MV/m to quench from 1010 at 35 MV/m. The details of this and two subsequent electropolished JLab 7-cell cavities will be reported.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
North Carolina State University, Raleigh, North Carolina
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
As SRF cavities approach fundamental material limits, there is increased interest in understanding the details of topographical influences on performance limitations. Micro-and nano-roughness are implicated in direct geometrical field enhancements and complications of the composition of the 50 nm surface layer in which the super-currents flow. Interior surface etching (BCP/EP) to remove mechanical damage leaves surface topography, including pits and protrusions of varying sharpness. These may promote RF magnetic field entry, locally quenching superconductivity, so as to degrade cavity performance. A more incisive analysis of surface topography than the widely-used average roughness is needed. In this study, a power spectral density (PSD) approach based on Fourier analysis of surface topography data acquired by both stylus profilometry and atomic force microscopy (AFM) is being used to distinguish the scale-dependent smoothing effects. The topographical evolution of the varied starting state Nb surface (CBP/ EBW) as a function of applied etching, polishing steps and conditions is reported, resulting in a novel qualitative and quantitative description of Nb surface topography.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Future accelerators require unprecedented cavity performance, which is strongly influenced by interior surface nanosmoothness. Electropolishing is the technique of choice to be developed for high-field superconducting radiofrequency cavities. Electrochemical impedance spectroscopy (EIS) and related techniques point to the electropolishing mechanism of Nb in a sulfuric and hydrofluoric acid electrolyte of controlled by a compact surface salt film under F- diffusion-limited mass transport control. These and other findings are currently guiding a systematic characterization to form the basis for cavity process optimization, such as flowrate, electrolyte composition and temperature. This integrated analysis is expected to provide optimum EP parameter sets for a controlled, reproducible and uniform surface leveling for Nb SRF cavities.
JLAB, Newport News, Virginia
CEA, Gif-sur-Yvette
PKU/IHIP, Beijing
ORNL, Oak Ridge, Tennessee
Funding: Notice: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
A new surface treatment technique for niobium (Nb) Superconducting Radio Frequency (SRF) cavities called Buffered Electropolishing (BEP) has been developed at JLab. It was found that BEP could produce the smoothest surface finish on Nb samples ever reported in the literature. Experimental results revealed that the Nb removal rate of BEP could reach as high as 4.67 μm/min. This is significantly faster* than that of the conventional electropolishing technique employing an acid mixture of HF and H2SO4. An investigation is underway to determine the optimum values for all relevant BEP parameters so that the high quality of surface finish achieved on samples can be realized within the geometry of an elliptical RF cavity. Toward this end, single cell Nb cavities are being electropolished by BEP at both CEA-Saclay and JLAB. These cavities will be RF tested and the results will be reported through this presentation.
*Xiangyang Lu et al, to be published.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Much remains to be learned regarding the details of SRF performance effects with material variation, including niobium treated in different ways, and different bulk/thin film materials that are fabricated under different conditions. A facility that can measure small samples’ RF properties in a range of 0~180mT magnetic field and 2~20k temperature is necessary in order to answer this question. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. The SIC system uses a sapphire-loaded cylindrical Nb cavity at 7.5GHz with 50mm diameter flat sample placed on a non-contacting end plate and a calorimetric technique to directly measure the rf dissipation in the sample in response to known rf fields over ~1 cm2. We report on the commissioning of this system and its first uses for characterizing materials. Preliminary tests with Nb thin film sample sputtered on Cu substrate, and bulk Nb sample have been done at low field. The presently available hardware is expected to enable tests up to 20 mT peak magnetic field on the sample CW. Paths to higher field tests have been identified.
JLAB, Newport News, Virginia
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
SRF cavities are observed to be limited by non-linear localized effects. The variation of local material parameters between "hot" and "cold" spots is thus of intense interest. Such locations were identified in a BCP etched large-grain single-cell cavity and removed for examination by high resolution electron microscopy (SEM), electron-back scattering diffraction microscopy (EBSD), and scanning Auger electron spectroscopy (SAM). Pits with clear crystal facets were observed on both "Hotspot" and "Coldspot" specimens. The pits were found in-grain and on "Y"-shaped junction of three crystals. They are interpreted as etch pits induced by surface crystal defects (e.g., dislocations). All "Coldspots" examined had obvious low density of etching pits or very shallow tri-crystal boundary junction. EBSD revealed crystal structure surrounding the pits via crystal phase orientation mapping. This study suggests a mechanism by which BCP etching creates pits on large-grain Nb cavity surfaces and sharp-edged topography in fine-grain Nb. Field enhancements at very deep, sharp and densely populated etching pits may then cause distributed hotspots and limit cavity performance.
AES, Medford, NY
Funding: Work supported by The Department of Energy under SBIR Contract DE-FG02-08ER85054
From past work we found that within the cost of the String Assembly that dominates the overall cost of the cryomodules for ILC, the greatest cost elements are the helium vessel with the 2 phase pipe assembly, the niobium material, and the SRF cavity fabrication*. The cost of niobium is dependant upon market supply and demand and is essentially out of our control. We have carried out an aggressive study to reduce the cost of cavity fabrication in a high production environment**, which leaves the helium vessel for further investigation. It is recognized that significant cost savings may be realized if the helium vessel could be constructed of stainless steel instead of titanium material as is currently planned. To facilitate this change (AES) has designed a niobium to stainless steel transition assembly that will interface the helium vessel to the SRF Cavity at each end. Details of the design and analysis of the low cost helium vessel assembly are discussed along with potential cost reductions for the ILC high production run.
