Akai, K

Ezura, E

Furuya, T

Hosoyama, K

Mitsunobu, S

Tajima, T

CERN Document Server

THE SUPERCONDUCTING CAVITY SYSTEM FOR KEKBT. Tajima1, K. Akai, E. Ezura, T. Furuya, K. Hosoyama, S. MitsunobuKEK, Tsukuba, Ibaraki, 305-0801, Japan1 Email:  tsuyoshi.tajima@kek.jpAbstractThe superconducting cavity (SCC) for KEKB is 508 MHzsingle-cell cavity that has large beam pipes (22 cm and 30cm i.d.) so that higher-order modes propagate out of thecavity and be absorbed by a lossy material. The inputcoupler is the TRISTAN-type coaxial one with somemodifications such that dc bias voltage can be applied toavoid multipactoring during beam operation, fins toefficiently cool the outer conductor and a heater to removecondensed gases.  The higher-order mode absorber is madeof ferrite directly sinter-bonded on the inner surface of thecopper pipe using a technique called Hot Isostatic Press(HIP). One prototype cavity was tested up to 0.57 A atTRISTAN Accumulation Ring (AR) in 1996. Then, fourcavities were constructed for KEKB. One of the cavitiesachieved an accelerating field of 19 MV/m at a test in avertical cryostat; this field is the world record at thisfrequency to our knowledge. No degradation of the fieldafter assembly into horizontal cryostats was observed upto the available power of 300 kW that corresponds to ~12MV/m. These four cavities were installed in KEKBtunnel and are expected to supply 6 MV in total voltage tothe 1.1 A electron beam in high energy ring (HER).Since beam commissioning started in Dec. 1998, thesystem has been supplying 6 MV and working verysmoothly without any trouble.  The maximum current hasbeen ~ 240 mA  and power delivered to beam per cavity is~200 kW up to the end of Feb., 1999.1  INTRODUCTIONBased on the successful beam tests at TRISTANAccumulation Ring (AR) in 1996 [1-4], we startedconstruction of four superconducting cavity modules forKEKB HER from the beginning of 1997. This paperdescribes its construction, performance of the cavities andsome of the first data on the beam operation.2  CONSTRUCTIONTable 1 gives the parameters of the cavity module.  Figure1 shows the whole module. It has two 300 liter/s ionpumps connected to the end cones and the diameter of theend gate valves is 15 cm.  Figure 2 is a picture of the fourmodules installed in the Nikko D11 tunnel of KEKB.Each module was installed between two quadrupolemagnets. We named the cavities RA, RB, RC and RDafter the names of Klystrons from left to right in thetunnel.Table 1: Parameters of SCC moduleFrequency 508.887 MHzGap length 243 mmR/Q 93 ΩGeometrical factor 251 ΩEsp / Eacc 1.84Hsp/Eacc 40.3 Gauss/(MV/m)Loaded Q 6-8 x 104Cryostat LHe volume 290 literHOM Absorber (SBP)HOM Absorber (LBP)Waveguide , input couplerLHe Transfer LineLHeTotal length: 3.7 mTuner300 L/s Ion pump 300 L/s Ion pumpFigure 1: Superconducting cavity module for KEKBFigure 2: A picture of four SCC modules in Nikko-D11tunnel0-7803-5573-3/99/$10.00@1999 IEEE. 440Proceedings of the 1999 Particle Accelerator Conference, New York, 19992.1  Fabrication of CavitiesThe cavities are made of 2.5 mm thick niobium withRRR being ~200. Cavity cells were spun from sheetmaterial and electron beam welded. The procedure ofsurface treatment was as follows: 1) electropolishing 80µm, 2) degassing at 700˚C for 1.5 hours and 3)electropolishing 15 µm and rinsing with 3-4 ppmozonized water.2.2  Assembly with Cryostat, etc.After cavity-alone tests in a vertical cryostat, cavities werechipped-off at flange valves and transported to a class 100clean room, vented with filtered nitrogen gas andassembled with extension pipes made of stainless steel atboth ends.  Then, they were assembled with cryostats, andfinally end cones that had been pre-assembled withdampers and gate valves were mounted at both ends.3  PERFORMANCE OF THECAVITIESFigure 3 shows unloaded quality factor, Q0, of the fourcavities as a function of accelerating field. In the figures,open circles are the results of cavity themselves measuredin a vertical cryostat. Open squares are the results ofbench tests after fully equipped with horizontal cryostatand other parts before installation in the tunnel. Solidtriangles and solid circles are the results measured in thetunnel on Nov. 27, 1998, before first operation and onJan. 