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Cryogenic system for TRISTAN superconducting RF cavities
Fusion Engineering and Design 20 (1993) 491-498 North-Holland
491
Cryogenic system for TRISTAN superconducting RF cavities K. H o s o y a m a a, K. H a r a and K. M a t s u m o t o b
a,
A. K a b e a, y . Kojima
a,
T. Ogitsu a, y . S a k a m o t o a, S. K a w a m u r a b
"KEK, National Laboratory for High Energy Physics, Tsukuba 305, Ibaraki, Japan h Hitachi Ltd. Kasado Work, Kudamatsu, Yamaguchi, Japan
A large cryogenic system has been designed, constructed and operated in the TRISTAN electron-positron collider at KEK for 508 MHz, 32 x 5-cell superconducting RF cavities. A 6.5 kW, 4.4 K helium refrigerator with 5 turbo-expanders on the ground level supplies liquid helium in parallel to the 16 cryostats in the TRISTAN tunnel through about 250 m long multichannel transfer line. Two 5-cell cavities are coupled together, enclosed in a cryostat and cooled by about 830 L pool boiling liquid helium. A liquid nitrogen circulation system with a turbo-expander has been adopted for 80 K radiation shields in the multicbannel transfer line and the cryostats to reduce liquid nitrogen consumption and to increase the operation stabilty of the system. The cryogenic system has a total of about 18000 hours of operating time from the first cool down test in August 1988 to November 1991. The design principle and outline of the cryogenic system and the operational experience are presented.
1. Introduction The TR I S TAN electron-positron collider at KEK was commissioned successfully in November 1986 [1]. The electron-positron beam was accelerated up to 27 GeV x 27 GeV by conventional copper cavities. Installation of superconducting RF cavities to the T R I S T A N ring was proposed for further upgrading of the beam energy and authorized as two years project in April 1986. The design study of the cryogenic system for 508 MHz, 32 × 5-cell superconducting RF cavities was started in cooperation with industries in August 1986 [2]. Before the construction of the cryogenic system, reliablity study of the superconducting RF cavity and its cryogenic system was performed. As a part of this study a cryogenic system with 600 W, 4.4 K helium refrigerator was designed, constructed in the TRISTAN Accumulator ring for 2 × 5-cell 508 MHz prototype superconducting R F cavities in 2 cryostats and operated successfully with beam [3]. A cryogenic system with 4 kW, 4.4 K helium refrigerator was constructed and 16 cavities in 8 cryostats were installed in the T R I S T A N tunnel during the summer shutdown of 1988. The first cooldown test of the cryogenic system with 16 x 5-cell cavities in 8
Correspondence to: Dr. K. Hosoyama, KEK, National Laboratory for High Energy Physics, Tsukuba 305, Ibaraki, Japan.
cryostats was performed in Octorber 1988 [4]. After about one year long physics run at 30.7 GeV beam energy, the 4 kW helium refrigerator was upgraded to 6.5 kW by installation of a supercritical turbo-expander and additional helium compressors [5]. This was done to support an additional 16× 5-cell cavities in 8 cryostats to take the beam energy to 32 GeV. At the same time the 80 K turbo-expanders were installed to reduce liquid nitrogen consumption. In December 1990 a turbo-expander was installed in the liquid nitrogen circulation system to reduce the liquid nitrogen consumption.
2. Cryogenic system overview The flow diagram of the cryogenic system for the TRI S TA N superconducting RF cavities is shown in fig. 1. A helium refrigerator cold box and compressor unit are installed in a building at ground level. The 32 x 5cell cavities in 16 cryostats are installed in the 11 m deep underground tunnel of the TRISTAN ring. The liquid helium produced by the helium refrigerator and stored in a 12000 L liquid helium storage dewar is transferred to the 16 cryostats through a 250 m long multichannel transfer line. The cryostats and the transfer line are thermally shielded by 80 K liquid nitrogen which is supplied by the liquid nitrogen circulation
0 9 2 0 - 3 7 9 6 / 9 3 / $ 0 6 . 0 0 © 1993 - E l s e v i e r S c i e n c e Publishers B.V. All rights r e s e r v e d
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Fig. 1. Flow d i a g r a m of T R I S T A N s u p e r c o n d u c t i n g R F cavity c r y o g e n i c system.
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Vent Line
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Fig. 2. Schematic picture of superconducting RE cavities in the cryostat. The location of platinum-cobalt thermometers from QI to Q5 are shown as small circles.
system. The entire helium inventory can be stored as gas in medium pressure (1.9 MPa) storage tanks and high pressure (15 MPa) storage vessels.
3. Superconducting RF cavities
4. Heat load Figure 3 shows the total heat load of the cryogenic system with 32 × 5 cell cavities in 16 cryostats, as a
8 -
/ Figure 2 shows schematic drawing of 2 × 5-cell 508 MHz superconducting RF cavities in the cryostat. Two 5-cell cavities made of about 2 mm thick pure niobium sheet are coupled together, enclosed in a liquid helium vessel with inner diameter of 700 mm. The resonance frequency change due to the helium bath pressure fluctuation in the cryostat and beam loading is compensated by adjusting the cavity length with piezo electric tuner. The superconducting RF cavities and the liquid helium vessel (about 1000 kg cold mass) are mildly cooled down by cold helium gas from the helium refrigerator by convection in the vessel. The amount of liquid helium in a cryostat is about 830 L. A 300 W electric heater installed at the bottom of the vessel, is used to warm up and to compensate the RF losses of the superconducting RF cavities during the operation. Six platinum-cobalt thermometers from Q1 to Q6 are installed in the vessel to monitor the temperature distribution during the cool down process. The 80 K radiation shield is cooled by liquid nitrogen pipe cooling.
