The XFEL and TTF facilities are planning to use section with a few third harmonic cavities (3.9GHz) upstream of the bunch compressor to improve beam performance [1-2]. Fermilab is developing superconducting third harmonic section for the TTFII upgrade. This section will include four cavities equipped with couplers and blade tuners installed in cryostat. Up to now, two cavities are complete and one of them is under test. The status of the cavity development and preliminary test results are presented in this paper

Edwards, Helen

Foley, Mike

Gonin, Ivan

Harms, Elvin

Khabiboulline, Timergali

Mitchell, Donald

Olis, Daniel

Rowe, Allan

Solyak, Nikolay

UNT Digital Library

Fermilab FERMILAB-PUB-06-196-TDTHE PROGRESS IN DEVELOPING SUPERCONDUCTING THIRD HARMONIC CAVITY Nikolay Solyak, Helen Edwards, Mike Foley, Ivan Gonin, Elvin Harms, Timergali  Khabiboulline, Donald Mitchell, Daniel Olis, Allan Rowe. FNAL, Batavia, IL 60510, U.S.A.Abstract The XFEL and TTF facilities are planning to use section with a few third harmonic cavities (3.9GHz) upstream of the bunch compressor to improve  beam performance [1 -2]. Fermilab is developing superconducting third harmonic section  for  the  TTFII upgrade. This section will inclu de four cavities equiped with couplers and blade tuners installed in cryostat. Up to now, two cavities are complete and one of them is under test. The status of the cavity development and preliminary test results are presented in this paper. INTRODUCTION The 3 rd harmonic 3.9 GHz cavity was proposed to  linearize energy distribution along the bunch before the bunch compressor [2 ]. These cavities operate in the TM010 mode and will be located downstream of the 1.3 GHz TESLA cavities.  The required o perating gradient is 14 MV/m [3 -4].  Fermilab has agreed to provide DESY with a cryomodule containing a string of four of the se cavities.  In addition, a second cryomodule  with one cavity will be fabricated for installation in  the Fermilab photo-injector, which will be upgraded for the ILC accelerator test facility.  In the scope  of this project Fermilab is developing cavities, couplers, blade tuners and cryostat [ 4-8]. The construction and successful test results of copper and  niobium cavity prototypes, helium vessels and blade tuners allowed us to start cavity production after several minor modifications in design . The status of the production and the first results of the cold test are discussed below.  STATUS OF CAVITY DEVELOPMENT Cavities The overall objective is to build eight 3 rd harmonic cavities.  The first cavity, a prototype design, was completed in December 2005.  Lessons learned suggested several minor design changes that were incorporated into the remaining seven cavit ies.  Fabrication of the cavities is a collaborative effort.  Fermilab will build four cavities (including the prototype), and JLAB  will build the remaining four cavities (using parts supplied by Fermilab).    Figure 1: Cavity No.2 before etching Cavity N o. 2 was completed at Fermilab in January 2006.  It is currently under test.  Parts for the remaining six cavities are complete, and fabrication is in progress.  Cavities No. 3-6 are approximately 80% welded at JLAB , while cavities No. 7 -8 are approximatel y 40% welded at Fermilab.  It is expected that cavities No. 3 -6 will be completed by late June or early July.  Cavities No. 7 -8 are scheduled for completion by late August.  Main Coupler Delivery of the first pair of 3.9GHz power input couplers is expected  June 23, 2006  (fig.2) .  Upon receiving, the couplers will be mechanically measured, leak checked, tested, and processed.   The station for coupler processing and te st is assembled and ready.   To reduce design complexity, the 3.9GHz couplers do not include a tuning mechanism for adjustment of antenna coupling to the cavity.   As procured, the antenna length on the coupler's cold end assembly is 2mm longer than required by simulations to accommodate possible simulation errors and fabrication tolerances.   