KEK, Ibaraki
Higher-order-mode (HOM) free superconducting (SC) single cell cavities were developed for the rf system of high luminosity storage ring colliders. Because of the successful results of these cavities under ampere-class beams, the components and technology of the SC cavities have immediately been applied to the middle sized storage rings upgrading the beam intensity by using a few SC cavities. Beside the storage ring rf, a SC based high intensity proton linac was commissioned for neutron physics. Recently, the feasibility study of energy recovery linacs has been carried at various laboratories aiming for the 4th generation light source. Status of these developments will be described.
CLASSE, Ithaca, New York
The talk will provide an overview of the latest developments in rf systems for cw operated SRF linacs, such as CEBAF (in particular, 12 GeV Upgrade), Cornell ERL injector, ELBE, and ERLP at Daresbury.
Fermilab, Batavia
High-beta superconducting rf elliptical cavities are being developed in large numbers for several accelerator projects including the International Linear Collider (ILC). In recent years, the understanding of cavity performance limitations has improved significantly, leading to better than 40 MV/m in some cavities. However, further improvement is needed to reach reliably the 31.5 MV/m operating gradient proposed for the ILC Main Linac cavities. World-wide R&D on the cavity gradient frontier includes improved surface cleaning and smoothing treatments, development of alternative cavity shapes and materials, and novel cavity manufacturing techniques. Substantial progress has been made with diagnostic instrumentation to understand cavity performance limitations. Some highlights of the efforts in superconducting rf R&D toward achieving higher gradients in high-beta elliptical cavities are reviewed.
IPN, Orsay
The talk will provide an overview of the SRF development toward the acceleration of light and heavy ions including QWRs, HWRs, spoke and CH cavities.
ORNL, Oak Ridge, Tennessee
Funding: This work has been sponsored by ORNL-SNS. The Spallation Neutron Source is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.
Normally, reactive loading is employed to construct accelerating cavities in order to slow the phase velocity of the electromagnetic wave. However, due to their non-uniform cross section, they tend to be difficult to machine, requiring complicated welding or brazing processes which increase the total cost. Although empty straight waveguides can only support faster-than-light propagation, empty twisted waveguides can support propagation at or below c. Because twisted structures have a uniform cross section in the transverse plane, they offer several potential advantages over dielectric loaded structures or other types of periodic structures. Of particular interest are twisted structures whose longitudinal cross section has been selected to resemble well-known accelerating structures, such as the iris-loaded accelerating structure and the TESLA type elliptical cavity. Comparisons are drawn between these conventional cavities and their twisted counterparts. Specifically, the phase velocity and dispersion relationship are discussed, the accelerating mode is found and analyzed, and R/Q is calculated. Design guidelines for the design of twisted structures are given.
Plansee Metall GmbH, Reutte
Nb-RRR sheet material is one of the key components of super conducting linear particle accelerator projects (e.g. XFEL, ILC). The high quality requirements led to sophisticated quality systems in the manufacturing line. A major aspect is the development of non-destructive inspection methods for the detection of surface defects, delaminations, and inclusions. Up to now the standard inspection technologies for quality assurance of Nb-RRR sheet material are based on electromagnetic techniques, e.g. SQUID and eddy current. For these methods the detection limit is in the range of 0.1 mm. Ultrasonic microscopy (USM) in the frequency range of 1 GHz is a well established and economic technique for non-destructive surface inspection. For volume inspection of sheet material ultrasonic frequencies of 50 to 100 MHz are applied. For Nb-RRR sheets with typical thickness of 2.8 mm a detection limit of 0.1 mm is expected. First results of USM on Nb-RRR sheet material are presented.
TRIUMF, Vancouver
University of Toronto, Toronto, Ontario
TRIUMF is proposing to build a 50 MeV electron linac as a driver to produce radioactive ion beams through photofission. The present design calls for the use of nine-cell 1.3 GHz Tesla type cavities. A 1.3 GHz Superconducting RF (SRF) program has been initiated with the goal to produce and test one nine cell cavity by the end of 2009. The program will utilize the existing clean room and SRF test facilities that support the ISAC-II heavy ion superconducting linac. A vertical cryostat has been modified with a new insert to allow single cell testing. Pumps for 2 K sub-atmospheric operation have been tested. A single cell fabrication program is being initiated with a local company. A RRR measurement program is on-going to test cavity welds. The goal of the 1.3 GHz upgrade is to not only produce cavities for the in house project but to broaden TRIUMF's technical base for future potential collaborations. The paper will report the progress and plans of the 1.3 GHz SRF program.