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Fig. 3. £otal heat load of the TRISTAN superconducting RF cavity cryogenic system as a function of accelerating field Eac c .
494
K. Hosoyama et al. / Cryogenic" system for TRISTAN
Table l Main parameters of the TRISTAN helium refrigerator Cold Box cooling capacity turbo-expanders 5 gas bearing rotor diameter (ram) fabricator Compressor Unit 6 oil flooded screw pressure (MPa) outlet/inlet/inlet flow rate (Nm3/h) input power (kW) fabricator
6.5 kW at 4.4 K T1, T2, T3, T4, T5 35/55/35/35/35 Hitachi 320Lx 3 + 3205 x 1 250Lx 1 + 250S x 1
1.9/0.4/0.11 14,60[)/2,300/12,200 373×3+913+ 127+423 Mayekawa
function of the accelerating field E~cc at Q = 0.5 x 10 '~, 1 × 10 '~, 2 × 10 ~. In designing the helium refrigeration system, we assumed the Q-value of the cavity to be I x 10 '~ at the design accelerating field of 5 M V / m and obtained an estimated total heat load of about 4 kW including about 1 kW static heat load from the transfer lines, 16 cryostats, cold valves and bayonet joints. Degradation of the Q-values of the cavities during the long term operation will cause heavier heat load to the refrigerator. There is also a possibility of improving the E,,~ up to 7 M V / m in the near future by technology development of the cavity surface treatment. In this case total heat load is about 6.5 kW at Q = 1 x 10
5. Helium refrigerator At the first phase of the project, a 4 kW at 4.4 K refrigeration system with 2 turbo-expanders T1, T2 and liquid nitrogen precooling system was constructed for 16 x 5 - c e l l cavities in 8 cryostats. Later for further installation of 16 × 5-cell cavities in 8 cryostats this system was upgraded to 6.5 kW without liquid nitrogen precooling by installation of a supercritical turbo-expander T3, 80 K turbo-expanders T4, T5 and additional C5, C6 compressors in tandem. The main parameters of the helium refrigerator are listed in Table 1.
At the 6.5 kW version the helium gas at 8 K expands from 1.6 to 0.4 MPa supercritical state in the supercritical turbo-expander. During the operation of the supercritical turbo-expander the outlet pressure is kept supercritical, higher than 0.23 MPa, by adjusting the bypass valve. The refrigeration capacity measurements of 4 and 6.5 kW version were performed by an electric heater in the 12000 L dewar. The maximum heater power of 4.16 and 8.3 kW at 4.4 K were achieved with the liquid helium level kept increaseing slightly for the 4 and 6.5 kW version respectively. A combination of 6 widely used commercially available oil flooded screw type compressors was chosen for the helium gas compressor unit because they have the highest reliability among all types of helium compressors currently available. The isothermal efficiency of the compressor unit is about 50% and the residual oil aerosol contamination is less than 0.025 ppm (vol.).
6. Liquid nitrogen circulation system We adopted a rather complicated liquid nitrogen circulation system to attain economical and stable operation of the 16 parallel pipe cooling heat loads. This system consists of widely used commercially available screw type air compressors, aluminum plate-fin type heat exchangers and a gas bearing turbo-expander. The turbo-expander is used to reduce the liquid nitrogen consumption during the steady state operation. The main parameters of the system are listed in table 2.
Table 2 Main parameters of the 6.5 kW liquid nitrogen circulation system Cold Box cooling capacity turbo-expander 1 gas bearing rotor diameter (mm) fabricator Compressor Unit 2 oil flooded screw pressure (MPa) outlet/inlet flow rate (Nine/h) input power (kW) fabricator
6.5 kW at 79 K
80 Kobe Steel EXT 80 KST55A-E 0.7/0.105 5411x 2 55 × 2 Kobe Steel
K Hosoyama et al. / Cryogenic system for TRISTAN
Gas He (Return)
/ ~
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lated transfer lines with bayonet joints are used from the end box of the main transfer line to the refrigerator cold box and the 12000 L liquid helium dewar, and also between the connection boxes in the header transfer line and the cryostats in the TRISTAN tunnel.
LN2
8. Helium gas recovery and storage system
LN2
The total amount of liquid helium required for the steady operation of the all cavities (32 x 5-cell cavities in 16 cryostats) is about 15 500 L including about 2000 L back-up in the 12000 L dewar. At the end of the operation the liquid helium in the system was recovered in the medium pressure tanks by the heaters in the cryostats and 12000 L dewar. And the warm up gas from the cryostats are stored in the gas bag through the helium gas recovery line and then stored in high pressure impure storage vessels by 5 stage air-cooled oil lubricated reciprocating compressor. The recovery helium gas is purified by high pressure low temperature charcoal adsorbent type (15 MPa, 80 K) purifier and stored in high pressure pure storage vessels. During the shutdown of the cryogenic system all liquid helium in the system is stored in medium pressure storage tanks (1.8 MPa, 100 m 3 x 9) and high pressure storage vessels (15 MPa, 9 m 3 x 4).
Liq. He [Supply) 0
10
20
30
I
I
1
I
Scale (cm) Fig. 4. Cross section of multi-transfer line. The refrigeration capacity of 6.5 kW at 80 K was measured by electric heater in the dummy cryostat temporarily connected to the system.
