The development of the superconducting machines, such as superconducting magnets for nuclear fusion, electric machines, high energy physics, magnetically levitated train, and the Josephson Junction, should be a driving force for a technical innovation. The reliable helium refrigerator system is indispensable for practical utilization of these superconducting machine. The requirements for a practical use of the helium refrigerator system are considered as follows. (1) A long and continuous operation is required. (2) The contamination in the circulating helium should be avoided. (3) The efficiency of the refrigerator system should be as high as possible. (4) The control method must be established. The helium compressor-one part of helium refrigerator system-should be required the above condition. However, the helium compressor is a week point of the helium refrigeration system and the many type of compressor have been developed. In this special issue, the construction, characteristics and performance of the helium compressors, which is used the helium refrigerator system in Japan as shown Table, is reported. Table Helium Compressor
This paper reviews the recent ternds in the space communication and also describes the historical review of the low noise amplifier (LNA) that operates using the cryogenic technology. The world-wide and domestic satellite communications which use the geostationary satellite located at a height of about 35, 800km above the equator, are the one of the most remarkably developing field in the space development program. The international commercial satellite communication began with the operation of the Intelsat I satellite launched in 1965. As of today, Intelsat V satellites each having the traffic capacity of 25, 000 one-way voice channels plus two TV channels, are in operation orbited over the Atlantic, Pacific and Indian Oceans. And 60% of the world's international telephone communication is handled by communication satellite. Intelsat trafflc projection through the year 2000 shows the anual growth rate of avarage over 20%. On the other hand, development of the domestic satellite communication system of which typical business is program distribution to cable TV terminal in the United States and direct satellite broadcasting in near future, is producing a unprecedented market. This rapidly growing demand motivates the satellite communication system having large-scaled, powerful and multi-functional satellites and a large number of small-size, low cost earth terminals. The unique ability of a communication satellite is multiple access which establishes simultaneous communication traffic between a number of earth stations. In order to increase the traffic capacity in a allocated frequency band, effective and repeated use of the limitted frequency band is adapted for the large scale satellite applying new techniques such as time division multiple access, multi-beam antenna, dual polarization, digital speech interpolation etc. The LNA such as maser, parametric amplifier and FET (field effect transistor) amplifier is the front-end amplifier of the receiving system in the satellite communication earth station of which distinctive feature is in it's low noise performance that is enhanced by cooling the LNA. Though the noise temperature (synonymous with noise figure) of the maser cooled by liquid helium is the lowest in the LNA's, it had been replaced by the parametric amplifier cooled by gaseous helium because of easy maintenance of the latter cryogenics and wide-band performance of the parametric amplifier. After that, in compliance with the requirement for the maintenance free and low cost LNA, thermoelectric cooling insted of cryogenic cooling and low noise FET have been developed. Now, thermoelectrically cooled parametric amplifier and FET amplifier, and even those amplifiers operated in uncooled condition are used in earth stations or terminals for international or domestic satellite communications.
This paper briefly reviews the present helium market situation: short supply, price increase and its probable causes in the U.S., the largest producer and exporter of helium in the world. After that, the paper will discuss the future supply-demand prospects and will stress the need for a long-range projection of the future helium availability.