13, 1999, before second operation.Although the designed voltage for operation is 1.5 MVor 6 MV/m per cavity, we set our target at 10 MV/m andQ0 ≥ 1x109 to have enough margin for stable operationand in case of the operation with fewer modules. CavityRA could not reach this target due to a defect on theequator. After the defect was ground off, the cavityshowed nearly 10 MV/m.  Cavity RB surpassed our recordon vertical tests (15 MV/m) [5], reaching 19 MV/m.Using the numbers given in Table 1, one can obtainsurface peak field of 35 MV/m and magnetic field of 750Gauss.  To our knowledge, this is the highest value everreached with 500 MHz range cavities at 4.2 K. CavityRC degraded during first vertical test due possibly to somedamage created by discharge.As to the results after full assembly and installation,the remarkable point is that very little degradation fromcavity-alone tests occurred compared to the results ofTRISTAN cavities [6]. Note that the highest fields arenot marked in Fig. 3 since it is difficult to measure Q0.Cavity RA got even better in Q0 for some reason. As forcavity RB, we did not try to go much higher because thefield is more than enough for operation and due toavailable power.10810910100 2 4 6 8 10 12 14Cavity single testInitial aft. fully-equippedNov. 27, 1998Jan. 13, 1999Eacc (MV/m)RAQ0O.P.10810910100 2 4 6 8 10 12 14 16Eacc (MV/m)Q0RCO.P.1st test10810910100 2 4 6 8 10 12 14Eacc (MV/m)Q0RDO.P.Figure 3: Unloaded Q vs. accelerating field of all thecavities.  O.P. stands for operation point.4  BEAM OPERATION4.1 ConditioningBefore cooling down the cavities, we condition the inputcoupler up to 300 kW with perfect reflection condition,and up to 200 kW with dc bias voltages applied on the10810910100 5 10 15 20Eacc (MV/m)Q0RBO.P.441Proceedings of the 1999 Particle Accelerator Conference, New York, 1999inner conductor up to ± 2 kV. Normally, it takes oneday with one operator for each module.After the LHe vessel is filled up to 90 %, we try toraise coupler power up to 300 kW in off-resonancecondition, and raise the cavity voltage, Vc, up to about 3MV that corresponds to 12 MV/m or till breakdownoccurs.  Then, we shift phase of cavity up to ± 30˚ so thatthe field profile in the coupler changes to condition theless conditioned parts of the coupler. When we see somedegradation on the attainable field, we apply pulsed powerconditioning. It takes only a few hours at most torecover. This also takes about one day.Finally, we measure Q0 at certain cavity voltages, e.g.at 2.0, 2.5 MV, with LHe consumption rate. This takesabout 6 hours, depending on how many data we want totake.Table 2 summarizes the present status of the modules.Table 2 : Present performance of the modulesMar. 18-19, 1999Cavity RA RB RC RDQ0 @ 2MV (x109) 2.1 2.0 1.3 1.4Eacc,max (MV/m) 12.3 13.0 10.7 11.3Static loss (W) 31 771) 32 321)Due to insufficient insulation vacuum caused by leak4.2 VacuumFigure 4 shows the time evolution of pressure in thecavities and at the adjacent beam ducts. Considering theimportance of making the amount of gas flowing into thecavity as small as possible for stable operation, weinstalled five 400 liter/s NEG pumps and one 300 liter/sion pump on the duct between each module. The effectivepumping speed at the neighboring ducts is ~77 liter/s·m[7]. As one can see in Fig. 4, the base pressure of theducts is normally 0.6 to 0.7 nTorr at present.  