7. Transfer line, distribution system and cryostats Figure 4 shows the cross section of the multi-transfer line. The helium lines (supply and return) are thermally shielded by 80 K thin aluminum pipe which is connected to the liquid nitrogen lines (supply and return). One end of the main transfer line is terminated by the end box near the helium refrigerator cold box, the other end is connected to the header transfer line in the TRISTAN tunnel. The header transfer line has the 16 connection boxes to 16 cryostats. Each connection box has 4 bayonet joint ports and 4 control valves. For easy to handle single channel vacuum insu-
9. Control system A distributed process control system composed of commercially available Hitachi Ex 1000 was used to control and monitor the whole cryogenic system. The main feature of this control system is its reliability and programming ease. All controls of the system are han-
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Fig. 5. Cooldown curves of TRISTAN 32 x 5-cell cavities in 16 cryostats.
3:00
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496
K. Hosoyama et al. / Cryogenic system for TRISTAN
died with color graphic operator's consoles. All input and output data are logged on hard disk of the process computer and the trends of data can be generated on the graphic screen. These logged data are transfer and recorded to 4 floppy disks every 2 days. The important data for the operation of the TRISTAN ring are transferred to the accelerator computer HITAC 80E.
10. Cooldown of 32 ties
×
5-cell superconducting RF cavi-
The 32 x 5-cell superconducting RF cavities, in 16 cryostats, with a total cold mass of about 16000 kg, were cooled down in parallel by cold helium gas from room tempeteture. Figure 5 shows a typical cooldown curves obtained in one of the 16 cryostats (locations of the sensors from Q1 to Q5 are shown in fig. 2). At the first stage of the cooling the cold helium gas about 80 K, which was precooled by liquid nitrogen, was suppled through the transfer line. The turbo-expanders T1, T2, T4 and T5 were started after the cavities in the cryostats reached about 200 K. After all the cavities reached about 5 K, we stopped the supply of cold helium gas to the cryostats and started the liquefaction in the 12000 L dewar with supercritical turbo-expander. We then started the transfer of liquid helium from the dewar to the cryostats for further cooldown and liquid helium filling to the cryostats. The liquid helium level in the 12000 L dewar is shown in figs. 5 and 6. In this cooldown casc, the liquid helium level in the dewar was filled up to about 50% (about 7000 L of liquid helium) before the cooldown of the cavities started. Figure 6 shows the liquid helium filling curves in the typical cryostats, where LI, LS, R1 and R8 indicate
the liquid helium level in the left side cryostats number 1 and 8 and right side cryostats number 1 and 8, respectively. We filled up the liquid helium in the left side cryostats first and then right side cryostats. During filling of the left side cryostats, cold helium gas leakage to the cavity occurred at left side cryostat number L8. The liquid helium transfer to left side cryostats was stopped immediately and the liquid helium levels were lowered to 30% to inspect the helium leakage. After the inspection, we made a decision to disconnect the cryostat L8 and warm up to room temperature to repair the leakage. Then we restarted the liquid helium filling to the left side cryostats except LB. During the liquid helium filling process a depression of the liquid levels occurred in the right side cryostats duc to the low pressure in the 12 000 L dewar. These depressions were conqured by increasing the dewar pressure up to 135 kPa. It took about 4 days to cooldown 32 × 5-cell superconducting RF cavities from room temperature to liquid helium temperature and 1 day to fill the 16 cryostats. The cooldown time of the cavities was limited by the criterion of the temperatur differences in the cryostats to be kept less than 50 K.
11. Operation of cryogenic system The liquid helium levels in the cryostats must be kept at desired level (830 L level) during the operation of the superconducting RF cavities. This was performed by controlling the supply valves at the end box of header transfer line. These valves are automatically loop controlled by the liquid helium level signals of the cryostats.
100 80 o<
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L1
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Fig. 6. Liquid helium filling curves of the 16 cryostats.
9:00
K~ Hosoyama et al. / Cryogenic system for TRISTAN T h e R F loss of the s u p e r c o n d u c t i n g R F cavities in one cryostat increased from a b o u t 30 W at b e a m injection to a b o u t 120 W at the b e a m top energy in a b o u t 2 min a n d c o n t i n u e d a b o u t 1.5 h long. D u r i n g this the pressure fluctuations due to the heat load c h a n g e must keep as small as possible, preferably less t h a n 1 kPa at an o p e r a t i n g pressure of 120 kPa~ W e used electric h e a t e r s in the cryostats to c o m p e n s a t e for the R F losses in the cryostats. T h e R F losses in the cavities were calculated from the m o n i t o r e d electric field in the cavities by the R F pick up p r o b e at the s u p e r c o n d u c t i n g R F cavities a n d the m e a s u r e d Q-values. By this t e c h n i q u e the total heat load of the cryogenic system could keep c o n s t a n t during the superconducting R F cavities operation. T h e liquid h e l i u m level in the 12000 L helium dewar was controlled by the electric h e a t e r in the dewar to keep the liquid helium level c o n s t a n t at set values, a b o u t 2000 L at n o r m a l operation.
