The 1Ith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Cool-down by Compact Refrigerators System for SRF Cavities in JAERI FEL
Nobuhiro KIKUZAWA, Ryoji NAGAI, Masaru SAWAMURA, Nobuyuki NISHIMORI and Eisuke MINEHARAFree Electron Laser Laboratory, Japan Atomic Energy Research Institute,Tokai-mura, Naka-gun, lbaraki, 319-11 JAPAN
Abstract
We have developed a built-in cryogenic systemusing compact refrigerators for the JAERI FELThe cryogenic
superconducting accelerator modules.
system consists of a refrigerator operated at 4.2 K to keepliquid helium level and a Gifford-McMahon cycle
refrigerator to cool duplex heat shields. By applying thecompact refrigerators, the cryogenic system can be operated
successfully without an operator of the cryogenic system.
A Cool-down of the JAERI superconducting
accelerating cavities by the compact refrigerators system
was carried out without liquid N2 or cold helium gas. The
cool-down will be reported and discussed in detail.

1. Introduction
The JAERI FEL driven by a superconducting rf
linac has been developed to demonstrate a high average
power and long pulse far-infrared FEL. By applying thesuperconducting rf linac, long pulse or quasi-cw and highaverage power may be readily attained at the JAERI FEL.

The superconducting rf linac consists of two pre-accelerator
modules of single-cell cavity and two main accelerator
modules of five-cell cavity. The resonant frequency of thecavities is 499.8 MHz and design values of the accelerating
field strength and Q-value for the cavities are 5 MV/m, and2x109 , respectively.
We have developed a built-in cryogenic systemusing compact closed-cycle helium gas refrigerators for thesuperconducting accelerator modulesPl. The cryogenic
system design goal was to realize low cost and easy
The cryogenic system consists of a 4Koperation.
refrigerator (Sumitomo SRJ-2008) and 20K/80K shieldrefrigerator (Sumitomo SRD-220). The 4K refrigerator
has the disadvantage of limitation of cooling power, but has
advantages of compact size and easy operation. The 4K
refrigerator is adopted to re-condense evaporated helium
gas inside the liquid helium container and to maintainliquid helium. The shield refrigerator that is a two stage
Gifford-McMahon cycle refrigerator cools duplex heatshields directly by heat conduction without liquid N2 or cold
helium gas.

He port

4K refrigcralor

Shield Refrigeralor

TunerFigure 1. Cross sectional view of the main accelerator module.

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2. Cryostat Design
Figure 1 shows a cross sectional view of a cryostat
for the main accelerator module. The 4K refrigerator
consists of a refrigerator unit, compressor unit and gas
helium line. The refrigerator unit is supported by asupporting frame over the accelerator module. A heatexchanger of the refrigerator unit is directly inserted in theliquid helium container and re-condenses evaporated
helium gas inside. Therefore a transport system for liquid
helium is not required in the JAERI FEL. Cooling powerof the 4K refrigerator is 11.5 watts at 4.3 K.
The shield refrigerator is attached to theaccelerator module and fixed with the supporting frame inorder to isolate vibration of the shield refrigerator. The
shield refrigerator cools duplex thermal radiation shields
and thermal anchors placed in the cryostat to reduce static
heat loads£21. The heat shields are made from 3 mm thickcopper sheet connected with cold heads of the shieldrefrigerator and cooled directly without liquid N2 • Cooling
capacities of the shield refrigerator are 20 watts at 20 K and
120 watts at 80 K.
Static heat load of the cryostat is about 3.5 watts andthe rfloss of the main accelerator module is for example 1.5
watts at Eacc=5MV/m and 3% duty pulse mode operation,
so the total refrigeration power of about 5 watts isrequired[ 3l. The 4K refrigerator has enough cooling power
for the main accelerator module.
For stable operation of the cryogenic systemespecially for pressure sensitive superconducting cavities, itis necessary to keep the pressure constant in the liquid
helium container. A heater that fixed on the bottom of theliquid helium container is used to compensate fluctuation ofheat load.
By the compensation, fluctuation of thepressure in the liquid helium container could be reduced
less than 2 hPa during operation of the JAERI FEL£41.

refrigerator were measured in pre-accelerator module #1.
The temperatures were about 42 K (first stage) and 10 K
(second stage). Static heat loads for the cold heads
estimated by the temperatures and cooling power diagram
were about 42 watts (first stage) and 6 watts (second stage).
(a)
JOO r-----,----,----~-----.-----r----,-----,II- - LHeVeuelBottom~
250 1-\\-\\---+-----t---- --t-----H ------- 20K Shield Bottom

\\\

~-

200I!