*E. Bonnema, J. Sredniawski,"ILC RF Unit Industrial Cost Study Methodology & Results"
**A. Favale, J. Sredniawski, M. Calderaro, E. Peterson,"ILC Cavity Fabrication Optimization for High Production"
CLASSE, Ithaca, New York
Funding: Work supported by NSF, New York State, and Cornell University
A cryomodule design for the Cornell Energy Recovery Linac (ERL) will be based on TTF technology, but must have several unique features dictated by the ERL beam parameters. The main deviations from TTF are that the HOM loads must be on the beamline for sufficient damping, that the average power through the RF couplers is low, and that cw beam operation introduces higher heat loads. Several of these challenges were addressed for the Cornell ERL Injector, from which fabrication and operational insight was gained. A baseline design for the Cornell ERL Linac cryomodule will be presented that includes fabrication and operational considerations along with thermal and mechanical analyses.
NSRRC, Hsinchu
The feasibility of SRF operation at 2K using the remaining refrigeration capacity of an operating 4.5K cryogenic plant at NSRRC is examined. A refrigeration configuration with warm compression is proposed under an assumption that a reasonable amount of cryogenic heat load is required at 2K. The expectation of the efficacy of the cold and warm heat exchangers (HEX) is evaluated in terms of the corresponding equivalent cryogenic heat load on the 4.5K cold box. A factor approximately 9.5 or 6.0 is required to convert the cryogenic loss, 12 W at 2K, into our 4.5K cold box operated in a refrigeration mode without or with the cold heat exchanger (efficiency 85 %), respectively. An additional benefit is that the required volumetric pumping speed of the warm compressor can be greatly decreased. Moreover, a considerable cold capacity from the sub-atmospheric cold return helium gas can be ultimately converted by combining the cold HEX working together with a highly effective warm HEX, to a conversion factor 3.8 with an efficiency 95 %. Special attention must be devoted to minimize the risk of contamination or impurity for a turbine refrigerator.
TU Darmstadt, Darmstadt
Funding: Supported by DFG via SFB 634
Operational Experience at the Darmstadt superconducting linac (S-DALINAC) showed unexpected effects on beam dynamics and beam quality. So operators could observe transverse beam deflections by changing phases of the SRF-Cavities. Furthermore there has been occurred a growth of normalized tranverse emittance by a factor of 2. The beam current in the S-DALINAC does not exceed 60 μA so space-charge effects could be eliminated to be the reason for the observations. In this work the effect of misalignment of the SRF-Cavities in the linac has been examined using beam-dynamic simulations with the tracking code GPT and measurements on the electron beam of the S-DALINAC. By measuring the transverse deflection of the beam by changes of the phases of the SRF-Cavities and comparing results with GPT-simulations a misalignment of the 5-cell capture cavity and first 20-cell cavity of several mm in both transverse directions could be found. This misalignment can explain transverse deflections as well as emittance growth. A correction of misalignment has been carried out using the described results. First measurements showed no more emittance growth and less beam-deflections by SRF-Cavities.
ORNL, Oak Ridge, Tennessee
Funding: SNS is managed by UT-Battelle, LLC, for the U. S. Department of Energy under Contract No. DE-AC05-00OR22725.
The paper presents a new variety of one-dimensional nonlinear integrable accelerator lattices with periodic and exponential invariants in coordinates and momenta. Extension to two-dimensional transverse motion, based on a recently published approach*, is discussed.
*V. Danilov, “Practical Solutions for Nonlinear Accelerator Lattice with Stable Nearly Regular Motion”, Phys. Rev. ST AB 11, 114001 (2008)
NSRRC, Hsinchu
Stability of photon beam intensity, I0, is one of the most important performance merits of a modern light source. The photon intensity measured at dragon beam line (BL11) is routinely used as a reference signal for I0 stability measurements. At NSRRC, a highly stable I0 intensity is maintained in most percentage of the user beam time. Meanwhile, glitches of I0 intensity up to few tens of percentage had been observed once every few operating hours, which was a puzzle before its reason had been identified later. A spontaneous large variation of photon intensity (I/I0) caused difficulties for users operating their experiments. Here, we report our development of a dedicated electronic circuit with functionality of single-gate, which was very helpful to clarify the puzzle of I0 glitches observed at NSRRC.
SLAC, Menlo Park, California
BNL, Upton, Long Island, New York
Funding: This work was supported by DOE Contract No. DE-AC02-76SF00515 and used resources of NERSC supported by DOE Contract No. DE-AC02-05CH11231, and of NCCS supported by DOE Contract No. DE-AC05-00OR22725.
SLAC's Advanced Computations Department (ACD) has developed the parallel 3D Finite Element electromagnetic Particle-In-Cell code Pic3P. Designed for simulations of beam-cavity interactions dominated by space charge effects, Pic3P solves the complete set of Maxwell-Lorentz equations self-consistently and includes space-charge, retardation and boundary effects from first principles. Higher-order Finite Element methods with adaptive refinement on conformal unstructured meshes lead to highly efficient use of computational resources. Massively parallel processing with dynamic load balancing enables large-scale modeling of photoinjectors with unprecedented accuracy, aiding the design and operation of next-generation accelerator facilities. Applications include the LCLS RF gun and the BNL polarized SRF gun.
NSRRC, Hsinchu
To improve the reliability of a contemporary synchrotron as light source, a diagnostic system is crucial. A satisfactory diagnostic system must enable a clear presentation of the reason for a system fault, and provide sufficient information to the data analyzer for system recovery and improvement. To identify a fault and to monitor the operation of a RF system, many diagnostic utilities have been adopted. The architecture for the diagnostics of the TLS RF system is here reported.