12. Down time of cryogenic system T h e cryogenic system d o w n e d 8 times during a b o u t 3 years operation. T a b l e 3 shows the d o w n t i m e of the
497
cryogenic system a n d of the T R I S T A N e l e c t r o n positron ring due to lack of refrigeration a n d the reasons of the failures. In the table * indicates the r e q u i r e d recovery time of the cryogenic system to the original condition. In No. 2 and No. 3 cases, during the exchange of the mechanical seals for C3 and C2 compressors, one of the t h r e e first stage compressors of the main compressor unit, only one c o m p r e s s o r was s t o p p e d and the cryogenic system was o p e r a t e d in the r e d u c e d capacity m o d e in o r d e r to m a i n t a i n the liquid helium levels in the cryostats. In case No. 4, t h e r e is no recovery time loss because the electric power failure occurred just after the compressor unit started. In case No. 6, the main compressors stopped by a trigger signal f r o m ' a t h e r m a l switch on an oil cooler. T h e t h e r m a l switch was influenced by hot a t m o s p h e r e in the compressor room due to the incorrect setting of it. In case No. 9, t u r b o - e x p a n d e r T1 tripped due to excess amplitude of turbine radial vibration presumably caused by an impurity in the process helium gas. T h e a m p l i t u d e of vibration increased gradually after power up operation of the t u r b o - e x p a n d e r . T h e t u r b o - e x p a n d e r could be r e s t a r t e d 2 hours later without trouble a n d continued the o p e r a t i o n stably. F o r t u n a t e l y all these cryo-
Table 3 Down time of TRISTAN superconducting RF cryogenic system no. Item Date
Failure (Action)
1
Low bearing gas pressure of T2 turbo-expander 4 h + 36 h* (Restart) Oil leakage at the mechanical seal of 1st stage of compressor unit 14 h (Exchange of mechanical seals) Oil leakage at the mechanical seal of 1st stage of compressor unit 8 h* (Exchange of mechanical seals) High discharge pressure of the compressor unit 2 h + 7 h* (Restart) Electric power outage due to thunderstorm 2h (Restart) High temperature at the oil cooler of C1 1st stage of helium compressor 7 h + 59 h* unit due to incorrect set of thermal switch (Set in correct position and Restart) Erroneous removal of a control relay during the investigation for the 1 h + 2 h* 5 min cause of the noise(Restart) High discharge gas temperature at the compressor unit due to incorrect 4 h + 35 h* 39 h set value of thermal switch (Set correct value and Restart) Turbo-expander T1 trip due to excess amplitude of turbine radial 3 h + 21 h* vibration (Restart)
2 3 4 5 6
7 8 9
Cold box Nov. 7, 1988 Helium compressor (C3) Dec. 29, 1988 Helium compressor (C2) Jan. 14, 1989 Helium compressor Jan. 26, 1989 Electric power May 3, 1989 Helium compressor Jun. 22, 1990 Cold box May 29, 1991 Helium compressor Jul. 8, 1991 Cold box Oct. 1, 199l
Cryogenics Downtime
TRISTAN Downtime
498
I( Hosoyama et al. / Cryogenic system for TRISTAN
genic system failures had not severe influence upon the TRISTAN operation except case No. 8.
tem worked satisfactorily and stably without major problem.
13. Summary
Acknowlegements
A 6.5 kW at 4.4 K cryogenic system for the TRISTAN 32 x 5-cell superconducting RF cavities in 16 cryostats has been designed and constructed following R &D study of the cryogenic system for superconducting RF cavities using medium size helium refrigerator. The system was constructed stepwise. The first step was the construction of 4 kW cryogenic system with the 16 x 5-cell cavities in 8 cryostats to confirm the reliability of the large scale cryogenic system and superconducting cavities in the TRISTAN ring as quick as possible. Next step was upgrading the cooling capacity up to 6.5 kW and installation of additional 16 × 5-cell cavities in 8 cryostats to take the TRISTAN beam energy to the maximum. At the same time two turboexpanders for 80 K precooling for helium refrigerator and one turbo-expander for the liquid nitrogen circulation system were installed to reduced the consumption of liquid nitrogen during steady state operation. By this the amount of liquid nitrogen consumption reduced from 8000 to 1700 L/day. The cryogenic system now has a total of about 18000 hours of operating time from the first cooldown test in August 1988 to December 1991, including three operating periods of approximately five months duration each. During this period the whole cryogenic sys-
The authors wish to thank Professers Y. Kimura and S. Kurokawa for their continuous support and encouragement and staff of the superconducting cavity group and cryogenic operation group of Hitachi Ltd. for their continuous support. They express thier thanks to O. Morioka and K. Shinkai of Kobe Steel Ltd. for their many useful discussions about the liquid nitrogen circulation system.
References [1] Y. Kimura, TRISTAN project and KEK activities, in: Proc. 13th the Int. conf. on high energy accelerators, Novosibirsk, USSR (1986). [2] K. Hara et al., Cryogenic system for TRISTAN superconducting RF cavity, Adv. Cryogen. Engrg. 33 (1988) 615. [3] K. Hosoyams et al., Cryogenic system for TRISTAN-AR superconducting RF cavity, Adv. Cryogen. Engrg. 35 (1990) 925. [4] K. Hosoyama et al., Cryogenic system for the TRISTAN superconducting RF cavities: performance test and present status, Adv. Cryogen. Engrg. 35 (1990) 933. [5] K. Hosoyama et al., Cryogenic system for TRISTAN superconducting RF cavities: upgrading and present status, to be published in Adv. Cryogen. Engrg. 37 (1992).
491
Cryogenic system for TRISTAN superconducting RF cavities K. H o s o y a m a a, K. H a r a and K. M a t s u m o t o b
a,
A. K a b e a, y . Kojima
a,
T. Ogitsu a, y . S a k a m o t o a, S. K a w a m u r a b
"KEK, National Laboratory for High Energy Physics, Tsukuba 305, Ibaraki, Japan h Hitachi Ltd. Kasado Work, Kudamatsu, Yamaguchi, Japan
A large cryogenic system has been designed, constructed and operated in the TRISTAN electron-positron collider at KEK for 508 MHz, 32 x 5-cell superconducting RF cavities. A 6.5 kW, 4.4 K helium refrigerator with 5 turbo-expanders on the ground level supplies liquid helium in parallel to the 16 cryostats in the TRISTAN tunnel through about 250 m long multichannel transfer line. Two 5-cell cavities are coupled together, enclosed in a cryostat and cooled by about 830 L pool boiling liquid helium. A liquid nitrogen circulation system with a turbo-expander has been adopted for 80 K radiation shields in the multicbannel transfer line and the cryostats to reduce liquid nitrogen consumption and to increase the operation stabilty of the system. The cryogenic system has a total of about 18000 hours of operating time from the first cool down test in August 1988 to November 1991. The design principle and outline of the cryogenic system and the operational experience are presented.
