2006 Fiscal Year Final Research Report Summary
Measurements and analysis of magnetic noises arising from the second-stage pulse-tube refrigerator for the development of liquid helium less LTS-SQUID biomagnetic fields measurement system
| Project/Area Number |
15206048
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Measurement engineering
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| Research Institution | KYUSHU UNIVERCITY |
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Principal Investigator |
SASADA Ichiro Kyushu University, Faculty of Eng. Sci., Prof., 大学院総合理工学研究院, 教授 (20117120)
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| Co-Investigator(Kenkyū-buntansha) |
ENPUKU Keiji Kyushu University, Faculty of Information Science and Electrical Engineering, 大学院システム情報科学研究院, 教授 (20150493)
TASHIRO Kunihisa Kyushu University, Faculty of Eng.Sci., Research Associate, 大学院総合理工学研究院, 助手 (50325487)
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| Project Period (FY) |
2003 – 2006
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| Keywords | 4.2 K Pulse tube refrigerator / magnetic shield / biomagnetic field measurement / SQUID gradiometer / DROS / Conducted EMI / Fluxgate magnetometer / CFRP composited magnetic shield |
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| Research Abstract |
We have developed a second-stage pulse-tube refrigerator (hereafter PT) with a cooling power greater than 0.1 W at 4.2 K. Its unique aspect is that the PT and a valve motor is separated by about 2 m with three extended flexible hoses to avoid strong low- frequency magnetic interferences from the motor. We have characterized magnetic noises arising from the PT in operation with the temperature, T, of the second stage. In order to perform this, several key issues have been addressed : cleaning conducted EMI from an inverter to the PT that destroys the operation of the SQUID, fabrication of highly stable and sensitive (20 nT【square root】Hz) orthogonal fluxgate magnetometers (hereafter FG) of a miniature-size, and fabrication of a CFRP composited cylindrical magnetic shield of high shielding factor to reduce static environment magnetic field around the PT. A miniature FG was installed on the inner wall of an Aluminum Dewar so that the FG should not vibrate and be kept at the room temperatu
… More
re. From the measurements of the 1.8 Hz (pumping frequency of the He-gas) component of the magnetic noise with FG, we found that there are two components : one becomes prominent below about T=10 K and increases with 1/T and the other is smaller than the first and almost constant over wide range of the temperature above about 10 K. The former is attributed to the periodical variation of the magnetic susceptibility of the paramagnetic regenerative materials such as Er3Ni and HoCu2, on the other hand the latter is attributed to any mechanical variation of the constructing parts made of stainless steel. We further characterized by using a planar gradiometer of DROS (double relaxation oscillation SQIUD prepared by KRISS) attached to the second-stage. The 1.8 Hz component observed was as large as 0.5 nT and followed by the second and the third harmonics. It would be interesting to note that the harmonics of the magnetic noises gradually disappear beyond the orders of 5 and 6 and appear again around 20 Hz (near 10th order) which may be due to mechanical vibration of the SQUID in the static magnetic field. Less
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