TRIUMF, Vancouver
PAVAC, Richmond, B.C.
The medium beta section of the ISAC-II superconducting linac (β=5.7% and 7.1%) has been operational since April 2006 providing 20 MV of accelerating potential at 106 MHz. The ‘high beta' extension to the linac, in progress, will see the addition of twenty 141 MHz quarter wave cavities at β=11%. The design specification calls for cw operation at a voltage gain of at least 1.1 MV/cavity for no more than 7 W of power dissipated in the cavity. This operation point corresponds to challenging peak surface fields of 30 MV/m and 60 mT. The cavity design is similar in concept to the medium beta cavities except for the addition of a drift tube to render symmetric the accelerating fields. A prototyping and qualification program was initiated with PAVAC Industries Inc. of Richmond, B.C. Two full size models in copper and two in niobium have been completed. The niobium cavities have been warm and cold-tested and characterized for frequency, rf performance and mechanical stability. The cold performance of both cavities exceeds the specification and the final frequency is within tuning range. The design, fabrication details and test results will be presented.
TRIUMF, Vancouver
The ISAC-II linac extension requires an additional 20 rf amplifiers to power twenty 141 MHz quarter wave superconducting cavities. Solid state amplifiers will be used for this extension as compared to tube amplifiers which have been employed for the existing ISAC-II linac section, operational since 2006. The amplifiers are specified to run with an output power of 600 W. The first amplifier of the production series has been tested for gain and phase linearity. Phase noise of this amplifier has been measured on a 141 MHz superconducting cavity and compared with phase noise measured with a tube amplifier. The test results and general rf, interlock and interface requirements are verified against tendered specification before series production of the remaining amplifiers can proceed. Benchmarking tests of the prototype amplifier will be reported.
GANIL, Caen
IPN, Orsay
The construction of the new Spiral 2 facility has started in Caen (France) at the National Heavy Ions Accelerator Center (GANIL). The SPIRAL 2 project is based on a multi-beam Superconducting Linac Driver delivering 5 mA deuterons up to 40 MeV and 1 mA heavy ions up to 14.5 MeV/u delivering different Radioactive Isotope Beams (RIB). The LINAC is composed of 2 cryomodule families. The low energy family (cryomodules A) is composed of 12 cryomodules housing a single cavity at beta=0.07. The "high" energy family (cryomodules B) is composed of 7 cryomodules housing 2 cavities at beta=0.12. In between cryomodules are located the focalisation quadrupoles and the diagnostic boxes. Strong beliefs forbid the use of interceptives diagnostics around superconducting cavities. We simulated the use of wires for diagnostics in the linac, sublimating 14 wires of tungsten, Niobium and carbon while operating the B cavity at full performances. The first results describe in this paper looks promising.
CEA, Gif-sur-Yvette
In the framework of the European CARE-HIPPI program we develop components for superconducting high pulsed power proton linacs at 704 MHz. We have designed, fabricated and tested a beta 0.47 5-cell elliptical cavity with an optimized stiffening to reduce its sensitivity to Lorentz forces. A fast piezo tuner has been developed in order to be able to operate the cavity in pulsed mode in our horizontal test cryostat CryHoLab. We also have carried out the development of a fundamental power coupler. It is designed to transmit a power up to 1 MW at a 10% duty cycle. A high power test area has been setup consisting of a 1.2 MW klystron, a pulsed high voltage power supply and a coupler test stand.