1. Introduction The TR I S TAN electron-positron collider at KEK was commissioned successfully in November 1986 [1]. The electron-positron beam was accelerated up to 27 GeV x 27 GeV by conventional copper cavities. Installation of superconducting RF cavities to the T R I S T A N ring was proposed for further upgrading of the beam energy and authorized as two years project in April 1986. The design study of the cryogenic system for 508 MHz, 32 × 5-cell superconducting RF cavities was started in cooperation with industries in August 1986 [2]. Before the construction of the cryogenic system, reliablity study of the superconducting RF cavity and its cryogenic system was performed. As a part of this study a cryogenic system with 600 W, 4.4 K helium refrigerator was designed, constructed in the TRISTAN Accumulator ring for 2 × 5-cell 508 MHz prototype superconducting R F cavities in 2 cryostats and operated successfully with beam [3]. A cryogenic system with 4 kW, 4.4 K helium refrigerator was constructed and 16 cavities in 8 cryostats were installed in the T R I S T A N tunnel during the summer shutdown of 1988. The first cooldown test of the cryogenic system with 16 x 5-cell cavities in 8
Correspondence to: Dr. K. Hosoyama, KEK, National Laboratory for High Energy Physics, Tsukuba 305, Ibaraki, Japan.
cryostats was performed in Octorber 1988 [4]. After about one year long physics run at 30.7 GeV beam energy, the 4 kW helium refrigerator was upgraded to 6.5 kW by installation of a supercritical turbo-expander and additional helium compressors [5]. This was done to support an additional 16× 5-cell cavities in 8 cryostats to take the beam energy to 32 GeV. At the same time the 80 K turbo-expanders were installed to reduce liquid nitrogen consumption. In December 1990 a turbo-expander was installed in the liquid nitrogen circulation system to reduce the liquid nitrogen consumption.
2. Cryogenic system overview The flow diagram of the cryogenic system for the TRI S TA N superconducting RF cavities is shown in fig. 1. A helium refrigerator cold box and compressor unit are installed in a building at ground level. The 32 x 5cell cavities in 16 cryostats are installed in the 11 m deep underground tunnel of the TRISTAN ring. The liquid helium produced by the helium refrigerator and stored in a 12000 L liquid helium storage dewar is transferred to the 16 cryostats through a 250 m long multichannel transfer line. The cryostats and the transfer line are thermally shielded by 80 K liquid nitrogen which is supplied by the liquid nitrogen circulation
0 9 2 0 - 3 7 9 6 / 9 3 / $ 0 6 . 0 0 © 1993 - E l s e v i e r S c i e n c e Publishers B.V. All rights r e s e r v e d
Screw Compressor Unit
Jm
7 Transfer Line
>
1
400 kPa
)ra ro( nk ,0C
IL .•
15 MPa
I"
16 Units
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~
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#a~~gT~0m'
LN2 Screw Compressor Unit
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6.5 kW at 80 K
'Am
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Cold Box
LN2 Circulation System
Purifier High Pressure Gas Storage 0.5m3x 18x 4 = 3 6 m 3
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Fig. 1. Flow d i a g r a m of T R I S T A N s u p e r c o n d u c t i n g R F cavity c r y o g e n i c system.
Medium Pressure Gas
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K. Hosoyama et aL / Cryogenic system for TRISTAN
Vent Line
RecoveryLine I [A I I • Recovery I {l~' LT +lIT I LIne I Header Transfer l~, ~' (~7~--I~~Lh I LineEndBox I ' T ~l~:-~"l I . . ~ . ~ I I11 II]11] .. Liq. He (Supply) I oo oo. I I III~iNn2eII]JIU He (Return) Safty -~' Sa~vYe ~ ~l '_~ I~ __Liq. HeLevel
~
80 K LN2 Shield
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Superconductng RF cavity Cryostat
Fig. 2. Schematic picture of superconducting RE cavities in the cryostat. The location of platinum-cobalt thermometers from QI to Q5 are shown as small circles.
system. The entire helium inventory can be stored as gas in medium pressure (1.9 MPa) storage tanks and high pressure (15 MPa) storage vessels.
3. Superconducting RF cavities
4. Heat load Figure 3 shows the total heat load of the cryogenic system with 32 × 5 cell cavities in 16 cryostats, as a
8 -
/ Figure 2 shows schematic drawing of 2 × 5-cell 508 MHz superconducting RF cavities in the cryostat. Two 5-cell cavities made of about 2 mm thick pure niobium sheet are coupled together, enclosed in a liquid helium vessel with inner diameter of 700 mm. The resonance frequency change due to the helium bath pressure fluctuation in the cryostat and beam loading is compensated by adjusting the cavity length with piezo electric tuner. The superconducting RF cavities and the liquid helium vessel (about 1000 kg cold mass) are mildly cooled down by cold helium gas from the helium refrigerator by convection in the vessel. The amount of liquid helium in a cryostat is about 830 L. A 300 W electric heater installed at the bottom of the vessel, is used to warm up and to compensate the RF losses of the superconducting RF cavities during the operation. Six platinum-cobalt thermometers from Q1 to Q6 are installed in the vessel to monitor the temperature distribution during the cool down process. The 80 K radiation shield is cooled by liquid nitrogen pipe cooling.
