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Installation of superconducting gravimeter in the Antarctica
Physica C 426–431 (2005) 759–763 www.elsevier.com/locate/physc
Installation of superconducting gravimeter in the Antarctica H. Ikeda a
a,*
, K. Doi b, Y. Fukuda c, K. Shibuya b, R. Yoshizaki
a
Research Facility Center for Science and Technology Cryogenics Division, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan b National Institute of Polar Research, Tokyo, Itabashi-ku 1-9-10, Kaga 173-8515, Japan c Department of Geophysics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Received 23 November 2004; accepted 16 February 2005 Available online 11 July 2005
Abstract In February 2003 to January 2004, a new superconducting gravimeter with a cryocooler was installed to replace the former one in Syowa Station on the Antarctica. It has a high sensitivity of one nano-Gal to survey inside the earth in the Global Geodynamics Project (GGP network). A new type of diaphragm was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. Real time remote monitoring system from Japan is also established. Ó 2005 Elsevier B.V. All rights reserved. PACS: 04.80.y; 43.40.Tm; 93.30.Sq; 93.85.+q Keywords: Superconducting gravimeter; Refrigerator; Diaphragm; Monitoring system; Antarctica
1. Introduction The superconducting gravimeter is the worldÕs most sensitive and stable gravimeter. With a sensitivity of one nano-Gal of surface gravity, precise measurements of earth tide parameters and the nearly diurnal free wobble of the earth can be made [1,2]. This high sensitivity will enable the
*
Corresponding author. Tel.: +81 29 853 2484; fax: +81 29 853 2482. E-mail address: [email protected] (H. Ikeda).
Global Geodynamics Project to search for internal gravity waves in the earthÕs liquid core and ‘‘slow or silent’’ earthquakes, especially causes of longterm gravity variations and the influence of environmental effects on gravity. The stability of the superconducting gravimeter approaches a few micro-Gal per year which makes it invaluable for geodetic purposes, such as monitoring sea-level changes and tectonic deformations [3]. In 1993 the first superconducting gravimeters was introduced in Syowa Station in the Antarctica by the 34th Japanese Antarctic research expedition members. The superconducting gravimeter can run
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.078
760
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
more than 10 years continuous and discovered the EarthÕs free oscillations [4]. From February 5, 2003 to January 31, 2004, a new superconducting gravimeter with a cryocooler was installed to replace the former one in Syowa Station on the Antarctica. We developed a new type of diaphragm which was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. Real time remote monitoring system from Japan is also established.
2. Superconducting gravimeter In 1980 the first superconducting gravimeters became commercially available and since then they have been operated very successfully worldwide [3]. These instruments provide long-term observations series, which are essential to improved understanding of the time-dependent gravity field, especially causes of long-term gravity variations and the influence of environmental effects on gravity. The excellent results were obtained result from the very low noise and drift characteristics of the superconducting gravimeter. Superconducting gravimeter consists of two basic components: (1) the Gravimeter Sensing Unit which includes: the superconducting magnets, the sphere, circuitry for energizing the coils, temperature control circuitry and magnetic shielding and (2) the liquid helium Dewar and refrigeration system which keeps the gravimeter sensing unit close to 4.2 K to maintain the superconducting state. As shown in Fig. 1, the gravimeter sensing unit contains a 2.54 cm diameter spherical proof mass. The sphere (Nb) is levitated by the forces produced by magnetic fields generated from a pair of superconducting coils. Since the sphere is superconducting, it behaves as a perfect diamagnetic so that surface currents are generated which exactly cancel and exclude any applied magnetic field from its interior. It is the interaction between the sphereÕs surface currents and the applied magnetic field that produce the levitation force. The vertical magnetic gradient (‘‘spring constant’’) can be made very weak by adjusting the ratio of currents in two field coils. The use of trapped persistent
Fig. 1. Gravity-sensing unit inside the Dewar vessel.
super-currents to produce an ultra stable levitation force accounts for the unprecedented long-term stability of the superconducting gravimeter in comparison to mechanical spring type gravimeters. A capacitance bridge network consisting of three spherical capacitor plates positioned around the sphere senses the position of the sphere. The sphere capacitively couples these excitation signals to the center plate of the bridge. When the sphere is equidistant from the upper and lower plates, the drive signals cancel and the resulting signal on the center plate is zero. When changes in gravity cause the sphere to move from its null position, it produces an error signal that is linear in displacement. During operation, the position of the sphere is held close to its null position by a feedback circuit, which applies a magnetic force through a separate feedback coil. Since the force from feedback coil is linear with current, measuring the current through the feedback coil provides a linear measurement of the force of gravity.