IPN, Orsay
SPIRAL2 aims at building a multi-purpose facility dedicated to nuclear physics studies, including the production of rich-neutrons isotopes. The multi-beam linear accelerator is composed of superconducting accelerating modules and warm focusing magnets. IPN Orsay is in charge of the high energy accelerating modules, each hosting two superconducting (β = 0.12) quarter-wave resonators operating at an accelerating field of 6.5 MV/m at 88 MHz. The static and dynamic frequency tuning is achieved by the insertion and motion of a niobium plunger into the magnetic field area. The efficiency of the tuning (1 kHz/mm) has been validated during the tests of the cryomodule. In this paper we discuss the impact of such a tuning system, based on experimental results on Spiral2 cavities, on the different aspects: maximum accelerating field, Qo slopes, quench, multipacting and microphonics.
IPN, Orsay
GANIL, Caen
LPSC, Grenoble
The SPIRAL2 project, installed in GANIL for Radioactive Ion Beam physics purposes requires the manufacturing of a multi beam driver. This driver is based on a superconducting Linac featuring two 88 MHz Quarter Wave Resonator families. IPN Orsay is in charge of the study and the assembly of the 7 high energy (beta = 0.12) cryomodules. Each cryomodule is composed of two QWRs, specified to operate at 4.2 K with a nominal accelerating gradient of 6.5 MV/m. A first qualifying cryomodule has been manufactured and tested at the beginning of 2008 in order to validate the resonator and the cryostat design before launching the serial production of the 6 remaining cryomodules. The paper presents the main results of this test and the cryomodule design in its final version.
BESSY GmbH, Berlin
Funding: Work partially funded by the EU Commission in the sixth framework programme, contract no 011935 EURO-FEL-DS5, BMBF and Land Berlin.
All mechanical tuning systems are subject to hysteresis effects: For coarse tuning with a stepper motor, the exercised forces lead to a visco-elastic deformation of the tuner body. In piezo-based fine tuning, even if the smaller deformations of tuner and cavity can be regarded as fully elastic, the piezo-actuators themselves suffer from remanent polarization effects. The extent of these nonlinearities has been measured in three different tuning systems (Saclay I, Saclay II and Blade Tuner) utilizing high-voltage as well as low-voltage piezo actuators. An estimate of the resulting tuner-resolution and performance degradation with respect to microphonics compensation is given. Experiments were performed in the HoBiCaT facility at BESSY.
IAP, Frankfurt am Main
Funding: GSI, BMBF contr. No. 06F134I, EU contr. No. 516520-FI6W, RII3-CT-2003-506395, EFDA/99-507ERB500CT990061
Worldwide there is an increasing interest in new high intensity proton and ion driver linacs with beam powers up to several MW. A very challenging part of these accelerators is the low and medium energy section up to 100 MeV. Depending on the duty cycle room temperature or superconducting options are favoured. In both cases the Crossbar-H-mode (CH)-structure, developed at the IAP in Frankfurt is an excellent candidate. Room temperature as well as superconducting prototype cavities have been developed and tested successfully. A superconducting 19 cell low energy (beta=0.1) CH-cavity at 360 MHz reached effective gradients of 7 MV/m corresponding to an accelerating voltage of 5.6 MV. This cavity could be used for high intensity, cw operated linacs like accelerator driven systems (ADS, EUROTRANS) or the international fusion material irradiation facility (IFMIF). Additionally, the new proton injector for FAIR (325 MHz, 70 mA, 70 MeV) will use room temperature CH-cavities. Recent developments of this new type of a multi-cell drift tube cavity, beam dynamics issues and the tests of the prototypes will be presented.
DESY, Hamburg
For more than 10 years DESY has been operating a high resolution eddy-current scanning installation with rotating table for nondestructive flaw detection on niobium sheets for SRF cavities. More than 2000 sheets have been examined up to now, several types of defects have been detected and identified using different supplementary methods such as EDX, X-ray fluorescence, neutron activation analysis etc. In order to scan Nb-sheets needed for XFEL-cavity production, new scanning devices have to be build. One option of the eddy-current installations could be an application of SQUID-sensors due to much higher sensitivity instead of conventional probes. A SQUID based scanner system was built and is in evaluation at DESY. A status report will be given.
DESY, Hamburg
Funding: We acknowledge the support of the European Community Research Infrastructure Activity under FP6 'Structuring the European Research Area' program (CARE, contract number RII-CT-2003-506395
Dry-Ice cleaning offers a dry and waterless cleaning option removing hydrocarbons and particles without residues. Complex excavations like Cu rf gun cavities and Nb multicell cavities in horizontal installation position can be cleaned in an effective way. In the recent past rf gun cathodes and cathode transportboxes could be cleaned satisfactory. A status report will be given.