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i
4
I
5
I
6
I
7
J
(MV/rn)
Fig. 3. £otal heat load of the TRISTAN superconducting RF cavity cryogenic system as a function of accelerating field Eac c .
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K. Hosoyama et al. / Cryogenic" system for TRISTAN
Table l Main parameters of the TRISTAN helium refrigerator Cold Box cooling capacity turbo-expanders 5 gas bearing rotor diameter (ram) fabricator Compressor Unit 6 oil flooded screw pressure (MPa) outlet/inlet/inlet flow rate (Nm3/h) input power (kW) fabricator
6.5 kW at 4.4 K T1, T2, T3, T4, T5 35/55/35/35/35 Hitachi 320Lx 3 + 3205 x 1 250Lx 1 + 250S x 1
1.9/0.4/0.11 14,60[)/2,300/12,200 373×3+913+ 127+423 Mayekawa
function of the accelerating field E~cc at Q = 0.5 x 10 '~, 1 × 10 '~, 2 × 10 ~. In designing the helium refrigeration system, we assumed the Q-value of the cavity to be I x 10 '~ at the design accelerating field of 5 M V / m and obtained an estimated total heat load of about 4 kW including about 1 kW static heat load from the transfer lines, 16 cryostats, cold valves and bayonet joints. Degradation of the Q-values of the cavities during the long term operation will cause heavier heat load to the refrigerator. There is also a possibility of improving the E,,~ up to 7 M V / m in the near future by technology development of the cavity surface treatment. In this case total heat load is about 6.5 kW at Q = 1 x 10
5. Helium refrigerator At the first phase of the project, a 4 kW at 4.4 K refrigeration system with 2 turbo-expanders T1, T2 and liquid nitrogen precooling system was constructed for 16 x 5 - c e l l cavities in 8 cryostats. Later for further installation of 16 × 5-cell cavities in 8 cryostats this system was upgraded to 6.5 kW without liquid nitrogen precooling by installation of a supercritical turbo-expander T3, 80 K turbo-expanders T4, T5 and additional C5, C6 compressors in tandem. The main parameters of the helium refrigerator are listed in Table 1.
At the 6.5 kW version the helium gas at 8 K expands from 1.6 to 0.4 MPa supercritical state in the supercritical turbo-expander. During the operation of the supercritical turbo-expander the outlet pressure is kept supercritical, higher than 0.23 MPa, by adjusting the bypass valve. The refrigeration capacity measurements of 4 and 6.5 kW version were performed by an electric heater in the 12000 L dewar. The maximum heater power of 4.16 and 8.3 kW at 4.4 K were achieved with the liquid helium level kept increaseing slightly for the 4 and 6.5 kW version respectively. A combination of 6 widely used commercially available oil flooded screw type compressors was chosen for the helium gas compressor unit because they have the highest reliability among all types of helium compressors currently available. The isothermal efficiency of the compressor unit is about 50% and the residual oil aerosol contamination is less than 0.025 ppm (vol.).
6. Liquid nitrogen circulation system We adopted a rather complicated liquid nitrogen circulation system to attain economical and stable operation of the 16 parallel pipe cooling heat loads. This system consists of widely used commercially available screw type air compressors, aluminum plate-fin type heat exchangers and a gas bearing turbo-expander. The turbo-expander is used to reduce the liquid nitrogen consumption during the steady state operation. The main parameters of the system are listed in table 2.
Table 2 Main parameters of the 6.5 kW liquid nitrogen circulation system Cold Box cooling capacity turbo-expander 1 gas bearing rotor diameter (mm) fabricator Compressor Unit 2 oil flooded screw pressure (MPa) outlet/inlet flow rate (Nine/h) input power (kW) fabricator
6.5 kW at 79 K
80 Kobe Steel EXT 80 KST55A-E 0.7/0.105 5411x 2 55 × 2 Kobe Steel
K Hosoyama et al. / Cryogenic system for TRISTAN
Gas He (Return)
/ ~
495
lated transfer lines with bayonet joints are used from the end box of the main transfer line to the refrigerator cold box and the 12000 L liquid helium dewar, and also between the connection boxes in the header transfer line and the cryostats in the TRISTAN tunnel.
LN2
8. Helium gas recovery and storage system
LN2
The total amount of liquid helium required for the steady operation of the all cavities (32 x 5-cell cavities in 16 cryostats) is about 15 500 L including about 2000 L back-up in the 12000 L dewar. At the end of the operation the liquid helium in the system was recovered in the medium pressure tanks by the heaters in the cryostats and 12000 L dewar. And the warm up gas from the cryostats are stored in the gas bag through the helium gas recovery line and then stored in high pressure impure storage vessels by 5 stage air-cooled oil lubricated reciprocating compressor. The recovery helium gas is purified by high pressure low temperature charcoal adsorbent type (15 MPa, 80 K) purifier and stored in high pressure pure storage vessels. During the shutdown of the cryogenic system all liquid helium in the system is stored in medium pressure storage tanks (1.8 MPa, 100 m 3 x 9) and high pressure storage vessels (15 MPa, 9 m 3 x 4).
Liq. He [Supply) 0
10
20
30
I
I
1
I
Scale (cm) Fig. 4. Cross section of multi-transfer line. The refrigeration capacity of 6.5 kW at 80 K was measured by electric heater in the dummy cryostat temporarily connected to the system.
