Vibration-Free Pulse Tube Cryocooler System for Gravitational Wave Detectors I

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1 1 Vibration-Free Pulse Tube Cryocooler System for Gravitational Wave Detectors I - Vibration-Reduction Method and Measurement - T. Tomaru A, T. Suzuki A, T. Haruyama A, T. Shintomi A, N. Sato A, A. Yamamoto A, Y. Ikushima B, R. Li B, T. Akutsu C, T. Uchiyama C and S. Miyoki C A High Energy Accelerator Research Organization (KEK) 1-1 Oho, Tsukuba, Ibaraki, , Japan B Sumitomo Heavy Industries Ltd Yato, Nishitokyo, Tokyo, , Japan C Institute for Cosmic Ray Research (ICRR), The University of Tokyo 5-1-5, Kashiwanoha, Kashiwa, Chiba, , Japan ABSTRACT We developed a vibration-free cryocooler system based on a 4K pulse tube (PT) cryocooler for a cryogenic interferometric gravitational wave detector. In this system, we introduced a vibration-reduction system, which consist of a firm support for the cold head and a rigid cold table thermally linked to the cold stage, to reduce both vibrations of the overall cold head and the cold stage. Our cryocooler system reduced the cold-head vibrations of the based PT cryocooler by over three orders of magnitude, where the vibration level was on the order of 10 9 / f 2 m/ Hz. INTRODUCTION A vibration-free cryocooler can be useful. Especially, it is essential in cryogenic interferometric gravitational wave detectors 1, since vibration of the optical mirrors is one of the sensitivity limitations of the detectors. Figure 1 shows a schematic diagram of a cooling system of the Cryogenic Laser Interferometer Observatory (CLIO) 2, which is a proto-type cryogenic gravitational wave interferometer being constructed in Japan. An optical mirror is cooled by conduction cooling using a vibration-free cryocooler. Vibration-free can be defined as the seismic vibration level, since all types of apparatuses are always moved by the seismic vibration, which can not be eliminated unless using special vibration isolators. The requirement concerning the vibration of the cryocooler in the CLIO is the seismic vibration level in Kamioka mine (the site of the CLIO; seismic vibration level, 10 9 / f 2 m/ Hz ), and the required cooling power of the cryocooler is 15W at 40K for the first stage and 0.5W at 4.2K for the second stage.

2 2 Vibration Isolation System Vibration Isolater (Pendulum) Cryostat 4.2K Heat Conductor Intermediate Mass 10K Vibration Isolater for Heat Link Cryocooler Suspension Fiber Heat Link Wire Heat Flow Laser Beam Mirror 20K Vacuum Cold Head Cylinder Laser Power Absorption Radiation Shield Cold Stage Figure 1. Schematic diagram of a mirror cooling system used in cryogenic interferometric gravitational wave detectors. Since the mirror has small laser power absorption (heat generation), it must be cooled continuously through heat links. In our previous study 3, we confirmed that the vibration of a 4K pulse tube (PT) cryocooler was much smaller than that of a 4K Gifford-McMahon (GM) cryocooler i. However, even for the PT cryocooler, the observed vibration levels were too large for the requirement of the CLIO. Therefore, we developed a vibration-reduction system of the PT cryocooler for the CLIO. In this system, vibrations of the overall cold head and the cold stage, which are different vibration components observed in our previous study, were reduced. (The names of parts of the cryocoolers used in this paper are defined in Fig. 1.) In this paper, we report on the vibration-reduction method and the measurement for a vibration reduction of the overall cold head. Measurements of the cooling performance and the vibration reduction of the cold stage are reported by Dr. R. Li in Vibration-Free Pulse Tube Cryocooler System for Gravitational Wave Detectors II -Cooling Performance and Vibration-. VIBRATION-REDUCTION SYSTEM Investigation of Vibrations for a 4K PT Cryocooler Figure 2 shows vertical vibration spectra for a 4K PT cryocooler (SRP-052A, Sumitomo Heavy Industries Ltd. 4 ) measured in our previous study 3. The spectra can be described based on the displacement density 5, which is defined as 1 T / 2 x ( ω) = lim x t T ( ) e i ω t dt T T 2 [ m Hz], (1) where ω is the angular frequency, T is the time length and x(t) is the displacement as a function of time i The vibration of the cold head for the 4K PT cryocooler was two orders of magnitude less than that of the 4K GM cryocooler. On the other hand, the cold-stage vibrations for both cryocoolers were at the same level. In this measurement, we used a setup of the PT cryocooler such that the rotary valve unit was separated from the cold head and they were connected by a rigid copper tube, which was anchored onto the ground.

