KEK, Ibaraki
A 6-m cryomodule, which includes four Tesla-like 9-cell cavities, was assembled and installed in the STF tunnel in April, 2008. After cooldown of the cryomodule, high power tests of four cavities had been carried out at 2 K from September to December, 2008. A cavity package consists of a 9-cell niobium cavity with two HOM couplers, an input coupler with a cold and a warm rf window, and a frequency tuning system with a mechancal and a piezo tuner. The performance as a total sc cavity system was checked in the cryomodule test with high rf power. One of the cavities was achieved a stable pulsed operation at 32 MV/m higher than the specific operating gradient (31.5 MV/m) in ILC. The maximum accelerating gradients (Eacc,max) obtained in the vertical cw tests was maintained or slightly improved in the cryomodule tests with a pulsed operation of 1.5 msec and 5 Hz. Compensation of Lorentz force detuning at 31 MV/m was successfully demonstrated by using piezo tuner and pre-detuning.
IHEP Beijing, Beijing
KEK, Ibaraki
TIPC, BeiJing
Funding: Supported by National Natural Science Foundation of China (10525525)
1.3 GHz single cell large grain (LG) cavities have been studied in our research programs on the superconducting cavity for the International Linear Collider (ILC) in the last three years and five LG cavities were fabricated at IHEP and KEK. Three cavities were dealt with by surface treatments based on electro polishing (EP) and the maximum gradient of 47.90 MV/m was achieved. The other two cavities were treated based on chemical polishing (CP) and both reached the accelerating gradients higher than 35 MV/m with the maximum gradient of 40.27 MV/m. In this paper, the performance comparison of the large grain cavities will be presented and discussed.
Fermilab, Batavia
Muons, Inc, Batavia
Funding: Supported in part by FRA DOE contract number DE-AC02-07CH11359
RF cavities below 800 MHz are large, so alternative cavities at low frequencies are needed. Novel dielectric loaded RF cavities will allow smaller diameter cavities to be designed; changing the frequency of a cavity design would be as simple as changing the dielectric cylinder insert material or inner radius of the dielectric in the cavity. This paper discusses RF cavities loaded with dielectric material that could be used in various ways for muon facilities. The examples given are for 400 and 800 MHz cavities. Our initial motivation was to use dielectric to reduce the radial size of gas-filled cavities in helical cooling channels, but dielectric-loading has potential use in vacuum cavities for suppression of dark current emission. We also studied cavities that can be used for the phase rotation channel in the front end of a muon collider or neutrino factory.
IIT, Chicago, Illinois
Fermilab, Batavia
LBNL, Berkeley, California
ANL, Argonne
Funding: The United States Department of Energy
The accelerating gradient in a RF cavity is limited by many factors such as the surface material properties, RF frequency, the external magnetic field and the gas pressure inside the cavity. In the MuCool Program, RF cavities are studied with the aim of understanding these basic mechanisms and improving their maximum stable accelerating gradient. These cavities are being developed for muon ionization cooling channel for a Neutrino Factory or Muon Collider. We report studies using the 805 MHz and 201 MHz RF cavities in the MuCool Test Area (MTA) at Fermilab. New results include data from buttons of different materials mounted in the 805 MHz cavity, study of the accelerating gradient in the 201 MHz cavity and X-ray background radiation from the cavities due to Bremsstrahlung. The 201 MHz cavity has been shown to be stable at 19 MV/m at zero magnetic field, well in excess of its 16 MV/m design gradient. We will also discuss results from the 201 MHz cavity study in magnetic field and introduce the test of E × B effects with the 805 MHz box cavity.
CLASSE, Ithaca, New York
Funding: Work Supported by the NSF and DOE
Superconducting RF cavity quench detection is presently a cumbersome procedure requiring two or more expensive cold tests. One cold test identifies the cell-pair involved via quench field measurements in several 1.3 GHz TM010 pass-band modes. A second test follows with numerous fixed thermometers attached to the culprit cell-pair to identify the particular cell. A third measurement with many localized thermometers is necessary to zoom in on the quench spot. We report here on a far more efficient alternative method which utilizes a few (e.g. 8) oscillating superleak transducers (OST) to detect the He-II second sound wave driven by the defect induced quench. Results characterizing defect location with He-II second sound wave OST detection, powering multiple modes of the 1.3GHz TM010 passband to locate multiple defects, and corroborating measurements with carbon thermometers will be presented.