DESY, Hamburg
In preparation for the series production of roughly 800 superconducting accelerating structures, several tests with an industrial-like production sequence have been tested for their accelerating gradient and quality factor. The main part of the surface preparation is being done with electropolishing. with ethanol rinse. For the two different final preparation steps namely electropolishing and etching the performance is compared. The results will be also put into the perspective of earlier cavity production cycles that were tested at DESY.
DESY, Hamburg
The removal of the Q-drop without field emission by a low temperature (app. 120C) bake procedure is essential in order to achieve the full performance in both electropolished (EP) and chemically etched (BCP) high gradient SCRF Nb accelerator cavities. A simplified procedure applying an open 120C bake out in an Argon atmosphere is presented. First results are compared to the well-established bake-out procedure under vacuum conditions.
DESY, Hamburg
The local thermal breakdown (quench) behavior of one- and nine-cell SCRF Nb accelerator cavities is investigated systematically. For more than 50 cavities, temperature mapping data have been analyzed with respect to surface preparation, Nb material etc. Results on quench location and characteristic correlations are presented.
IUAC, New Delhi
The frequency control of the superconducting quarter wave resonator at IUAC is currently accomplished by mechanical and electronic tuners which are operated in the time scale of seconds and hundreds of milliseconds to a few tens of microseconds respectively. Due to presence of microphonics, input rf power in the range 200-300 W was required to control the resonator for a typical field of 3-5 MV/m achieved with 6 watts dissipation. Implementation of a novel idea to damp the mechanical vibration with the help of SS-balls has helped to reduce rf power below 100 W. Though resonators are working fine at this power level, we are investigating whether further reduction of rf power is possible using a piezo actuator to control the drift of frequency. The piezo tuner working in hundreds of milli seconds range with the dynamic phase control scheme will share a substantial load from the electronic tuner. As a result, the resonator's phase lock loop will remain locked for less rf power. The initial test results of the piezo tuner will be presented.
INFN/LASA, Segrate (MI)
DESY, Hamburg
BESSY GmbH, Berlin
Cavity tuners are needed to precisely tune the narrow-band resonant frequency of superconducting cavities. The Blade Tuner presented is installed coaxially to the cavity and changes the resonator frequency by varying its length. Piezoceramic actuators add dynamic tuning capabilities, allowing fast compensation of main dynamic instabilities as Lorentz Forces, under pulsed operations, and microphonic noise. A prototype piezo Blade Tuner has been assembled on a TESLA cavity and extensively cold tested inside the horizontal cryostats CHECHIA (DESY) and HoBiCaT (BESSY). Then, as suggested by results, a few minor modifications have been implemented thus achieving the current Blade Tuner design. The introduction of thicker blades re-distributed along the circumference allows to increase its stiffness and fulfill European and American pressure vessel codes, while ensuring requested performances and cost. The paper will present the successful characterization tests performed on the prototype, the extensive mechanical analyses made to validate the final model and the results from qualification tests of first revised Blade Tuner produced, to be installed in the second module of ILCTA at FNAL.
INFN/LASA, Segrate (MI)
Fermilab, Batavia
DESY, Hamburg
The third harmonic cavities that will be used at the injector stage in the XFEL to linearize the rf curvature distortions and minimize beam tails in the bunch compressor are based on the rf structures developed at FNAL for the DESY FLASH linac. The design and fabrication procedures have been modified in order to match the slightly different interfaces of XFEL linac modules and the procedures followed by the industrial production of the main (1.3 GHz) XFEL cavities. A revision of the helium vessel design has been required to match the layout of the cryomodule strings, and a lighter version of the tuner has been designed (derived from the 1.3 GHz ILC blade tuner activities). The main changes introduced in the design of the XFEL cavities and the preliminary experience of the fabrication of three industrially produced and processed third harmonic rf structures are described here.