7. Transfer line, distribution system and cryostats Figure 4 shows the cross section of the multi-transfer line. The helium lines (supply and return) are thermally shielded by 80 K thin aluminum pipe which is connected to the liquid nitrogen lines (supply and return). One end of the main transfer line is terminated by the end box near the helium refrigerator cold box, the other end is connected to the header transfer line in the TRISTAN tunnel. The header transfer line has the 16 connection boxes to 16 cryostats. Each connection box has 4 bayonet joint ports and 4 control valves. For easy to handle single channel vacuum insu-
9. Control system A distributed process control system composed of commercially available Hitachi Ex 1000 was used to control and monitor the whole cryogenic system. The main feature of this control system is its reliability and programming ease. All controls of the system are han-
350
100
280 "
_J
Q1-Q2 Q3 . . . . . . .
~"~':-.~..~-:.::.:... ....
z . : ~ 2 ~ - . ~ ....
c~4
211
.
.
.
.
.
.
80
.
~"-'~[-
60
Dewar Level
& 142
40
~-
2O
73 4 15:00
'i ............ ; .........
3:00
15:00 Oct. 20
3:00
15:00 Oct. 21
3:00
15:00 Oct. 22
Fig. 5. Cooldown curves of TRISTAN 32 x 5-cell cavities in 16 cryostats.
3:00
15:00
.J
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K. Hosoyama et al. / Cryogenic system for TRISTAN
died with color graphic operator's consoles. All input and output data are logged on hard disk of the process computer and the trends of data can be generated on the graphic screen. These logged data are transfer and recorded to 4 floppy disks every 2 days. The important data for the operation of the TRISTAN ring are transferred to the accelerator computer HITAC 80E.
10. Cooldown of 32 ties
×
5-cell superconducting RF cavi-
The 32 x 5-cell superconducting RF cavities, in 16 cryostats, with a total cold mass of about 16000 kg, were cooled down in parallel by cold helium gas from room tempeteture. Figure 5 shows a typical cooldown curves obtained in one of the 16 cryostats (locations of the sensors from Q1 to Q5 are shown in fig. 2). At the first stage of the cooling the cold helium gas about 80 K, which was precooled by liquid nitrogen, was suppled through the transfer line. The turbo-expanders T1, T2, T4 and T5 were started after the cavities in the cryostats reached about 200 K. After all the cavities reached about 5 K, we stopped the supply of cold helium gas to the cryostats and started the liquefaction in the 12000 L dewar with supercritical turbo-expander. We then started the transfer of liquid helium from the dewar to the cryostats for further cooldown and liquid helium filling to the cryostats. The liquid helium level in the 12000 L dewar is shown in figs. 5 and 6. In this cooldown casc, the liquid helium level in the dewar was filled up to about 50% (about 7000 L of liquid helium) before the cooldown of the cavities started. Figure 6 shows the liquid helium filling curves in the typical cryostats, where LI, LS, R1 and R8 indicate
the liquid helium level in the left side cryostats number 1 and 8 and right side cryostats number 1 and 8, respectively. We filled up the liquid helium in the left side cryostats first and then right side cryostats. During filling of the left side cryostats, cold helium gas leakage to the cavity occurred at left side cryostat number L8. The liquid helium transfer to left side cryostats was stopped immediately and the liquid helium levels were lowered to 30% to inspect the helium leakage. After the inspection, we made a decision to disconnect the cryostat L8 and warm up to room temperature to repair the leakage. Then we restarted the liquid helium filling to the left side cryostats except LB. During the liquid helium filling process a depression of the liquid levels occurred in the right side cryostats duc to the low pressure in the 12 000 L dewar. These depressions were conqured by increasing the dewar pressure up to 135 kPa. It took about 4 days to cooldown 32 × 5-cell superconducting RF cavities from room temperature to liquid helium temperature and 1 day to fill the 16 cryostats. The cooldown time of the cavities was limited by the criterion of the temperatur differences in the cryostats to be kept less than 50 K.
11. Operation of cryogenic system The liquid helium levels in the cryostats must be kept at desired level (830 L level) during the operation of the superconducting RF cavities. This was performed by controlling the supply valves at the end box of header transfer line. These valves are automatically loop controlled by the liquid helium level signals of the cryostats.
100 80 o<
~eo ~4o
L1
"
'- " R~
2O 9:00
15:00 Oct.24
21:00
3:00 Oct. 25
Fig. 6. Liquid helium filling curves of the 16 cryostats.
9:00
K~ Hosoyama et al. / Cryogenic system for TRISTAN T h e R F loss of the s u p e r c o n d u c t i n g R F cavities in one cryostat increased from a b o u t 30 W at b e a m injection to a b o u t 120 W at the b e a m top energy in a b o u t 2 min a n d c o n t i n u e d a b o u t 1.5 h long. D u r i n g this the pressure fluctuations due to the heat load c h a n g e must keep as small as possible, preferably less t h a n 1 kPa at an o p e r a t i n g pressure of 120 kPa~ W e used electric h e a t e r s in the cryostats to c o m p e n s a t e for the R F losses in the cryostats. T h e R F losses in the cavities were calculated from the m o n i t o r e d electric field in the cavities by the R F pick up p r o b e at the s u p e r c o n d u c t i n g R F cavities a n d the m e a s u r e d Q-values. By this t e c h n i q u e the total heat load of the cryogenic system could keep c o n s t a n t during the superconducting R F cavities operation. T h e liquid h e l i u m level in the 12000 L helium dewar was controlled by the electric h e a t e r in the dewar to keep the liquid helium level c o n s t a n t at set values, a b o u t 2000 L at n o r m a l operation.