3 3 Figure 2. Vertical vibration spectra for a Sumitomo 4K PT cryocooler. The black lines show the vibration of the cryocooler and the gray lines show the sensor noise (background noise). The features of the vibrations include two points: one is the cold-stage vibration with the driving frequency (1Hz) and its higher harmonics; other is the cold-head vibration with the frequency components above about 50Hz. From a spectrum analysis of the pressure oscillation of the working gas and an ANSYS simulation, we identified that this cold-stage vibration came from an elastic deformation of the cylinder due to the pressure oscillation of the working gas. Therefore, this is an inevitable problem for cryocoolers using oscillating gas. The cold-head vibration mainly came from reactions of a connecting tube between the cold head and the rotary valve unit, and so on. Vibration-Reduction System Based on the above study, we designed a vibration-reduction system for the 4K PT cryocooler. The design principle of this system is to separate all vibration sources from the cryostat and to anchor them to the ground. Figure 3 shows a schematic diagram of the system and Fig. 4 shows photographs of the system. Figure 3. Schematic diagram of the vibration-reduction system for a 4K PT cryocooler.

4 4 Figure 4. Photographs of the developed vibration-reduction system. (a) Support frame of the cold head and the rotary valve table; (b) Vibration-reduction stage for the cold stage. To reduce the cold-head vibration of the PT cryocooler, we introduced a rigid frame to support the cold head, named support frame. The cold head was mounted on the support frame and a welding bellows was connected between the cold head and a cryostat. A rotary valve unit, which could generate large vibrations, was not attached to the cold head directly, but was fixed onto an independent heavy table (rotary valve table). A connecting tube between the cold head and the rotary valve unit was also fixed on the rotary valve table. Flexible tubes from the compressor were converted to rigid pipes, and connected to the rotary valve unit. The pipes were also clamped onto the rotary valve table. To reduce the cold-stage vibration, we used a rigid table, named vibration-reduction stage. The vibration reduction for the cold stage by using a vibration-reduction stage has been reported by Lienerth et al. 6 An improved point concerning our vibration-reduction stage is that we set it under a lower flange of the bellows (named lower flange ), which was installed on the cryostat. Since the lower flange has been quiet owing to being separated from the cold head, we can expect an absolute reduction of the cold-stage vibration by the vibration-reduction stage. The vibration-reduction stage consisted of eight alumina-frp pipes, a copper plate and about forty heat-link cables for each stage. The reason why we used alumina-frp is that it has a smaller thermal conductivity at 4K, and a two times larger Young s modulus than those of glass-frp. As the heat link, we used copper stranded cables for the first stage and pure aluminum stranded cables (purity, 99.99%; size of each wire, φ0.1mm; total number of wires in a cable, 1666) for the second stage, because the thermal conductivities of the metals strongly depend on their purities around 4K at the second stage; in contrast, they had a weak dependence of their purities around 40K at the first stage. The reason why stranded cables with many thin wires were used is because the spring constant of the heat link is inversely proportional to the number of wires under the condition of a constant heat flow; therefore, the vibration conduction of the heat link could be reduced. VIBRATION MEASUREMENT OF OVERALL SYSTEM Since the vibration level of our system has been below the seismic vibration level in typical urban areas in our preliminary measurements, we checked the vibrations of the overall cryocooler system in Kamioka mine in Japan to reduce the background vibration noise. It is known that the seismic vibration in Kamioka mine is extremely small, the spectrum of which is about 10 9 / f 2 m/ Hz above a few hertz and two orders of magnitude smaller than that in the typical urban area.

5 5 Figure 5. Schematic diagram of the experimental setup for vibration measurements of the overall cryocooler system (See Fig. 4). Vibrations of our cryocooler system were measured by a laser accelerometer through a jig. Figure 5 shows a schematic diagram of the experimental setup. The cold head was set onto a support frame through four posts of φ100mm in diameter and 90mm in height. The support frame was 690mm in height and 670mm in width, and consisted of H-beams of 100mm in width. The lower flange of the bellows was mounted onto a cryostat. The length of the connecting tube between the rotary valve unit and the cold head was about 400mm, and the weight of the rotary valve table was about 100kg. The cryostat, the support frame and the rotary valve table were anchored onto the ground by thick bolts. The flexible tubes were 20m in length and put on the ground in a coil. The distance between the cold head and the compressor was about 5m. During the measurements, vacuum pumps were stopped so as to eliminate any influence of their vibration. We used a laser accelerometer (LA-50, Rion Co.) to measure the vibrations. The measured positions were on the ground and on the lower flange. Since the laser accelerometer had a large size, and it was difficult to set it onto the lower flange directly, we set it there by using a jig, which was a stainless-steel plate of 50mm wide, 20mm thick and 190mm long. Vibration spectra were acquired by a spectrum analyzer (Agilent 35670A). To achieve sufficient frequency resolutions, we acquired the data by dividing them into three frequency ranges: 0.031Hz resolution below 12.5Hz, 0.25Hz resolution between 12.5Hz and 100Hz, and 2Hz resolution between 100Hz and 800Hz. In this paper, we described the spectra in the displacement density defined by Eq. (1). Figure 6 shows the measured results. The vibration level at the lower flange was two orders of magnitude smaller than the seismic vibration level in a typical urban area (our laboratory in KEK), and almost the same as the seismic vibration level in Kamioka mine. Since we observed that the seismic vibration in the Kamioka mine sometimes changed due to a change of the surrounding situations, and the maximum difference in the change was about one order of magnitude, the floor level in the seismic vibration spectrum could change by about one order of magnitude. Several jumps at the floor in the spectra also came from such a change of the surrounding situations, because the data were not acquired at the same time for all frequency ranges. A large peak at around 100Hz came from a resonance of the jig for the laser accelerometer; we confirmed that this vibration did not come from the cryocooler system. We observed sharp peaks at the driving frequency (1.2Hz) and its higher harmonics in the vibration spectrum of the lower flange. After moving the compressor to outside of the experimental room, which were partitioned by a concrete-block wall, these sharp peaks disappeared. Figure 7 shows a comparison of the vibrations at the lower flange when the compressor was set inside and outside of the experimental room. Based on this result, these sharp peaks could come from the motion of the compressor and the flexible tubes.