CLASSE, Ithaca, New York
Fermilab, Batavia
Funding: Work Supported by the U.S. Department of Energy
To build the ~14,000 cavities required for the ILC each of the three world regions must have a sizable industrial base of qualified companies to draw cavities from. One of these companies, Niowave Inc., recently manufactured six 1.3 GHz single-cell cavities for qualification purposes. All six cavities achieved gradients above 25 MV/m before they were limited by the available RF power (Q-slope) or quenched. This paper will report the results of cold tests for all six cavities and on the causes of quench determined by 2nd sound detection and optical inspection.
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: Work Supported by the NSF and the DOE
An innovative reentrant cavity design instigated the initial, highly successful, superconducting niobium reentrant-single-cell cavity tests at Cornell and KEK. Prompted by the success of the single cell program a joint effort of Cornell University and Advanced Energy Systems (AES) fabricated two multiple-cell reentrant cavities: a three-cell and a nine-cell cavity. This paper reports the development status of these two cavities. First, the results of cold tests, superfluid helium defect location and repair work on the reentrant nine-cell cavity will be presented. Second, the results of cold tests, including defect location and repair efforts of the reentrant three-cell cavity will be presented.
CLASSE, Ithaca, New York
The superheating magnetic field of a superconducting niobium 1.3 GHz reentrant cavity was measured at several points in the temperature range from (1.7 to 4.4) K. This experimental data is used to discriminate between two competing theoretical s for the temperature dependent behavior of the RF superheating field. Measurements were made with <250 us high power pulses (HPP, ~1MW) to avoid defect initiated thermal breakdown from contaminating the data. Our test incorporated oscillating superleak transducers to determine the cavity quench locations and characterize changes and the migrations of the quench locations during processing. This information provides insight into the factors which limit the ultimate achievable RF surface magnetic field.
DESY, Hamburg
ACCEL, Bergisch Gladbach
W.C. Heraeus GmbH, Materials Technology Dept., Hanau
A test program of 1.3 GHz TESLA shape 9-cell large grain (LG) resonators for the European XFEL project was started at DESY. The main aim is to find out whether or not the choice of LG material could be an option for the fabrication of approx. 800 XFEL resonators. Several aspects are under investigation and will be compared with the conventional polycrystalline material option. One of the aspects is the material issue: could the required amount of LG niobium be produced at industry in a cost effective and reliable manner? The second issue is the fabrication of cavities: could the series production of resonators be done on the level of required accuracy and costs? The third one is the performance issue: what is the appropriate treatment for reproducibly achieving the specified XFEL accelerating gradients? Development of the LG disc production was done within the framework of the R&D program of DESY and W. C. HERAEUS. Eleven resonators are produced at the company ACCEL. Up to now three resonators are RF-tested vertically. The He-vessel was welded onto one of the resonators which passed the horizontal RF-test. The data and perspectives of the LG cavity application are discussed.
Fermilab, Batavia
Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359
The Fermilab High Intensity Neutrino Source (HINS) Linac R&D program is building a 60 MeV superconducting H- linac. The Linac incorporates SC solenoids, high power RF vector modulators and SC spoke-type accelerating cavities starting at 10 MeV. This will be the first application and demonstration of any of these technologies in a low-energy, high-intensity proton/H- linac. The HINS effort is relevant to a high intensity, SC H- linac that might serve the next generation of neutrino physics and muon storage ring/collider experiments. Three types of superconducting resonators are used in the linac front end. Single Spoke Resonators typs-1 (SSR1) at Beta=0.2, Single Spoke Resonators type-2 (SSR2) at Beta=0.4 and Triple Spoke Resonators (TSR) at Beta=0.6. In this paper we describe the Buffer Chemical Polishing (BCP) performed on SSR1-#2 and the results of the cold tests for this bare cavity. We also describe the inelastic tune performed on cavity SSR1-#1, during this operation we measured also the spring constant and the frequency sensitivity of the end walls. We have also completed the design for the helium vessel that will be used to jacket SSR1 resonators and we present its design here.
Fermilab, Batavia
Euclid TechLabs, LLC, Solon, Ohio
KEK, Ibaraki
The accelerating gradient required for the ILC project exceeds 30 MeV/m. With current technology the maximum acceleration gradient in SC structures is determined mainly by the value of the surface RF magnetic field. In order to increase the gradient, the RF magnetic field is distributed homogeneously over the cavity surface (low-loss structure), and coupling to the beam is improved by introducing aperture "noses" (re-entrant structure). These features allow gradients in excess of 50 MeV/m to be obtained for a singe-cell cavity. Further improvement of the coupling to the beam may be achieved by using a TW SC structure with small phase advance per cell. We have demonstrated that an additional gradient increase by up to 46% may be possible if a pi/2 TW SC structure is employed. However, a TW SC structure requires a SC feedback waveguide to return the few GW of circulating RF power from the structure output back to the structure input. Advantages and limitations of different techniques of exciting the traveling wave in this structure are considered, including an analysis of mechanical tolerances. We also report on investigations of transient processes in the SC TW structure.
