Yasuharu Kamioka

Subcooled liquid nitrogen refrigerator for HTS power systems

Subcooled liquid nitrogen is a good cooling medium of high temperature superconducting (HTS) electric power systems such as an electric power line and a power transformer. To produce subcooled liquid nitrogen, a cryocooler is used and a circulation pump is installed in the system. Several subcooled liquid nitrogen circulation systems were constructed and tested. Those are used as a refrigerator for HTS power systems. The pressure of subcooled liquid nitrogen is maintained at atmospheric pressure (0.108 MPa) and the working temperature is 68 K. One system of HTS power transformer was tested in distribution power line. In each case, the temperature of the cold head of the cryocooler is kept at 64 K little above nitrogen freezing temperature. For the stable operation, the system must work even in the case of shaking condition by earthquake, the pressure must be stable and be kept at atmospheric pressure.

Development of GM cryocooler separate type liquid-helium-free 3He-4He dilution refrigerator system

We developed the new liquid-helium-free dilution refrigerator system, in which the Gifford-McMahon (GM) cycle cryocooler and dilution refrigerator (DR) unit are separated. We obtained the base temperature below 50 mK in this DR system. In usual liquid-helium-free DR systems, the DR unit directly couples with GM-cryocooler in the same vacuum chamber. Therefore the mechanical vibration of GM-cryocooler is hardly removed from DR unit. In order to eliminate the vibration problem, the separated vacuum chamber contacting the GM-cryocooler is connected with the DR unit chamber by the flexible hose with length of about 1 meter. Thin flexible tubes used for circulation of the refrigerant gas and radiation shield are installed in the connection hose. The 4He gas, cooled in the GM-cryocooler unit, transfers to the DR unit throw the thin flexible tubes. After cooling the DR unit, the gas returns to GM-cryocooler unit with cooling of the radiation shield. We expect that our separate-type dilution refrigerator becomes a useful piece of apparatus for the low temperature experiments.

1 ATM subcooled liquid nitrogen cryogenic system with GM-refrigerator for a HTS power transformer

A subcooled liquid nitrogen cryogenic system with GM-refrigerators was developed. The system was operated successfully in a commercial distribution power grid for three consecutive weeks without additional liquid nitrogen supply. The system consists of two main units. One is a HTS transformer unit and the HTS transformer is installed in a G-FRP cryostat. The other one is a pump unit. The pump unit has a liquid nitrogen pump and two GM-refrigerators of 290 W at 64 K for 50 Hz operation in a stainless steel dewar. The refrigerator cold heads are immersed in liquid nitrogen and produce directly subcooled liquid nitrogen in the pump unit. Those two units are connected by transfer-tubes and 1 atmosphere (0.1 MPa) subcooled liquid nitrogen is circulated through the system. In the field test, the refrigerators were operated at 60 Hz and it took 12 hours to cool the transformer down to 70 K and 26 hours to 66 K. The refrigerator cold heads were controlled not to be below 64 K during operation. In spite of a heat generation by the HTS transformer, the subcooled liquid nitrogen temperature in the HTS transformer unit was kept lower than 68 K.

Performance of compact liquid helium free 3He-4He dilution refrigerator directly coupled with GM cooler in TES microcalorimeter operation

A superconducting transition edge thermosensor (TES) microcalorimeter was cooled by a compact liquid-helium-free 3He-4He dilution refrigerator with loading a Gifford-McMahon (GM) cooler for detection of LX-ray photons emitted from an 241Am source. The first and second stages of the GM cooler are directly coupled with the first and the second precool heat exchangers of a stick shaped dilution unit through copper plates in the vacuum chamber, respectively. The circulating 3He-4He gas through the precooled heat exchangers is condensed into a liquid of condense mixture by the isoenthalpic expansion through the Joule-Thomson impedance. A cascade of two mixing chambers are employed for achieving sufficient cooling power. The helium-free dilution refrigerator performs the cooling power of 20 μW at 100 mK. The TES and SQUID chips suffered from mechanical vibrations induced by a reciprocating motion of the displacer of the GM cooler. Detection signals of LX-ray photons emitted from 241Am source were observed by operating the TES microcalorimeter in severe noise environment induced by mechanical vibrations.

Development of a Flexibly Separated 4K Head Refrigerator for a Squid System

A refrigerator with a 4K helium recondensing head located 1.5 m from the main refrigerator unit has been developed. The 4K head is separated from the main refrigerator unit ( a G-M/J-T 4K refrigerator) by a 1m flexible vacuum insulated line. When the 4K head is inserted in a SQUID cryostat, the flexible line cuts the mechanical vibration from the main refrigerator unit and allows the cryostat to be easily handled. The refrigeration power was obtained as a function of the cold head temperature and of the J-T loop mass flow in the system. We also examined the refrigeration power both in the case that there is no extension in the J-T line (without the flexible line) and in the case that the cold head is 1.5 m from a J-T valve (with the flexible line). The experimental results show that a degradation of 0.5 W is caused by the extension of the flexible line. And it is concluded that a degradation of 0.3 W is caused by heat leak and a degradation of 0.2 W is caused by the decrease of mass flow. An additional 4K cold head will be separated by 5 m , and in the future the field noise of the SQUID system will be measured. In that case, a high pressure helium gas flow from the 70K heat exchanger of the refrigerator will be lead along the 4K-flow line as a thermal shield. In regard to the mass flow decrease; the J-T valve must be opened to keep a refrigeration power of 3 W. This increases the refrigeration temperature. A refrigeration temperature with the 5 m flexible line is predicted to be 4.61 K.