Kyoto ICR, Uji, Kyoto
KEK, Ibaraki
Inspections of superconducting rf cavities seem essential in achieving high achieving gradient. The inspection of interior surface of a superconducting rf cavity with high enough resolution to find defects more than several tens microns is achieved by our high resolution camera system. This system revealed undiscovered defects at just inner sides of the locations predicted by passband-mode and thermometry measurements. This system will help to improve cavity fabrication processs and their yield. This system will be delivered world wide for that purpose. We are planning to widen our activity in this field: developments of new termometry system with easy installation and less cabling and high sensitivity Eddy Current Surface Inspection system for bare niobium sheets. The detailed systems and some preliminary data obtained from the systems will be presented.
CERN, Geneva
The HIE-ISOLDE activity aims at the construction of a superconducting linac based on 101.28 MHz Nb sputtered Quarter Wave Resonators (QWRs), which will be installed downstream the present REX-ISOLDE linac. The present design considers two basic geometries of the cavities (geometric β0 = 6.26% and 10.84%) for which a mechanical, chemical treatment and Nb coating design study has been performed. We report here on the status of the prototype cavity and sputtering tests.
Cockcroft Institute, Lancaster University, Lancaster
SLAC, Menlo Park, California
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
Fermilab, Batavia
UMAN, Manchester
CERN, Geneva
Crab cavities have been proposed for a wide number of accelerators and interest in crab cavities has recently increased after the successful operation of a pair of crab cavities in KEK-B. In particular crab cavities are required for both the ILC and CLIC linear colliders for bunch alignment. Consideration of bunch structure and size constraints favours a 3.9 GHz superconducting, multi-cell cavity as the ILC solution, whilst bunch structure and beam-loading considerations suggest an X-band copper travelling wave structure for CLIC. These two cavity solutions are very different in design but share complex design issues. Phase stabilisation, beam loading, wakefields and mode damping are special issues for these crab cavities. Requirements and potential design solutions will be discussed for both colliders.
ANL, Argonne
A set of six new 109 MHz β=0.15 superconducting quarter-wave resonators (QWR) has been built at ANL as part of an upgrade to the ATLAS superconducting heavy-ion linac. The final cavity string assembly will also use many of the techniques needed for the next generation of large high-performance ion linacs such as the U.S. Department of Energy's FRIB project. Single-cavity cold tests at T=4.5 K have been performed for three cavities with moveable coupler, rf pickup, and VCX fast tuner as required for the full 6-meter cryomodule assembly. The average maximum accelerating gradient of 4 cavities (3 new + 1 prototype), is EACC=11.2 MV/m (BPEAK=65 mT). Clean cavity string assembly techniques, required here and for most future SRF ion linacs, are fairly well developed. Details on cavity performance including high-field cw operation, microphonics and fast tuning are presented.
ANL, Argonne
Fermilab, Batavia
New SRF cavity processing systems at ANL, including those for electropolishing (EP), high-pressure water rinsing (HPR), and single-cavity clean room assembly have been developed and operated at ANL for use with cavities for a range of electron and ion linac applications. Jointly with FNAL, systems for 1.3 GHz single- and multi-cell elliptical cavities for the linear collider effort have been developed. New systems for use with low-beta TEM-class cavities have also been built and used to process a set of new quarter-wave resonators as part of an upgrade to the ATLAS heavy-ion accelerator at ANL. All of the new hardware is located in a 200 m2 joint ANL/FNAL Superconducting Cavity Surface Process Facility (SCSPF) consisting of two separate chemical processing rooms, a clean anteroom, and a pair of class 10 and 100 clean rooms for HPR and clean assembly. Results of first cold tests for elliptical and TEM-class cavities processed in these facilities are presented.
Fermilab, Batavia
University of Udine, Udine
Funding: This work was supported by Fermilab Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
The 3.9 GHz 3rd harmonic cavities are designed to serve as compensation devices for improving the longitudinal emittance of the free-electron laser FLASH at DESY. These cavities operate in the TM010 mode, and will be located between the injector and the accelerating cavities. Fermilab is obligated to provide DESY with a cryomodule containing four 3rd harmonic cavities. In this paper we discuss the process of welding helium vessels to these cavities. Included will be a description of the joint designs and weld preparations, development of the weld parameters, and the procedure for monitoring the frequency spectrum during TIG welding to prevent the cavity from undergoing plastic deformation. Also discussed will be issues related to qualifying the dressed cavities as exceptional vessels (relative to the ASME Boiler and Pressure Vessel Code) for horizontal testing and eventual installation at DESY, due to the necessary use of non-ASME code materials and non-full penetration electron beam welds.
