Development of a Vibration-Reduction System of Cryocooler for a

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, Ibaraki
305-0801, 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 cryocooler
for a cryogenic interferometric gravitational wave detector, based on an experimental
vibration-analysis for commercial 4 K cryocoolers. In a preliminary test, the vibration
of the cold stage was reduced by two orders of magnitude using this vibration-reduction
system.
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,
−9
2
which is about 10 /f 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 vibrationreduction system of cryocooler for the CLIO.
2. Vibration Analysis for 4 K Cryocoolers
We measured the vibrations of a new commercial model of a 4 K pulse tube (PT)
cryocooler[3] and a widely used 4 K 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 Gravitationa
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 10 Hz. 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 Gravitationa
Figure 2. Power spectral densities of vibrations for the 4 K cryocoolers. The cold
head vibration for the GM cryocooler was measured by a piezo-electric accelerometer
and 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 cold
head, 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 Gravitationa
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 100 Hz. 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, 6 µmrms for the
original PT and 0.03 µmrms for the VRS at 1 Hz for the vertical direction.
4. Conclusion
We analyzed the vibrations of the 4 K 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.
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