Yoshida, T

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188JAERI TANDEM BOOSTER AND ITS CRYOGENIC SYSTEMT. YoshidaJapan Atomic Energy Research Institute. Tokai Research Establishment2-4 Shirakata shirane Tokai-mura Naka-gun Ibaraki-ken 319-1195. JapanAbstractThe JAERI tandem booster linac has been operated for the variousexperimental programs since 1994. During this seven years, the booster linacworked steadily without serious trouble. However, there are some smallproblems on the cryogenic system. This paper describes the operationalissues on the booster linac and its cryogenic system.1.  INTRODUCTIONThe JAERI tandem accelerator facility has a large tandem accelerator and a superconducting booster.The tandem accelerator, of which maximum performance in terminal voltage was 20 MV, wasmanufactured by National Electrostatics Corporation (NEC) in U.S.A. and started operations for theexperiments in 1982. In 1994, the superconducting booster was built for increasing 2 to 4 times the ionbeam energy from the tandem accelerator. We have a difficulty with the tandem booster such itscryogenic system loses its stable run sometimes in a few hours after starting the operation of thebooster. 2. THE TANDEM BOOSTERThe JAERI tandem booster linac consists of 40 1/4-wavelength type cavities of 130MHz in10 cryostats, and the total acceleration voltage is 30 MV. There is a buncher with 130 MHz and260 MHz cavities before the linac, and 60% of the continuous beam becomes available from the two-frequency buncher. A 130 MHz de-buncher is placed after the linac, which is for equalizing spreadenergy of the accelerated ion beam [1, 2]. Figure 1 shows the schematic diagram of the wholeaccelerator system, Fig. 2 and Fig. 3 show the cutaway view of the SC cavity and cavity assembled inline, respectively.Fig. 1:  Schematic diagram of the JAERI tandem accelerator and booster linac facilityInj.Inj.post s.fBUNCHER@260MHz QWRx2@130MHz QWRx210 LINAC UNITS@130MHz QWRx4x10@Q-DoubletHELIUM REFRIGERATOR@ @4.2 j @LHe 270W@ @80K      GHe 1.6KWTarget AreaDE-BUNCHER@130MHz QWRx2Qt     x9Qt Qd3 SNICS2 Decks2nd s.fs.fs.gTANDEM        ACCELERATOR1893. THE CRYOGENIC SYSTEM FOR THE TANDEM BOOSTERThe JAERI tandem booster is constituted by the accelerating cavities, cryogenic transfer lines and therefrigeration system. The cryogenic system is licensed under the refrigerator safety regulations of thehigh-pressure gas law in Japan. The cold-box is a model TCF-50 manufactured by Switzerland SuluzerCo. The cold-box has a refrigeration performance to cool down five cryostats (approximately 150W RFload in 20 accelerating cavities) and the thermal load of cryogenic transfer lines and quiescent loss ofthe whole system. The TCF-50 type cold-box was chosen by their operation records, performances, sizeand as at many other research institutes. Cold (80K) helium gas is also taken out from the cold-box andreturned to the cold-box for cooling radiation shields. It takes three days to cool five cryostats, bunchersand cryogenic transfer lines down to the steady state with full liquid helium. Figure 4 shows theschematic diagram of cryogenic system. Figure 5 shows the He gas flow of the cryostat.Fig. 4: Schematic diagram of the cryogenic systemFig. 2: Cutaway view of the SC cavity Fig. 3: Cavity assembled in lineBUNCHER 10 LINAC UNITSHE REFRIGERATORHE TRANSFER  LINEwith vacuum pumpand super insulator HE BAFFER TANK30mIONBEAM3 X2HE COMPRESSORDE-BUNCHER340KW/6.6KV4K @ @270W80K @1.6KWRem ote feedcontrol valve(every cryostat)190Fig. 5:  Schematic diagram of He gas flow4. OPERATIONAL ISSUES ON THE SUPERCONDUCTING ACCELERATOR Here, we discuss involved in the operations of the cryogenic system and the superconductingaccelerator itself4.1 The cryogenic systemThe main component of cryogenic system is cold box, which produces liquid helium from compressedgas helium. As is described above, the JAERI booster system is regulated by the high-pressure gas lawfor refrigerators. Under such regulations, it is not allowed to put a reservoir in the liquid helium loop.Gas flow in the cold box always changes quickly with changing thermal load. The time constant of heatexchangers is very long, because there are several large heat exchangers in the cold box. Control of coldbox, which has a different time constant, is complicated, because it does not respond immediately toabrupt fluctuation. Control processes of the cold box deviate little by little from an expected series ofoperations, and eventually loose the stable operation. The pressure supplying liquid to the cryostatschanges in such a situation to cause the cavities to deviate their oscillating frequencies. Another reasonon instability lies in operation of the pneumatic valves of the cold box. They are operated by usingcompressed air for the sake of safety in case of a power failure, but the valve control circuits and themechanical structure are very delicate. Therefore, some valves sometimes get out of control from thecontrol computer. In near future, we will change the control circuits to new reliable ones. 4.2 Acceleration unitsJAERI superconducting booster has been running stably during seven years, recently, however, weopened two cryostats because of helium leak from cavity adapter flanges. Indium wires for the vacuumseal were replaced. The leaks were due to frequent thermal cycles. Several thermal cycles have beenrepeated every year for many reasons such as the legal maintenance of the cryogenic system and thelaboratory power stations. In this occasion of opening, we