It is certainthat this good vacuum is contributing to the stableoperation.Figure 5 shows the pressure increase with current. Thepressure goes up linearly with current. Cavity RD isshowing relatively higher pressure as seen in Fig. 5 forsome reason, although the slope is same as others. Theextrapolated pressure at 1.1 A is 5.7-6.8 nTorr. Comparedwith the last test in TRISTAN AR where the pressurewent up to ~10 nTorr with 400 mA, this lookspromising. However, taking into account that the beamloss will be much higher with shorter bunch length andpressure might increase non-linearly, we should keepwatching it very carefully and it will be important to findappropriate indicators on the condition of trips in terms ofpressure or amount of gas condensed on the cold surface,although it is of course desirable if there will be few tripsin the future as it is now.050100150200250Beam current (mA)Feb. 26, 1999  3:00 - 18:000.1110Cavity P (nTorr)0.1110TimeDuct P (nTorr)3:00 6:00 12:00 15:009:00 18:00Figure 4 : Typical trend of pressure during beam operation00.511.522.50 50 100 150 200 250Beam current (mA)K99030602-5P (nTorr)Data on Feb. 26, 1999Cavity RDFigure 5 : Pressure at SCC versus beam current.4.3 HOM DampersHOM dampers are made of ferrite [8] and tested up to 5kW and 7 kW for SBP and LBP, respectively, with a 508MHz coaxial line, then baked at 150˚C for about onemonth to reduce outgassing rate. The expected rate afterbaking is less than 1.5 x 10-11 Torr·liter/s·cm2 at roomtemperature, which corresponds to a total outgassing rateof the dampers being 5.2 x 10-8 Torr·liter/s.442Proceedings of the 1999 Particle Accelerator Conference, New York, 1999The damper has 3/8" or 1/2" o.d. copper cooling pipepress-inserted on the outer layer made of oxygen freecopper.  Each cavity module has its own cooling unit fordampers and its capacity is 11.6 kW at 20˚C. The normaloperating condition is 5 liters/min at 23˚C. The expectedtotal power to be absorbed at 1.1 A with 5000 bunches isabout 5 kW.Figure 6 shows the power calculated from the differenceof inlet and outlet water temperature and the flow rate as afunction of beam current. This data was taken when thebeam is coasting in order to minimize the inaccuracy dueto delay of thermal response compared to the change ofcurrent. The fluctuation of the signal is caused by thefluctuation of inlet temperature to regulate thetemperature.  It relatively widens at lower temperature dueto small temperature difference between inlet and outletwater. If one takes the lower envelope of each curve, hecan obtain correct values since power should not benegative at zero current. All the data are shifted so thatthe power becomes zero with no current.This data was taken with 8 trains of 40 bunches. Thebunch separation was 5 RF buckets. If the loss factor isconstant, i.e. bunch length does not change, and thenumber of bunches is unchanged during the coasting,power should increase quadratically with current as shownin the formula below. One can see this dependence inFig. 6.Figure 7 shows the loss factor, k, calculated with thefollowing formula and the measured power, Ploss , togetherwith a prediction drawn by a solid line.k [V/pC] =P kW] N f kHz]I mA]       (1)loss b rb[ [[⋅ ⋅{ }2where Nb and fr are the number of bunches and therevolution frequency, respectively. The error bar of themeasured value corresponds to the distribution of all thefour modules.  As seen in Fig. 7, the measured data wereslightly lower than the calculation at bunch length of 6mm.  Whether it will stay lower than the calculation orwill go higher than that is not clear yet, although theresults of a beam test carried out at TRISTAN showed thatit tends to become higher than calculation with shorterbunches [9].00.20.40.60.810 50 100 150 200 250Beam current (mA)Power absorbed (kW)K99032001-1SBP00.20.40.60.810 50 100 150 200 250Beam current (mA)Power absorbed (kW)K99032001-2LBPFigure 6 : Power absorbed at the dampers S and L versusbeam current. Data of 4 modules are put in the samefigure.  Lower envelopes of the curves give correct values.Fluctuation is due to the temperature of inlet water beingregulated.00.511.520 5 10 15 20 25 30Loss factor (V/pC)Bunch length (mm)K99030601-2Figure 7 : Loss factor versus bunch length.  