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
761
The gravity sensor is surrounded by a superconducting magnetic shield for eliminate effects from changes in the external magnetic field. It is also enclosed in a vacuum can and temperature regulated to a few mK. This makes the sensor completely insensitive to environmental effects such as changes in external temperature, humidity and barometric pressure. The vacuum can is sealed inside the liquid helium Dewar during assembly at the factory. Electrical leads are brought out through the top end of the Dewar through a fiberglass neck tube and terminated at the head of the instrument. Great care is taken in design and manufacturing of the neck, and signal leads to insure that heat leaks into the liquid helium reservoir are minimized. High quality environmental connectors allow interfacing the gravity sensor and its subsystems to an electronics package that resides external to the Dewar. The Dewar refrigeration system consists of a newly designed Dewar interfaced with a cryocooler capable of obtaining temperatures below the vaporization point of liquid helium. The system is based on the Coolpower 4.2 LAB cryocooler manufactured by Leybold Vacuum Products Inc. This Gifford–Mcmahon (GM) type cryocooler uses a mechanically driven piston and offers superior cooling performance, as well as greatly reduced noise and vibration compared to previous gas driven cryocoolers. The lower stage of the
cryocooler can cool below the vaporization temperature of liquid helium. This allows boiled off helium gas to condense at this stage. Therefore, during normal operation the system consumes no liquid helium and will operate indefinitely. This two-stage cryocooler delivers a maximum cooling power of 4 W at 60 K to its upper stage and 0.25 W at 4.2 K to its lower stage.
Fig. 2. Initial cool-down curve for superconducting gravimeter.
Fig. 3. Component of a new superconducting gravimeter.
3. Set up and initial cool-down In February 2003, a new superconducting gravimeter was installed in an observation Syowa Station at 69°00 0 S and 39°35 0 E on East Ongul Island, Lutzow-Holm Bay, East Antarctica. Fig. 2 shows initial cool-down curve for the superconducting gravimeter. The temperature distribution measured along the neck tube monitoring by using carbon resistance thermometer. We used pre-cool down liquid nitrogen and then transfer liquid helium. The results indicate that cooling by the liquid helium acts effectively. When the refrigerator working neck tube final temperature distribution, upper neck (1st stage) about 60 K
762
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
and lower neck (2nd stage) is 4 K. We waited the refrigerator is working until liquid helium level 88%. The superconducting gravimeter was carried out March 18, 2003 using upper and lower superconducting coil for the first levitation. The gravimeter is adjusted in position of the sphere. Finally the data recording started on April 18, 2003.
4. Diaphragm and remote monitoring system
Fig. 4. A communication course between the Antarctica and Japan by virtual private network (VPN) used by S-Box.
Fig. 3 shows schematically component of a new superconducting gravimeter. The isolation bellows provides a seal while isolating the Dewar from the vibrations of the cold-head. The diaphragm is made of rubber and attaches to the gravimeter head on one side and the cold-head support plate on the other side. However, air atoms penetrate a rubber diaphragm at room temperature. They result in a large amount of solid-air in a refrigerator growing in a long-term operation. To prevent
Fig. 5. Real-time remote monitoring system from Japan. Recently occurred earthquake in Japan at Nigata prefecture.