KEK, Ibaraki
Kyoto ICR, Uji, Kyoto
The inspections of the superconducting RF cavities seem essential in achieving high accelerating gradient. The Kyoto camera system is a good tool to survey a defect location and to be analysis a defect shape in the inner surface of the superconducting rf cavities. The cavity inspections of the AES, ACCEL, ZANON and STF Baseline cavities were inspected to study relations between a defect shape and a heating gradient of the superconducting rf cavities. The STF Baseline #5 and #6 cavities with each surface treatment (as received, after Pre-EP, after EP-1, and after vertical test with EP-2) were inspected to trace a changing spots shape. The full inspection of the EBW seam, the HAZ (heat affected zone) and hot spots region were carried out before EP-2 process and a vertical test then the shape analysis of a discovered spots was done. The vertical tests of these cavities with T-map of fixed 9-cell type were measured at STF from September 2008. The inspection and shape analysis of these cavities were made after vertical tests for based on T-map data. The result of vertical tests and changing a shape of a discovered spots with EP-2 process will be presented.
HRC, Santa Clara, California
Funding: Work performed under the auspices of the U.S. Department of Energy SBIR Grant No. DE-FG02-08ER85197.
The salient features and design parameters of a dual resonant ring system configured for evaluating the high gradient performance of 11.424 GHz TW linear accelerator structures are presented; and the inherent rapid protection mechanism that automatically limits energy deposition during breakdown of the structure, and minimizes RF source reflections, is discussed. The diagnostic characteristics of the RF bridge load monitors and their unique capability of detecting the power imbalance caused by a feedback loop phase change of less than 2 parts in 10000, representing a 2 to 3 degree phase change of the linac structure, is described. The transient and steady-state power apportionment within the ring system is analyzed; and, in considering initial high power tests using an 18-cavity CLIC/KEK/SLAC structure, the results indicate that the demonstration of an unloaded average accelerating gradient of 108 MV/m will require a source power of 26 MW.
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
ALICE (Accelerators and Lasers in Combined Experiments) incorporates two super-conducting radio frequency (SCRF) cryomodules each with two identical 9-cell cavities that are powered by 5 inductive output tubes (IOTs) from 3 different commercial suppliers. During the commissioning of the ALICE rf system numerous problems were encountered with the operation of the high voltage power supply and the auxiliary power supplies, which had to be resolved before the beam commissioning of the accelerator could commence. The issues encountered and measures taken to improve the operation of the rf system are described within this paper.
ORNL, Oak Ridge, Tennessee
University of Tennessee, Knoxville, Tennessee
MIT Lincoln Laboratory, Boston MA
Funding: ORNL/SNS is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.
Investigations of the rf properties of certain twisted waveguide structures show that they support favorable accelerating fields. This makes them potential candidates for accelerating cavities. Using the particle tracking code, ORBIT, We examine the beam - rf interaction in the twisted cavity structures to understand their beam transport and acceleration properties. The results will show the distinctive properties of these new structures for particle transport and acceleration, which have not been previously analyzed.
LANL, Los Alamos, New Mexico
ORNL, Oak Ridge, Tennessee
SLAC, Menlo Park, California
Penn State University, University Park, Pennsylvania
Funding: DTRA
We are currently testing the effect of coating Nb with a thin layer of another superconductor such as NbN and MgB2. Gurevich’s theory of multi-layered coating predicts an enhancement of the critical magnetic field, giving us hope to increase the achievable accelerating gradient to above 50 MV/m in elliptical cavities. CW test results of 3 GHz Nb single-cell cavities coated with ~100 nm NbN at LANL and 11.4 GHz <1 μs high-power pulsed test results of 2” Nb disk samples coated with ~100 nm MgB2 will be presented.
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.
ANL, Argonne
Euclid TechLabs, LLC, Solon, Ohio
Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-06CH11357.
The Argonne Wakefield Accelerator Facility is dedicated to the study of advanced accelerator concepts based on electron beam driven wakefield acceleration and RF power generation. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures as well as in metallic structures (iris loaded, photonic band gap, etc). Accelerating gradients as high as 100 MV/m have been reached in dielectric loaded structures, and RF pulses of up to 44 MW have been generated at 7.8 GHz. In order to reach higher accelerating gradients, and also be able to generate higher RF power levels, a photocathode with higher quantum efficiency is needed. Therefore, a new RF gun with a Cesium Telluride photocathode will replace the electron gun that has been used to generate the drive bunches. In addition to this, a new L-band klystron will be added to the facility, increasing the beam energy from 15 MeV to 23 MeV, and thus increasing the total power in the drive beam to a few GW. The goal of future experiments is to reach accelerating gradients of several hundred MV/m and to extract RF pulses with GW power level.
