Development of a Vibration-Reduction System of

Cryocooler for a Cryogenic Interferometric

Gravitational Wave Detector

Takayuki Tomaru†, Toshikazu Suzuki†, Tomiyoshi Haruyama†,Takakazu Shintomi†, Nobuaki Sato†, Akira Yamamoto†, Yuki

Ikushima‡, Tomohiro Koyama‡, and Rui Li‡† High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki305-0801, Japan‡ Sumitomo Heavy Industries Ltd., 2-1-1 Yato, Nishitokyo, Tokyo 188-8585, Japan

Abstract. We have developed a vibration-reduction system of pulse tube cryocoolerfor a cryogenic interferometric gravitational wave detector, based on an experimentalvibration-analysis for commercial 4 K cryocoolers. In a preliminary test, the vibrationof the cold stage was reduced by two orders of magnitude using this vibration-reductionsystem.

1. Introduction

The Cryogenic Laser Interferometer Observatory (CLIO)[1] is under construction in

Kamioka mine as a prototype of the Large-scale Cryogenic Gravitational wave Telescope

(LCGT)[2]. The goal displacement-sensitivity of the CLIO is on the same order as that

of the LCGT.

A technical issue concerning the CLIO is to cool the mirrors without conducting

mechanical vibrations to them. We introduce cryocoolers in the CLIO, since cryocoolers

are very useful owing to their convenient handling and long operation of the detector.

However, the cryocoolers can have large vibrations. The specification of vibration for

the cooling system in the CLIO is the ground-vibration level in the Kamioka mine,

which is about 10−9/f2 m/√

Hz.

At first, we investigated the vibration amplitude and the vibration mechanism for

commercial 4 K cryocoolers. Based on the measured results, we made a vibration-

reduction system of cryocooler for the CLIO.

2. Vibration Analysis for 4K Cryocoolers

We measured the vibrations of a new commercial model of a 4K pulse tube (PT)

cryocooler[3] and a widely used 4K Gifford-McMahon (GM) cryocooler[4], manufactured

by Sumitomo Heavy Industries Ltd. PT cryocoolers are expected to be quieter than

Development of a Vibration-Reduction System of Cryocooler for a Cryogenic Interferometric Gravitational Wave Detector2

Figure 1. Vibration measurement apparatus for cryocoolers. (a) Setup of the sensors,and (b) whole setup of the apparatus.

GM cryocoolers, since PT cryocoolers have no mechanical piston (displacer) in the cold

head.

Figure 1 shows the vibration measurement apparatus[5]. The cold stage vibration

was measured by an optical displacement-sensor. It was calibrated just before each

measurement by moving the sensor with a motor-driven X-stage. The vibration of the

whole cold head was measured by commercial accelerometers. For the PT cryocooler,

a rigid copper-tube, used to connect between the cold head and a rotary valve unit,

was anchored onto a block of 24 kg. For the GM cryocooler, flexible tubes were directly

connected to the cold head, and were also anchored onto the block. The measurement

apparatus and the compressor were partitioned by a steel door so as to reduce sound

vibration from the compressor.

Figure 2 shows the measured power spectral densities of the vibrations for the 4 K

cryocoolers at the cold stage and the cold head[6]. We found that the cold head of

the GM cryocooler had very large vibrations above 10Hz. The vibration was mainly

caused by the motion of the displacer. Since the PT cryocooler has no displacer, its

vibration amplitude of the cold head was two orders of magnitude smaller than that

of the GM cryocooler above 10 Hz. Unlike the cold head, the vibrations of the cold

stage, the sharp peaks of 1 Hz and their higher harmonics, for both cryocoolers were of

the same order of magnitude. From a spectral analysis of the pressure of the working

He gas and an ANSYS simulation, we found that the cold stage vibration came from

an elastic deformation of the ’pulse tube’ (cylinder) due to pressure oscillation of the

working gas.

3. Vibration-Reduction System Developed for a PT Cryocooler

The PT cryocooler is preferable to the GM cryocooler for the CLIO. However, its

vibration amplitude is still higher than the requirement for the CLIO. We have been

Development of a Vibration-Reduction System of Cryocooler for a Cryogenic Interferometric Gravitational Wave Detector3

Figure 2. Power spectral densities of vibrations for the 4 K cryocoolers. The coldhead vibration for the GM cryocooler was measured by a piezo-electric accelerometerand that for the PT cryocooler was measured by a laser accelerometer.

Figure 3. Vibration-reduction system we have been developing for the PT cryocooler.

developing a vibration-reduction system of the PT cryocooler (Figure 3).

To reduce the vibration of the cold head, the cold head was not fixed to the cryostat

directly, but to the support stage. The cold head was connected to the cryostat through a

soft bellows. Rubber sheets were put between folds of the bellows to damp the vibrations

through its surface. The rotary valve unit was separated by about 30 cm from the cold

head‡, and was fixed to a heavy table so as to reduce the reaction of the flexible tubes.

We are now measuring the vibration of the cold head of the PT cryocooler with the

vibration-reduction system.

‡ Since the cooling capacity of the cryocooler is reduced when the rotary valve unit is far from the coldhead, we set the valve unit as close as possible to the cold head.

Development of a Vibration-Reduction System of Cryocooler for a Cryogenic Interferometric Gravitational Wave Detector4

To reduce the vibration of the cold stage, a vibration-reduction stage (VRS), which

consists of new cold stages supported by rigid alumina-FRP rods, was introduced.

Another group also reported that the cold stage vibration was reduced by one order of

magnitude using a simple VRS[7]. The VRS was set below the top flange of the cryostat

so as to be independent of the cold head vibration. The lowest resonant frequencies of

the VRS was 100Hz. Heat link wires were connected between the cold stage and the

VRS. In a preliminary test, we observed a vibration-reduction of the cold stage by two

orders of magnitude smaller than that of the original PT cryocooler, 6µmrms for the

original PT and 0.03 µmrms for the VRS at 1Hz for the vertical direction.

4. Conclusion

We analyzed the vibrations of the 4K cryocoolers, and made a vibration-reduction

system of PT cryocooler for the CLIO. In this system, a VRS for the cold stage and a

support stage for the cold head were introduced. In a preliminary test, the cold stage

vibration was reduced by two orders of magnitude by using the VRS. The cold head

vibration is presenting being measured.

We plan to investigate the thermal and vibrational performances of one unit of the

cooling system, which is not only the cryocooler system, but also a main cryostat, in

the Kamioka mine. The cryostat of the CLIO is presenting being designed, and will be

manufactured in 2003. The performance of the cooling system will be investigated in

early 2004.

Acknowledgments

We express our appreciation to Dr. T. Shimonosono, Dr. Y. Ohtani and Dr. T.

Kuriyama at Toshiba Co., for their cooperation and advice during the early stage of

this work.

Reference

[1] M. Ohashi et al., Submitted to Class. Quantum Grav. (2003)[2] K. Kuroda et al., J. Mod. Phys. D 8, (1999) 557.[3] M. Y. Xu et al., Cryocoolers 12, Proc. of the 12th Int. Cryocooler Conf., (2003) 301.[4] Y. Ikeya et al., Cryocoolers 12, Proc. of 12th Int. Cryocooler Conf. (2003), 403.[5] T. Tomaru et al., Submitted to Meas. Sci. Tech. (2003)[6] T. Tomaru et al., J. of the Cryo. Soc. of Japan, 38 (2003)[7] C. Lienerth et al., Proc. of ICEC 18, (2000) 555.