re-tuned the resonant frequencies of thecavities of which natural resonant frequencies had been lowered too much during the thermal cycles.The cavity Q factors have not been degraded very much. However, the Q factors of many cavities builtearlier were lower than those made later. Figure 6 shows the result of each Q value. The low Q factorswere due to the Q-degradation during slow cool-down through the region of temperature from 130 K to90 K in which niobium-hydrides precipitate on the niobium cavity surface. We could improve the Qfactors by a fast cool-down using a sequential cooling process. The result is shown in Fig. 6 with adifferent notation.OIL SEPARATORCOMPRESSORBUFFER TANKHEXTURBINECOLD BOXLHE SUPPLYLHE RETURN80K SUPPLY80K RETURNCRYOSTATSC CAVITY80K SHIELDHE TANKSUMP80K RETURN80K SUPPLYLHE RETURNLHE SUPPLYNEXT CRYOSTAT191Fig. 6: Q factors under three conditions4.3 The peculiarity of superconducting linacTo maintain the good condition of the superconducting linac for a long time, we need meticuloushandling and considerations of following items; a) vacuum system, b) thermal cycle, c) maintenanceand d) Q degradation.4.3.1 Vacuum systemWith respect to the superconducting linac, very strong electrical fields are generated on the surfaces ofthe acceleration cavities. Therefore, the surface must be protected from any contamination such asmicro particles as much as possible. All vacuum components are oil free, and equipped with safetyinterlocks. Automatic-shut-off valves are inserted between cryostat and main pump, main pump andfore line pump. The main pumps used at JAERI are a magnetically suspended 1,000 L/s turbomolecular pump. 4.3.2 Thermal cycleComposite materials of niobium-clad copper are used in the cavities, and indium wires for the vacuumseal between flanges. Thermal cycles, then, cause strong stresses between the different materials. Wehave not experienced serious problems caused by material itself, although we repeated warming-up thecavities several times in a year. It worries us, however, that a big problem may happen in the future. Wehave to minimize the thermal cycles taking into consideration of the machine time and the powerstation’s maintenance time.4.3.3 MaintenanceOpening a cryostat for maintenance has been extremely few, but there have been several times that wehad to brake the vacuum until now. The vacuum break has a risk to sprinkle micro particles on thecavity surface. At the vacuum break, dry nitrogen was fed through a very fine filter into the cryostat asslow as possible, because micro-particles might cause electron field emission from the surface of thecavity. In case of opening a cryostat, we used a clean booth.0.11101000 4 8 12 16 20Cavity numbeQo at Eacc=1MV/moff-line2-modulesall-togetherx1081924.3.4 Q degradationHow to realize and maintain a high Q factor is a big theme for superconducting cavities. With respect tothe JAERI’s superconducting cavities, very high performances were realized after the improvements ofelectropolishing and rinsing technology. However, it is uncertain that we will be able to keep theirpresent performances for a long time. If the Q factors decrease, we have no good solution to recover theQ factors. Therefore, the best way to keep the present condition is to protect the cavities against anycontamination. Recovering Q factors by a fast cool-down, on the other hand, has a risk of seriousdamage to the cavities. 4.4 OperationsFor the operation of the JAERI tandem booster, it takes about one hour and a half to set the cavities forthe beam acceleration. The electric fields of cavities, which give expected ion beam energy, arecalculated before starting operation. The RF phase of each cavity is determined by getting the beamphase measured by a beam-bunch phase detector. A heater stabilizes the liquid level in the sumpcontainer of the cold box and the total thermal load to the refrigerator is kept constant. A few hoursprior to the cavity start-up, the power (approximately as much as the RF power to be inputted) is fed tothe liquid helium system using heaters placed in the Dewars of the cryostats. The heater power isdecreased with increasing RF input power in order to keep the load to the refrigerator as constant aspossible, also. A few hours later from starting the cavities, the operating condition is balanced, and thecondition will continue without any handling. 5. ANOTHER TOPICA job of exchanging acceleration tubes of the tandem accelerator has been scheduled as an upgradeproject of the tandem accelerator. The injection energy to the booster can be increased to make thematching between the tandem and its booster better for very heavy ions. In addition, rare gas ion beams(Ne, Ar, Kr, Xe) are accelerated, and utilized for the various researches. An installation program of RFQand interdigital-H type linacs may start from the next fiscal year in order to increase species andintensities of ion beams.6. CONCLUSIONThe JAERI superconducting linac has been continued stable operation, therefore, we have some settlingproblems in the above mentioned. Recovering of the Q factors and maintaining present performancesare extremely important subjects of JAERI tandem booster system. We have to find out best solutionagainst these problems.References[1] S. Takeuchi et al., Proc. of the 8th workshop on RF superconductivity in Abano Terme 1997, 6 -10 Oct. p247[2] S. Takeuchi et al., Nucl. Instr. and Methods A382(1996) 153-160

JAERI Tandem Booster and its Cryogenic System

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