As pa rt of testing, antenna coupling to the cavity will be measured.   Antennas will then be trimmed to final length using wire EDM.  This coupling test will take place at ambient temperature and pressure and will use much of the same hardware as designed for the coupler processing station.  Figure 2: Coupler for 3.9GHz cavity. Cryomodule  The design of the cryomodule has been completed  (Figure 3) . Components for the cryovessel, helium vessels, bladetuners, heat and magnetic shields, and cold mass supports have been ordered with delivery beginning in the summer of 2006. We are currently following up on the bid process for many of these main components and implementing some small design modifications as needed. DESY engineers will be designing and procurin g the RF and vacuum systems.  Assembly tooling for the cavity string and the cold mass is well underway. Many of the fixtures are common to the 1.3 GHz cryomodule which are currently being assembled and installed at Fermilab. Others, like cavity support po sts, will be slightly modified for the 3.9 GHz cavities.  Figure 3: Layout of the cryostat. The cavity string will be assembled at Fermilab but the cryomodule will be pre -assembled to check the fit of all components, disassembled, and then shipped to DESY  for final assembly. Ultimately, the  cryomodule will be installed in DESY’s TTF in 2007. COLD TEST RESULTS  Test of the 3-cell cavity.  The first niobium 3 -cell prototype was built to test cavity performances and develop tooling and technology.  There are  no  HOM or main coupler s on this  cavity. The first results of th e cold tests were reported in [5 -6].  A brief test history is shown in Table 1.  Table.1: History of cold tests of the 3-cell cavity BCP; HT; HPWR Test results No treatment R=2000nΩ, Ea=5.6 MV/m BCP-140   m; 10hrs@600C;  HPR:30’ R=60nΩ, Ea=11.5 MV/m; X-ray  BCP-30   m; HPR: 3x30 R=70nΩ;  E=12.5 MV/m; X-ray HPWR:1hrs, movable R=60nΩ;E=12.8MV/m;NoX-ray  BCP:20µm;HPR:7x90’  E=15MV/m - coupler problem BCP:5   m; HPR~2hrs  R=6 nΩ; Ea=19MV/m; NoX-ray  Bake: 48hrs@120C R=16 nΩ, E=19MV/m; NoX-ray  BCP:10   m;HPR:2.5hrs R=58 nΩ; Ea=12MV/m,  X-ray HPR:6.5 hrs, movable R=15nΩ;Ea~19MV/m; NoX-ray  Initially the cavity was etched ~140  m inside (measured by etching rate, not frequency) and 20  m outside and then treated 10hrs at 600ºC.  The first test showed a  field emission problem, which was fixed after appropriate high pressure rinsing. The b est achieved residual surface resistance was 6 nΩ (measured at 1.4 °K). Typical resistance at 1.8 °K of ~50 nΩ  corresponds Q0~6.e+9. The c avity was quenched at accelerating gradient of ~19 MV/m (surface peak magnetic field o f ~105mT) and this threshold can be explained by thermal breakdown model [ 9]. The 9 -cell cavity will have a gradient of ~21MV/m for the same surface magnetic field achieved in the 3-cell cavity. This is well above design gradient of 14MV/m. We are planning to use this cavity as a reference cavity to check performances of the buffered chemical polishing  ( BCP) and high pressure water rinsing (HPWR) facilities.  Test results of the 9-cell cavity  The first  9-cell Nb cavity  (prototype)  failed during the deep BCP. After ~130µm etch a hole developed in one of the HOMs  coupler s, which has thinner wall thickness than in the final design.  Cavity No. 2 was successfully manufactured, tuned and tested after BCP, high temperature treatment (10hrs at 600°K) and HPWR.  Initial measurements show ed a very high residual resistance Rres=3000 nΩ. After operating with high fields in “pi” and “0” modes, the residual resistance dropped off ~5 times to ~ 600 nΩ. After processing at high field level during the rest of the  day, the next day showed a residual resistance of ~80 n Ω. Processing was done at pulsed power 120  W and different duty factor from  1% to CW. Additional high field processing for 2 more days improved residual resistance to 40 n Ω. The h istory of RvsT measurements is shown in figure 4.  