Fermilab, Batavia
Funding: Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
Fermilab is involved in an effort to design, build, test and deliver four 3.9 GHz superconducting rf cavities within a single cryomodule to be delivered to DESY as a 'third harmonic' structure for the FLASH facility to improve the longitudinal emittance. In addition to an overall status update we will present recent results from single 'dressed' cavity horizontal tests and shipping and alignment measurements.
Fermilab, Batavia
Funding: Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
3.9 GHz SRF cavities have been built and tested at Fermilab for use in the DESY FLASH facility. Six cavities have undergone testing in various scenarios. Comparisons of performance in these different conditions, from bare cavities in a vertical dewar to 'dressed' in the horizontal test stand and intermediate test configurations are presented. We also report on analysis of expected maximum performance and an estimate of same.
Fermilab, Batavia
ANL, Argonne
The High Intensity Neutrino Source (HINS) project represents the current effort at Fermilab to develop 60 MeV Proton/H- Linac as a front end for possible use in the Project X. Eighteen superconducting β=0.21 single spoke resonators (SSR), operating at 325 MHz, comprise the first stage of the HINS cold section. Two SSR cavities have now been fabricated in industry under this project and undergone surface treatment that is described here. We report the results of high gradient tests of the first SSR in the Vertical Test System (VTS). The cavity successfully achieved accelerating gradient of 13.5 MV/m; higher than the design operating gradient of 10 MV/m. The history of multipacting and conditioning during the VTS tests will be discussed. Experimental measurements of the cavity mechanical and vibration properties including Lorenz force detuning and measurements of X-rays resulting from field emission are also presented.
NSCL, East Lansing, Michigan
INFN/LNL, Legnaro, Padova
FZJ, Jülich
A superconducting linac is being planned for re-acceleration of exotic ions produced by the Coupled Cyclotron Facility at Michigan State University. The re-accelerator will include a gas stopper, a charge breeder, a normal conducting radio-frequency quadrupole, and two types of superconducting quarter-wave resonators (QWRs) for re-acceleration to energies of up to 3 MeV per nucleon initially, with a subsequent upgrade path to 12 MeV per nucleon. The QWRs (80.5 MHz, optimum beta = 0.041 and 0.085, made from bulk niobium) are similar to existing cavities presently used at INFN-Legnaro. The re-accelerator's cryomodules will accommodate up to 8 cavities, along with superconducting solenoids for focussing. Active and passive shielding is required to ensure that the solenoids' field does not degrade the cavity performance. First prototypes of both QWR types have been fabricated and tested. A prototype solenoid has been procured and tested. A test cryomodule has been fabricated: one QWR, one solenoid, and two other beam line elements have been installed inside. This paper will cover the re-accelerator cavity and cryomodule prototyping efforts, results so far, and future plans.
CLASSE, Ithaca, New York
Funding: Work is supported by the National Science Foundation grant PHY 0131508.
Two high power 1300 MHz rf systems have been developed for the Cornell University ERL Injector. The first system, based on a 16 kWCW IOT transmitter, is to provide rf power to a buncher cavity. The second system employs five 120 kWCW klystrons to feed 2-cell superconducting cavities of the injector cryomodule. The sixth, spare klystron is used to power a deflecting cavity in a pulsed mode for beam diagnostics. A digital LLRF control stem was designed and implemented for precise regulation of the cavities' field amplitudes and phases. All components of these systems have been recently installed and commissioned. The first operational experience with the systems is discussed.
CLASSE, Ithaca, New York
Funding: Work is supported by the National Science Foundation grant PHY 0131508
Multipactor in beam-pipe transitions of superconducting rf cavities can be explained using rf potential well theory*. In this paper we present simulation results supporting this explanation for both rf cavities and transmission lines.