12. Down time of cryogenic system T h e cryogenic system d o w n e d 8 times during a b o u t 3 years operation. T a b l e 3 shows the d o w n t i m e of the
497
cryogenic system a n d of the T R I S T A N e l e c t r o n positron ring due to lack of refrigeration a n d the reasons of the failures. In the table * indicates the r e q u i r e d recovery time of the cryogenic system to the original condition. In No. 2 and No. 3 cases, during the exchange of the mechanical seals for C3 and C2 compressors, one of the t h r e e first stage compressors of the main compressor unit, only one c o m p r e s s o r was s t o p p e d and the cryogenic system was o p e r a t e d in the r e d u c e d capacity m o d e in o r d e r to m a i n t a i n the liquid helium levels in the cryostats. In case No. 4, t h e r e is no recovery time loss because the electric power failure occurred just after the compressor unit started. In case No. 6, the main compressors stopped by a trigger signal f r o m ' a t h e r m a l switch on an oil cooler. T h e t h e r m a l switch was influenced by hot a t m o s p h e r e in the compressor room due to the incorrect setting of it. In case No. 9, t u r b o - e x p a n d e r T1 tripped due to excess amplitude of turbine radial vibration presumably caused by an impurity in the process helium gas. T h e a m p l i t u d e of vibration increased gradually after power up operation of the t u r b o - e x p a n d e r . T h e t u r b o - e x p a n d e r could be r e s t a r t e d 2 hours later without trouble a n d continued the o p e r a t i o n stably. F o r t u n a t e l y all these cryo-
Table 3 Down time of TRISTAN superconducting RF cryogenic system no. Item Date
Failure (Action)
1
Low bearing gas pressure of T2 turbo-expander 4 h + 36 h* (Restart) Oil leakage at the mechanical seal of 1st stage of compressor unit 14 h (Exchange of mechanical seals) Oil leakage at the mechanical seal of 1st stage of compressor unit 8 h* (Exchange of mechanical seals) High discharge pressure of the compressor unit 2 h + 7 h* (Restart) Electric power outage due to thunderstorm 2h (Restart) High temperature at the oil cooler of C1 1st stage of helium compressor 7 h + 59 h* unit due to incorrect set of thermal switch (Set in correct position and Restart) Erroneous removal of a control relay during the investigation for the 1 h + 2 h* 5 min cause of the noise(Restart) High discharge gas temperature at the compressor unit due to incorrect 4 h + 35 h* 39 h set value of thermal switch (Set correct value and Restart) Turbo-expander T1 trip due to excess amplitude of turbine radial 3 h + 21 h* vibration (Restart)
2 3 4 5 6
7 8 9
Cold box Nov. 7, 1988 Helium compressor (C3) Dec. 29, 1988 Helium compressor (C2) Jan. 14, 1989 Helium compressor Jan. 26, 1989 Electric power May 3, 1989 Helium compressor Jun. 22, 1990 Cold box May 29, 1991 Helium compressor Jul. 8, 1991 Cold box Oct. 1, 199l
Cryogenics Downtime
TRISTAN Downtime
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I( Hosoyama et al. / Cryogenic system for TRISTAN
genic system failures had not severe influence upon the TRISTAN operation except case No. 8.
tem worked satisfactorily and stably without major problem.
13. Summary
Acknowlegements
A 6.5 kW at 4.4 K cryogenic system for the TRISTAN 32 x 5-cell superconducting RF cavities in 16 cryostats has been designed and constructed following R &D study of the cryogenic system for superconducting RF cavities using medium size helium refrigerator. The system was constructed stepwise. The first step was the construction of 4 kW cryogenic system with the 16 x 5-cell cavities in 8 cryostats to confirm the reliability of the large scale cryogenic system and superconducting cavities in the TRISTAN ring as quick as possible. Next step was upgrading the cooling capacity up to 6.5 kW and installation of additional 16 × 5-cell cavities in 8 cryostats to take the TRISTAN beam energy to the maximum. At the same time two turboexpanders for 80 K precooling for helium refrigerator and one turbo-expander for the liquid nitrogen circulation system were installed to reduced the consumption of liquid nitrogen during steady state operation. By this the amount of liquid nitrogen consumption reduced from 8000 to 1700 L/day. The cryogenic system now has a total of about 18000 hours of operating time from the first cooldown test in August 1988 to December 1991, including three operating periods of approximately five months duration each. During this period the whole cryogenic sys-
The authors wish to thank Professers Y. Kimura and S. Kurokawa for their continuous support and encouragement and staff of the superconducting cavity group and cryogenic operation group of Hitachi Ltd. for their continuous support. They express thier thanks to O. Morioka and K. Shinkai of Kobe Steel Ltd. for their many useful discussions about the liquid nitrogen circulation system.
References [1] Y. Kimura, TRISTAN project and KEK activities, in: Proc. 13th the Int. conf. on high energy accelerators, Novosibirsk, USSR (1986). [2] K. Hara et al., Cryogenic system for TRISTAN superconducting RF cavity, Adv. Cryogen. Engrg. 33 (1988) 615. [3] K. Hosoyams et al., Cryogenic system for TRISTAN-AR superconducting RF cavity, Adv. Cryogen. Engrg. 35 (1990) 925. [4] K. Hosoyama et al., Cryogenic system for the TRISTAN superconducting RF cavities: performance test and present status, Adv. Cryogen. Engrg. 35 (1990) 933. [5] K. Hosoyama et al., Cryogenic system for TRISTAN superconducting RF cavities: upgrading and present status, to be published in Adv. Cryogen. Engrg. 37 (1992).