The solid lineis the sum of ABCI calculation for the cavity structurewith cones without absorber and analytic calculation [10]for the absorber. Error bar denotes the difference betweenmodules.443Proceedings of the 1999 Particle Accelerator Conference, New York, 19994.4  Cryogenic SystemWe are reusing the cryogenic system constructed forTRISTAN [11] except that new transfer lines weredeveloped to reduce heat loss and to add more flexibility inconnecting with cavity modules. The system has beenrunning smoothly, thanks to the skilled operators.4.5  RF SystemBased on the successful test at the TRISTAN AR [4],direct RF feedback has been implemented [12] and used,contributing to stable operation.4.6  TripsSo far, up to the end of Feb., 1999, there have been onlytwo trips since the commissioning started in Dec. 1998.These trips were caused by breakdown of a cavity but wecould not identify the cause because it did not repeat. Inour past experiences, trips are strongly related to thecondensed gases on the input coupler or cavity near thecoupler. Therefore, we will be watching the behavior ofpressures at the cavity and at the adjacent beam ducts.5 SCHEDULE FOR NEXT MODULESWe are planning to install another four modules toincrease voltage and beam current to achieve 1.1 A. Table3 shows a tentative schedule for the construction of thenext four cavity modules. In our present plan, fourcavities will be installed in the tunnel in August, 2000.Table 3 : Schedule for the next 4 modulesActivity Name1 2 3 4 5 6 7 82000Manufacture of 4 cavities and cryostatsBench tests with vertical cryostat Full assembly with other partsBench tests with horizontal cryostat Installation in the tunnelLHe transfer line install.6  SUMMARYWe finished construction and installation of foursuperconducting cavity modules for KEKB-HER by thefall of 1998. Since the beam commissioning started inDec. 1998, all the modules have been operated at 1.5MV(6 MV/m) each or 6 MV in total. Up to the end ofFeb., 1999, maximum current is about 240 mA andmaximum power delivered to beam is about 200 kW percavity.The system has been running very smoothly with veryfew trips so far. As beam current gets higher and bunchlength gets shorter, some dedicated studies such as beamrelated loss and HOM searches will be planned.7  ACKNOWLEDGMENTThe authors are indebted to K.Kanazawa, Y.Suetsugu andtheir co-workers for design and installation of adjacentvacuum ducts and components, H.Nakanishi forwaveguide architecture, S.Isagawa, S.Yoshimoto and theirco-workers for maintaining Klystrons, K.Hara, A.Kabe,Y.Kojima, Y.Morita, H.Nakai and their co-workers forconstructing cryogenic system and its maintenance. Theyalso would like to thank Y.Kijima, T.Murai, S.Takagiand their co-workers of Mitsubishi Electric Company fortheir devotion to the construction. Finally, they aregrateful to Professors S.Kurokawa, K.Takata, Y.Yamazakiand M. Kihara for their continuous encouragement.8  REFERENCES[1] T. Furuya et al.; Proc. PAC97, Vancouver, B.C.. Canada, May 12-16, 1997. p. 3087[2] T.Tajima et al.; ibid p. 3092[3] S. Mitsunobu et al.; ibid p. 2908[4] K.Akai et al.; 11th symposium on Accelerator Tachnology andScience, Hyogo, Japan, Oct. 21-23, 1997. KEK Preprint 97-137.[5] T.Tajima et al.; 5th Workshop on RF Superconductivity, DESY,Hamburg, Germany, August 19-24,1991. KEK Preprint 91-193[6] S.Noguchi et al.; EPAC '94 London, UK ; 27 Jun - 1 Jul 1994. KEKPreprint 94-43.[7] Y.Suetsugu: private communication.[8] T.Tajima et al.; Proc. PAC95 p. 1620[9] T.Tajima et al.; KEK Report  96-7 (1996).[10] Code for analytic calculation was written by N.Akasaka.[11] K.Hosoyama et al.; Proc. 4th Workshop on RF Superconductivity,KEK, Tsukuba, Japan, Aug. 14-18, 1989, p. 755, KEK Report 89-21.[12] K.Akai et al.; EPAC98, Stockholm, June 22-26, 1998. KEKPreprint 98-82.444Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

The Superconducting Cavity System For Kekb

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