*S. Belomestnykh and V. Shemelin, "Multipacting-free Transitions between Cavities and Beam-pipes," submitted to Nuclear Instruments and Methods in Physics Research A.
CLASSE, Ithaca, New York
Cornell University, Ithaca, New York
Funding: DOE and NSF
Quench detection for 9-cell LLC cavities is presently a cumbersome procedure requiring two or more cold tests. One is to identify the cell-pair involved via quench field measurement in several pass band modes, followed by a second cold test with many fixed thermometers attached to the culprit cell-pair to identify the particular cell, and possibly a third measurement to zoom in on the quench spot with many localized fixed thermometers. We report here on a far more efficient alternative method which utilizes a few (e.g. 8) oscillating super-leak transducers to detect the He-II second sound wave driven by the defect induced quench. Results characterizing defect location on a 9-cell reentrant cavity with He-II second sound detection and corroborating measurements with carbon thermometers will be presented.
LANL, Los Alamos, New Mexico
NRL, Washington, DC
LLNL, Livermore, California
Stanford University, Stanford, Califormia
Beam-Wave Research, Inc., Union City
Funding: Supported by the High-Energy Laser Joint Technology Office
We are investigating spoke resonators that originally were proposed for moderate energy proton acceleration for application in high-average-current free-electron lasers (FEL). This structure holds the promise of alleviating the BBU limitations of conventional rf structures. Spoke resonator have several advantages: 1) strong coupling simplifies the access to higher order modes (HOM), 2) at the same frequency a spoke resonator is about half the size of an elliptical resonator, 3) the spokes provide additional mechanical stability and stiffening , 4) the power and HOM couplers can be attached to the cavity body and do not take up additional space along the length of the accelerator, 5) the presence of the spokes limits the polarizations of the HOMs to two orientations which facilitates the selection of HOM coupler positions. The rf performance of a spoke resonator specifically designed for high-current electron applications (beta=1.0) will be presented and compared with the expected performance of elliptical resonators designed for such applications. Besides the structure's effectiveness for acceleration also HOM properties will be presented.
SLAC, Menlo Park, California
Funding: Work supported by DOE contract DE-AC02-76SF00515.
The TESLA-shape cavity has been chosen as the baseline design for the 1.3 GHz SCRF linacs of the International Linear Collider. However, there is ongoing research to develop new cavity shapes that will support higher gradients and hence lower the machine cost. The critical magnetic flux (Bc) of the niobium, which is approximately 180 mT, ultimately limits the gradient achievable in a superconducting cavity. Thus far, the new designs have focused on minimizing the peak surface magnetic field (Bs) for a given on-axis gradient, while relaxing the requirement on the peak surface electric field (Es). For example, the Low Loss (LL) design reduces Bs by more than 10% relative to the baseline design, which should allow a gradient of up to 50 MV/m with a 20% reduction in cryogenics loss. However, Es is about 15% higher in this case, which enhances field emission that in practice is one of the main impediments to achieving the Bc-limited gradient. In this paper, we will present an optimized cavity shape that reduces both Bs and Es, and thus should have a better chance of reaching higher gradients. The design of HOM couplers for wakefield damping in this cavity will also be presented.
SLAC, Menlo Park, California
Funding: Work supported by DOE contract DE-AC02-76SF00515.
The shape deviation of a SRF cavity from the design shape may result in significant impact on cavity performance and wakefields that could lead to unexpected effects in beam dynamics. Yet, most of these deviations are unknown in the final cavity installation because of the complicated process of assembly and tuning. It is desirable to be able to uncover such distortions using measurable rf quantities. With these data, the cavity performance can be analyzed and realistic tolerance criteria may be implemented in the cavity design and manufacture for quality assurance. To perform such analyses, SLAC has developed advanced Shape Determination Tools, under the SciDAC support for high performance computing, that recover the real cavity shape by solving an inverse problem. These tools have been successfully applied to analyze shape distortions to many SRF cavities, and identified the cause of unexpected cavity behaviors. The capabilities and applications of these tools will be presented.