6 6 Figure 6. Vertical vibration spectra of the overall vibration-reduction cryocooler system. The black line shows the vibration at the lower flange, the gray line shows the seismic vibration (background noise) in Kamioka mine and the dotted line shows the seismic vibration in a typical urban area (KEK). Figure 7. Comparison of the vertical vibration spectra when the compressor located outside of the experimental room (black line) and inside (gray line), which were partitioned by a concrete-block wall.

7 7 Figure 8. Vertical vibration spectrum of the based PT cryocooler (black line). The dotted line shows the vibration spectrum at the lower flange of the vibration-reduction cryocooler system when the compressor was located outside of the experimental room, and the gray line shows the sensor noise of the piezo-electric accelerometer used in the vibration measurement of the based PT cryocooler. We also measured the cold-head vibration of the based 4K PT cryocooler in order to compare its vibration with that of our cryocooler system. We used the standard setup of the PT cryocooler, in which the rotary valve unit was attached to the cold head directly, the flexible tubes were connected to the rotary valve unit directly and the cold head was mounted on the cryostat directly. Figure 8 shows the measured result. Since we could not measure the vibration of the based PT cryocooler by the laser accelerometer, due to its large vibration, we used a piezoelectric accelerometer (Model 710, TEAC Co.) for the vibration measurement of the based PT cryocooler. Therefore, the sensor noise level of the piezo-electric accelerometer was larger than that of the laser accelerometer used in the measurement for the vibration-reduction cryocooler system. Above the 100Hz frequency range, the vibration of the based PT cryocooler was over three orders of magnitude larger than that of the vibration-reduction cryocooler system. The motions of the rotary valve unit and the flexible tubes could generate this large vibration. From the above results, we concluded that our vibration-reduction system reduced the coldhead vibration of the based PT cryocooler, and that the vibration level was almost the same as the seismic vibration level in Kamioka mine. CONCLUSION We developed a vibration-reduction system of a 4K PT cryocooler for the CLIO. The system consisted of a support frame and a vibration-reduction stage. The vibration sources in the cryocooler were separated from the cryostat and anchored onto the ground. From the vibration measurement for the overall system, we confirmed that the vibration of the system was more

8 8 than three orders of magnitude smaller than that of the based PT cryocooler. This vibration level was much smaller than the seismic vibration level in a typical urban area (10 7 / f 2 m/ Hz ), and was almost the same as the seismic vibration level in Kamioka mine (10 9 / f 2 m/ Hz ). Therefore, we concluded that our cryocooler system could be designated as being vibrationfree. ACKNOWLEDGMENT This study was supported by a grant-in-aid for Scientific Research on Priority Area ( ) prepared by Ministry of Education, Culture, Sports, Science and Technology in Japan. REFERENCES 1. Kuroda K. et al., Large-Scale Cryogenic Gravitational Wave Telescope, Int. J. Mod. Phys. D Vol. 8, (1999) p Ohashi, M. et al., Design and Construction Status of CLIO, Class. Quantum Grav. Vol. 20 No. 17 (2003) S599-S Tomaru, T. et al., Vibration Analysis of Cryocoolers, Accepted to Cryogenics (2004) 4. Xu M. Y. et al., Cryocoolers Vol. 12, Proc. of the 12th Int. Cryocooler Conf., (2003) p Bendat J. S. and Piersol A. G., "Random Data: Analysis and Measurement Procedures", Jown Wiley & Sons, Inc., C. Lienerth et al., Proc. of ICEC Vol. 18, (2000) p.555.