IAP/RAS, Nizhny Novgorod
A new two-beam accelerating structure based on periodic chain of rectangular shape multi-mode cavities was suggested recently*. The structure is aimed to increase threshold breakdown surface field and thus to provide a high gradient. This threshold increase is to be brought about by designing cavities of the structure to operate simultaneously in several harmonically-related TMn,n,0 modes, thereby reducing the effective exposure time of the cavity surface to the peak fields. The more number of the operating modes is the more reduction of the exposure time. Unfortunately, a big amount of modes leads to limitation for cavity length and practical limitation of filling factor. In order to avoid this, it is suggested to operate with several TMn,n,l modes with non-zero longitudinal indices. These modes are able to provide the long interaction of a moving bunch with RF fields along the cavity. Such regime requires for the longitudinal index l to be strictly proportional the mode frequency. A cylindrical shape cavity design is also considered.
*S.V. Kuzikov, S.Yu. Kazakov, M.E. Plotkin, J.L. Hirshfield, High-Gradient Multi-Mode Two-Beam Accelerating Structure, Proc. of EPAC’08 Conf., Genoa, June 23-27, 2008, WEPP133.
NRL, Washington, DC
ANL, Argonne
Euclid TechLabs, LLC, Solon, Ohio
Icarus Research, Inc., Bethesda, Maryland
Funding: Department of Energy, Office of Naval Research, and DoE SBIR Phase II grant DE-FG02-06ER84463
Dielectric-loaded accelerating (DLA) structures, in which a dielectric liner is placed inside a cylindrical metal tube, offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. A joint Naval Research Laboratory/Euclid Techlabs/Argonne National Laboratory study is under way to investigate the performance of X-band DLA structures using high-power 11.43-GHz radiation from the NRL Magnicon Facility*. The initial goal of the program has been to develop structures capable of sustaining high accelerating gradients. The two significant limitations that have been discovered relate to multipactor loading of the structures and rf breakdown at joints between ceramic sections. We will report the results of several recent structure tests that have demonstrated significant progress in addressing both of these issues. The longer-range goal of the program is to study electron acceleration in DLA structures. For this purpose, we are developing an X-band DLA test accelerator. We will also report the results of initial operation of a 5-MeV injector for the new accelerator.
*C. Jing, W. Gai, J. Power, R. Konecny, S. Gold, W. Liu and A. Kinkead, IEEE Trans. Plasma Sci., vol. 33, pp.1155-1160, August 2005.
MIT/PSFC, Cambridge, Massachusetts
SLAC, Menlo Park, California
Funding: Work supported by DoE HEP, under contracts DE-FG02-91ER40648 and DE-AC02-76-SF00515
In order to understand the performance of photonic bandgap (PBG) structures under realistic high gradient operation, an X-band (11.424 GHz) PBG structure was designed for high power testing in a standing wave breakdown experiment at SLAC. The PBG structure was hot tested to gather breakdown statistics, and achieved an accelerating gradient of 65 MV/m at a breakdown rate of two breakdowns per hour at 60 Hz, and accelerating gradients above 110 MV/m at higher breakdown rates, for a total pulse length of 320 ns. High pulsed heating occurred in the PBG structure, with many shots above 270K, and an average of 170K for 35 x 106 shots. Damage was observed in scanning electron microscope imaging. No breakdown damage was observed on the iris surface, the location of peak electric field, but pulsed heating damage was observed on the inner rods, the location of magnetic fields as high as 1 MA/m. Breakdown in accelerator structures is generally understood in terms of electric field effects. PBG structure results highlight the unexpected role of magnetic fields on breakdown. We think that relatively low electric field in combination with high magnetic field on the rod surface may trigger breakdowns.
USTC/NSRL, Hefei, Anhui
USTC, Hefei, Anhui
Photonic crystals have been suggested for use as laser driven particle accelerator structures with higher accelerating gradients and effective damping of unwanted higher order modes. Here we selected Photonic band gap (PBG) fibers with hollow core defects to design such an accelerating structure. To achieve this design, Out-plane-wave mode in photonic crystal fiber was selected for longitudinal electric field. The out-plane-wave plane wave expansion method was deduced for confinement and the dispersive curve versus variation of kz and speed of line for synchronization. Then super cell approximation was also introduced for calculating the defected photonic crystal structure. After the design of appropriate geometry and the dimensions of photonic crystal fiber accelerating structure, the field distribution was simulated with RSOFT Bandsolve software for this structure.