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
air penetration we employed an Al-coated polyurethane diaphragm. The materials no transmit helium and air molecule at every temperatures. We confirmed a new type of diaphragm to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly for eleven months [5]. The communications satellite (INTELSAT) data receiving system with an 11 m diameter parabolic antenna was installed at Syowa Station in February 2004. Fig. 4 shows the communication course between the Antarctica and Japan by virtual private network (VPN) used by S-Box. August 26, 2004 we started real-time remote monitoring system from Japan and it has also been established [6]. As shown in Fig. 5, recently occurred earthquake in Japan at Nigata prefecture observation at Syowa Station of the superconducting gravimeter. In addition Web camera monitoring from Japan is started November 2004. Then refrigerators sound real-time monitoring is possible. 5. Conclusion We have succeed in a new superconducting gravimeter with a cryocooler which was installed to replace the former one in Syowa Station on the Antarctica. The 4 K cryocooler re-condenses the helium gas as it boils off from the liquid helium bath stored inside the Dewar. In this manner, the
763
Dewar operates as a closed cycle system with no loss of coolant as long as power is maintained to the refrigerator. We developed a new type of diaphragm which was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. In addition real-time remote monitoring system from Japan is also established.
Acknowledgement We would like to thank the 44th Japanese Antarctic research expedition members, for the installation support.
References [1] B. Richter, Proc. IAG Hamburg, vol. 1, Dept. Geod. Sci. Surv., OSU, OH, 1983, p. 204. [2] B. Richter, in: Proc. Das supraleitende Gravimeter, Frankfurt Deut. Geod. Komm., 1987, p. 329. [3] R. Warburton, E. Brinton, in: Proc. Second Workshop: Non-Tidal Gravity Changes, Les Cahiers du Centre European de Geodynamique et Seismologie, 11, Luxembourg, 1995, p. 23. [4] K. Nawa, N. Suda, Y. Fukao, T. Sato, Y. Aoyama, K. Shibuya, Earth Planet Space 50 (1998) 3. [5] H. Ikeda, K. Doi, Y. Fukuda, T. Noguchi, Y. Tamura, T. Nakashima, K. Iimura, K. Shibuya, J. Cryo. Soc. Jpn. 39 (8) (2004) 348. [6] K. Iimura, T. Nakashima, IEEJ IIC-99-44 (1999) 69.
Installation of superconducting gravimeter in the Antarctica H. Ikeda a
a,*
, K. Doi b, Y. Fukuda c, K. Shibuya b, R. Yoshizaki
a
Research Facility Center for Science and Technology Cryogenics Division, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan b National Institute of Polar Research, Tokyo, Itabashi-ku 1-9-10, Kaga 173-8515, Japan c Department of Geophysics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Received 23 November 2004; accepted 16 February 2005 Available online 11 July 2005
Abstract In February 2003 to January 2004, a new superconducting gravimeter with a cryocooler was installed to replace the former one in Syowa Station on the Antarctica. It has a high sensitivity of one nano-Gal to survey inside the earth in the Global Geodynamics Project (GGP network). A new type of diaphragm was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. Real time remote monitoring system from Japan is also established. Ó 2005 Elsevier B.V. All rights reserved. PACS: 04.80.y; 43.40.Tm; 93.30.Sq; 93.85.+q Keywords: Superconducting gravimeter; Refrigerator; Diaphragm; Monitoring system; Antarctica
1. Introduction The superconducting gravimeter is the worldÕs most sensitive and stable gravimeter. With a sensitivity of one nano-Gal of surface gravity, precise measurements of earth tide parameters and the nearly diurnal free wobble of the earth can be made [1,2]. This high sensitivity will enable the
*
Corresponding author. Tel.: +81 29 853 2484; fax: +81 29 853 2482. E-mail address: [email protected] (H. Ikeda).
Global Geodynamics Project to search for internal gravity waves in the earthÕs liquid core and ‘‘slow or silent’’ earthquakes, especially causes of longterm gravity variations and the influence of environmental effects on gravity. The stability of the superconducting gravimeter approaches a few micro-Gal per year which makes it invaluable for geodetic purposes, such as monitoring sea-level changes and tectonic deformations [3]. In 1993 the first superconducting gravimeters was introduced in Syowa Station in the Antarctica by the 34th Japanese Antarctic research expedition members. The superconducting gravimeter can run
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.078
760
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
more than 10 years continuous and discovered the EarthÕs free oscillations [4]. From February 5, 2003 to January 31, 2004, a new superconducting gravimeter with a cryocooler was installed to replace the former one in Syowa Station on the Antarctica. We developed a new type of diaphragm which was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. Real time remote monitoring system from Japan is also established.