JLAB, Newport News, Virginia
Funding: Supported by US DOE Contract No. DE-AC05-06-OR23177
A new type of rf structure for the deflection and crabbing of particle bunches is introduced. It is comprised of a number of parallel TEM-resonant lines operating in opposite phase from each other. One of its main advantages is its compactness compared to conventional crabbing cavities operating in the TM110 mode, thus allowing low frequency designs. The properties and characteristics of this type of structure are presented.
JLAB, Newport News, Virginia
Fermilab, Batavia
KEK, Ibaraki
Funding: Supported by DOE
Jefferson Lab plays an active role in the ILC high-gradient SRF R&D. Eight 9-cell cavities have been processed and tested so far by using the state-of-the-art recipes. Five reached a maximum gradient of over 32 MV/m. However, not surprisingly, the high-gradient performance is not necessarily reached during the first test. Re-processing by progressively more material removal can improve performance ultimately, but the number of re-processing cycles needed is un-predictable. Some cavities are quench limited repeatedly at around 20 MV/m. The quench locations are near the equator weld of specific cells. Based on the non-trivial high-gradient experiences in the past two years, we come to the conclusion that new capabilities beyond the state-of-the-art must be added to the existing SRF infrastructures in order to reliably achieve high gradients at a low cost. Targeted R&D is required to identify and characterize gradient limiting defects and field emitters. An enhanced high-gradient R&D program is emerging at JLab for continued contribution to realize the ambitious ILC gradient yield goal.
DESY, Hamburg
JLAB, Newport News, Virginia
Funding: This manuscript has been partially authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
The technology of forming multi-cell seamless niobium cavities has been developed at DESY within the European CARE (Coordinated Accelerator Research in Europe) program. Three cell units have been manufactured successfully and a 9-cell cavity has recently been completed from three sub-sections and will be tested in the near future. Meanwhile, we have equipped two 3-cell units – one center unit of a 9-cell cavity and one end-unit – with niobium beam pipes, have tuned these units and carried out cryogenic rf tests after standard bcp surface treatments had been applied to these cavities. In addition, we will take temperature maps with Jlab's two-cell thermometry system; since in cavities fabricated by 'standard' methods such as deep drawing of half cells and electron beam welding cavity performance limitations have often been found at or near equator welds. It will be of particular interest to compare the seamless cavity quench locations to those from standard cavities. This contribution will report about the cryogenic test results and the T-mapping findings.
JLAB, Newport News, Virginia
SLAC, Menlo Park, California
Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Higher Order Modes generated by a particle beam passing through a superconducting accelerating cavity have to be damped to avoid beam instabilities. A coaxial coupler located in the beam pipes of the cavities provides for better propagation of HOMs and strong damping in appropriate HOM dampers. The whole damping device can be designed as a detachable system. If appropriately dimensioned, the rf currents can be minimized at the flange position. Additionally, the coaxial system also provides efficient coupling of fundamental mode rf power into the superconducting cavity. Compared to presently available solutions for HOM damping, this scheme provides for several advantages: stronger HOM damping, flangeable solution, exchangeability of the HOM damping device on a cavity, less complexity of the superconducting cavity, possible cost advantages. This contribution will describe the results of room temperature measurement and discuss modeling, which resulted in an optimized layout of a cavity-coupler system.
ORNL, Oak Ridge, Tennessee
University of Tennessee, Knoxville, Tennessee
Funding: This work has been sponsored by ORNL-SNS. The Spallation Neutron Source is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.
Normally, reactive loading is employed to construct accelerating cavities in order to slow the phase velocity of the electromagnetic wave. However, due to their non-uniform cross section, they tend to be difficult to machine, requiring complicated welding or brazing processes which increase the total cost. Although empty straight waveguides can only support faster-than-light propagation, empty twisted waveguides can support propagation at or below c. Because twisted structures have a uniform cross section in the transverse plane, they offer several potential advantages over dielectric loaded structures or other types of periodic structures. Of particular interest are twisted structures whose longitudinal cross section has been selected to resemble well-known accelerating structures, such as the iris-loaded accelerating structure and the TESLA type elliptical cavity. Comparisons are drawn between these conventional cavities and their twisted counterparts. Specifically, the phase velocity and dispersion relationship are discussed, the accelerating mode is found and analyzed, and R/Q is calculated. Design guidelines for the design of twisted structures are given.