In the cavity tests we observed strong multipactor ing which causes Q -slope at small field le vel. After high fields “processing” Q improved for small fields and the multipacting level shifted from ~0.5 MV/m acceler ating gradient to ~ 2.5 MV/m.  In pulsed regime accelerating gradient was ~8-11 MV/m, limited by available RF power due to low Q . Some DC cur rent was observed from pick -up antenna at the gradient higher than 10 MV/m (fig. 5).  Figure 4: Resistance vs. Tc/T.  Measurements were done in a few cold tests.  Figure 5: History of the QvsE measurements  “0”-mode measurements shows fairly high Q up to the quench level (Figure 6 ). Fields in the end cells  of the cavity are m uch lower in this mode. The result clearly  indicates that the problem is located in the cavity end s, not in the cells. Also some x-rays were observed in higher field operation.  A temperature sensor installed in one of the HOM couplers showed  heating during the  pulse.  In CW operation with a power level  of  ~100W the temperature at the sensor increased ~20K in one minute.   Preliminary 3D multipacting (MP) calculations show possible problem in the HOM coupler in the range of ~10 mT surface fields at the  in ner conductor ( Figure 7). MP activity is very sensitive to the shape of the conductor, for example, the increasing of the gap between the conductor and the HOM body will eliminate MP in the full of design fields range. T his modification can hopefully be done without rebuilding the HOM coupler.  Blade tuner design and tests A b lade tuner with a copper 9 -cell full size prototype cavity was tested at room temperature . The m easured sensitivity was ~2.5 MHz/mm [7]. This test was  repeated with the blade tuner submerged in 4.2K liquid helium. The goal of this test was to measure tuner performances together with  a  9 -cell cavity  in a cold environment . Because of helium  inside the cavity , the frequency  spectrum was lowered by 75 MHz and sensitivity of ~ 2 MHz/mm was measured. Due to fluctuations in  h elium pressure the frequency measurements were noisy . To reduce noise the major measurements were done with temporarily no h elium supply to the d ewar. A total of 30 full cycles o f compressing and stretching w ere done. T he tuner was carefully inspected after warm -up and no evidence of  damage or  cracks were found. Since during this test  the stepping  motor and gear system were submerged into liquid helium, we are going repeat this test in a horizontal test station under insulating vacuum. ACKNOWLEDGEMENTS    The authors thank Pierre.Bauer, Cristian.Boffo, Charlie Cooper, Tug Arkan, Harry Carter and  Eugueny Borissov for help and useful discussions and  John DeFord for providing code [10] and help with MP simulations.  REFERENCES [1]http://xfel.desy.de/e154/upload/upload_file/Docs/XFEL_TDR-Draft_2006-05-15.pdf [2] K.Floettmann et al., TESLA-FEL-01-06.  [3] J.Sekutowicz, et al., TESLA-FEL, 2002-05. [4] N.Solyak, et.al, Development of the third harmonic SC cavity at Fermilab, PAC’03, 2003, p1213.. [5] N.Solyak, et all, “Test results of the 3.9 GHz cavity at Fermilab” Linac’04, Luebeck, 2004, p. 797. [6] N.Solyak et.al.,”First Results of Testing 3.9 GHz Superconducting Cavity”, ASC’04, Jacksonville, 2004. [7] N.Solyak et.al.,”Development of the SC 3.9 GHz accelerating cavity at Fermilab”, PAC’05, Knoxville, p.3825. [8] http://www-td.fnal.gov/lc/sc/lc.html  [9] P.Bauer et al., “Evidence for non-linear BCS resistance in SRF Cavities”,  SRF’05, Ithaca, NY, July 2006 [10] ”, Simulation Technology & Applied Research, Inc, DOE SBIR Phase 2 Proposal: “A 3D Finite-Element Modeling Tool for Multipacting Analysis”   Figure 7: Multipacting 3D simulations.    Figure 6: Q and surface magnetic field measured in “0”-mode.   Figure 8: Blade Tuner Cold test results. 

The progress in developing superconducting third harmonic cavity

https://digital.library.unt.edu/ark:/67531/metadc889800/m2/1/high_res_d/901113.pdf

The progress in developing superconducting third harmonic cavity

Abstract

Similar works

Full text

Available Versions

UNT Digital Library