2. Superconducting gravimeter In 1980 the first superconducting gravimeters became commercially available and since then they have been operated very successfully worldwide [3]. These instruments provide long-term observations series, which are essential to improved understanding of the time-dependent gravity field, especially causes of long-term gravity variations and the influence of environmental effects on gravity. The excellent results were obtained result from the very low noise and drift characteristics of the superconducting gravimeter. Superconducting gravimeter consists of two basic components: (1) the Gravimeter Sensing Unit which includes: the superconducting magnets, the sphere, circuitry for energizing the coils, temperature control circuitry and magnetic shielding and (2) the liquid helium Dewar and refrigeration system which keeps the gravimeter sensing unit close to 4.2 K to maintain the superconducting state. As shown in Fig. 1, the gravimeter sensing unit contains a 2.54 cm diameter spherical proof mass. The sphere (Nb) is levitated by the forces produced by magnetic fields generated from a pair of superconducting coils. Since the sphere is superconducting, it behaves as a perfect diamagnetic so that surface currents are generated which exactly cancel and exclude any applied magnetic field from its interior. It is the interaction between the sphereÕs surface currents and the applied magnetic field that produce the levitation force. The vertical magnetic gradient (‘‘spring constant’’) can be made very weak by adjusting the ratio of currents in two field coils. The use of trapped persistent
Fig. 1. Gravity-sensing unit inside the Dewar vessel.
super-currents to produce an ultra stable levitation force accounts for the unprecedented long-term stability of the superconducting gravimeter in comparison to mechanical spring type gravimeters. A capacitance bridge network consisting of three spherical capacitor plates positioned around the sphere senses the position of the sphere. The sphere capacitively couples these excitation signals to the center plate of the bridge. When the sphere is equidistant from the upper and lower plates, the drive signals cancel and the resulting signal on the center plate is zero. When changes in gravity cause the sphere to move from its null position, it produces an error signal that is linear in displacement. During operation, the position of the sphere is held close to its null position by a feedback circuit, which applies a magnetic force through a separate feedback coil. Since the force from feedback coil is linear with current, measuring the current through the feedback coil provides a linear measurement of the force of gravity.
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
761
The gravity sensor is surrounded by a superconducting magnetic shield for eliminate effects from changes in the external magnetic field. It is also enclosed in a vacuum can and temperature regulated to a few mK. This makes the sensor completely insensitive to environmental effects such as changes in external temperature, humidity and barometric pressure. The vacuum can is sealed inside the liquid helium Dewar during assembly at the factory. Electrical leads are brought out through the top end of the Dewar through a fiberglass neck tube and terminated at the head of the instrument. Great care is taken in design and manufacturing of the neck, and signal leads to insure that heat leaks into the liquid helium reservoir are minimized. High quality environmental connectors allow interfacing the gravity sensor and its subsystems to an electronics package that resides external to the Dewar. The Dewar refrigeration system consists of a newly designed Dewar interfaced with a cryocooler capable of obtaining temperatures below the vaporization point of liquid helium. The system is based on the Coolpower 4.2 LAB cryocooler manufactured by Leybold Vacuum Products Inc. This Gifford–Mcmahon (GM) type cryocooler uses a mechanically driven piston and offers superior cooling performance, as well as greatly reduced noise and vibration compared to previous gas driven cryocoolers. The lower stage of the
cryocooler can cool below the vaporization temperature of liquid helium. This allows boiled off helium gas to condense at this stage. Therefore, during normal operation the system consumes no liquid helium and will operate indefinitely. This two-stage cryocooler delivers a maximum cooling power of 4 W at 60 K to its upper stage and 0.25 W at 4.2 K to its lower stage.
Fig. 2. Initial cool-down curve for superconducting gravimeter.
Fig. 3. Component of a new superconducting gravimeter.