- ·- ·-·- BOK Shield Bottom

1-~i~~~~~~-----+----~----,r-----t---__,\\\'\

i'·, \1\i \\ISO 1--;..\-\-1 +---___:\,1------f------1---~----~------1II

100 ~--~\~----+-~~+------1------1---~-----1·,..'\UJo Transler

5o~--~~~~,~~~~±~~~==~~~~==~~~~'< ~-~-~-ll~--- ·=~--~~-

93.10.21

93.10.23

93.10.25

91.10.27

93.10.31

93.11.2

93.11.4

(b)
300

250

200

_l

~
i\ 1\.
.,,,.,,,

II-~

I'
\\i

\- - LHe v.....1Bottom\\\ I\\\

100\-.

3. Cool-down\\------· 20K Shield Bottom-·-·-·- 80K Shield Bottom

I~\'\\'· ' ' --....:-·- -- -~~-

93.11.11~........

50

Cool-down from room temperature to 4 K was
carried out by the 4 K refrigerator and shield refrigerator
without liquid N2 or cold helium gas. Before the cooldown, air in the liquid helium container was pumped out
and helium gas was re-filled from helium gas bottles. The
heat shields were cooled down by the shield refrigerator byheat conduction and the helium gas was cooled down by the
4 K refrigerator and heat convection and radiation coolingby the inner heat shield.
Cool-down curve of the temperatures measured on
the beat shields and liquid helium vessel are shown in fig.
2. It took about 7 days for pre-cooling to transfer liquid
helium from dewars.
Liquid helium was required
approximately 160 liters for the pre-accelerator module andabout 480 liters for the main accelerator module in thisprocess. We have tried to use liquid N2 for pre-cooling toshorten the cooling time.
Temperatures of the cold heads of the shield

93.10.29

Day

93.11.13

93.11.15~ !"-,------- -----

93.11.17

93.11.19--\ -------

93.11.21

93.11.23

93.11.25

DayFigure 2. Cool-down curves of the accelerator modules:(a) pre-accelerator module #2 and (b) main accelerator
module #1.
4. Operation
The refrigerators system has been operated routinely
without a specialist for the cryogenics through the year.
The refrigerator should be overhauled once a year and timespent for the overhaul is about a week. When themalfunction of the 4K refrigerator occurs, the refrigerator
and compressor units of the 4K refrigerator are replaced byspare units. It usually takes about four hours to exchange
and to adjust the 4K refrigerator and liquid helium
evaporates about 20 liters during the recovery.
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modifications of the 4K refrigerator, troubles could be
reduced to a few times a year[ 51.
Current of the heater fixed on the bottom of theliquid helium container is controlled by a PID loop to keep
the pressure constant. We can determine surplus cooling
capacity of the 4K refrigerator by the heater power. Inorder to get enough cooling capacity of the 4K refrigerator,a Joule-Thomson (J-T) valve of the 4K refrigerator should
be adjusted manually so to increase the cooling power, thatis, to increase the heater power. After the adjustment, themaximum surplus cooling capacity of the 4K refrigerator isabout 6 watts at 4.2 K.
5. Summary
The compact refrigerators system for the accelerator
modules has been driven successfully without a specialist
about cryogenics. By modifications of the 4K refrigerator,
troubles could be reduced to a few times a year. Therefrigerators are halted only during failures of electricity orwater supply.
The refrigerators were used for cooling thesuperconducting cavities without liquid N2 or cold heliumgas. It has proved that it took about 7 days for cool-downby this manner. We have been tried to use liquid N2 forpre-cooling to shorten the cooling time.
References[1] N.Kikuzawa et al., Nucl. Instr. and Meth. A331 (1993)
276.
[2] N.Kikuzawa et al., "Multi-refrigerators system
integrated into the superconducting accelerating cavity
cryostat", Proc. of the 5th workshop on RF
superconductivity, Hamburg, Germany, (1991) 577.
[3] N.Kikuzawa et al., "Superconducting Accelerator
Modules for the JAERI FEL", Proc. of the 9th Symp. on
Accelerator Science and Technology, Tsukuba, Japan,(1993) 440forControl"Pressureal.,et[4] N.Kikuzawa
Superconducting Accelerators at JAERI FEL", Proc. ofthe 22nd Linear Accelerator Meeting in Japan, Sendai,Japan, (1997).
[5] E.Minehara et al., "Recent Status of theSuperconducting Rf Linac Driver for the JAERI FEL",
Proc. of the 21st Linear Accelerator Meeting in Japan,Tokyo, Japan, (1996) 18.

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