3. Set up and initial cool-down In February 2003, a new superconducting gravimeter was installed in an observation Syowa Station at 69°00 0 S and 39°35 0 E on East Ongul Island, Lutzow-Holm Bay, East Antarctica. Fig. 2 shows initial cool-down curve for the superconducting gravimeter. The temperature distribution measured along the neck tube monitoring by using carbon resistance thermometer. We used pre-cool down liquid nitrogen and then transfer liquid helium. The results indicate that cooling by the liquid helium acts effectively. When the refrigerator working neck tube final temperature distribution, upper neck (1st stage) about 60 K
762
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
and lower neck (2nd stage) is 4 K. We waited the refrigerator is working until liquid helium level 88%. The superconducting gravimeter was carried out March 18, 2003 using upper and lower superconducting coil for the first levitation. The gravimeter is adjusted in position of the sphere. Finally the data recording started on April 18, 2003.
4. Diaphragm and remote monitoring system
Fig. 4. A communication course between the Antarctica and Japan by virtual private network (VPN) used by S-Box.
Fig. 3 shows schematically component of a new superconducting gravimeter. The isolation bellows provides a seal while isolating the Dewar from the vibrations of the cold-head. The diaphragm is made of rubber and attaches to the gravimeter head on one side and the cold-head support plate on the other side. However, air atoms penetrate a rubber diaphragm at room temperature. They result in a large amount of solid-air in a refrigerator growing in a long-term operation. To prevent
Fig. 5. Real-time remote monitoring system from Japan. Recently occurred earthquake in Japan at Nigata prefecture.
H. Ikeda et al. / Physica C 426–431 (2005) 759–763
air penetration we employed an Al-coated polyurethane diaphragm. The materials no transmit helium and air molecule at every temperatures. We confirmed a new type of diaphragm to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly for eleven months [5]. The communications satellite (INTELSAT) data receiving system with an 11 m diameter parabolic antenna was installed at Syowa Station in February 2004. Fig. 4 shows the communication course between the Antarctica and Japan by virtual private network (VPN) used by S-Box. August 26, 2004 we started real-time remote monitoring system from Japan and it has also been established [6]. As shown in Fig. 5, recently occurred earthquake in Japan at Nigata prefecture observation at Syowa Station of the superconducting gravimeter. In addition Web camera monitoring from Japan is started November 2004. Then refrigerators sound real-time monitoring is possible. 5. Conclusion We have succeed in a new superconducting gravimeter with a cryocooler which was installed to replace the former one in Syowa Station on the Antarctica. The 4 K cryocooler re-condenses the helium gas as it boils off from the liquid helium bath stored inside the Dewar. In this manner, the
763
Dewar operates as a closed cycle system with no loss of coolant as long as power is maintained to the refrigerator. We developed a new type of diaphragm which was confirmed to well isolate the vibration from refrigerator cold-head and to prevent the solid air contamination perfectly. In addition real-time remote monitoring system from Japan is also established.
Acknowledgement We would like to thank the 44th Japanese Antarctic research expedition members, for the installation support.
References [1] B. Richter, Proc. IAG Hamburg, vol. 1, Dept. Geod. Sci. Surv., OSU, OH, 1983, p. 204. [2] B. Richter, in: Proc. Das supraleitende Gravimeter, Frankfurt Deut. Geod. Komm., 1987, p. 329. [3] R. Warburton, E. Brinton, in: Proc. Second Workshop: Non-Tidal Gravity Changes, Les Cahiers du Centre European de Geodynamique et Seismologie, 11, Luxembourg, 1995, p. 23. [4] K. Nawa, N. Suda, Y. Fukao, T. Sato, Y. Aoyama, K. Shibuya, Earth Planet Space 50 (1998) 3. [5] H. Ikeda, K. Doi, Y. Fukuda, T. Noguchi, Y. Tamura, T. Nakashima, K. Iimura, K. Shibuya, J. Cryo. Soc. Jpn. 39 (8) (2004) 348. [6] K. Iimura, T. Nakashima, IEEJ IIC-99-44 (1999) 69.