Howard O Mcmahon

Superconductive devices

Over the past 2 years there have been several suggestions advanced for computer devices which utilize the unique properties of superconductors. Some of these take advantage of the strong nonlinear dependence of resistance on magnetic field which makes possible the construction of high-speed switches. Other proposed devices utilize the ability of a superconductor to carry a so-called persistent current which offers the possibility of constructing memory elements.

Closed-cycle helium refrigeration

INTRODUCTION THE practicability and range of applications of superconductive circuits appear likely eventually to be determined by the ease and reliability of maintaining the required low temperature. Present knowledge indicates that a temperature near the boiling point of liquid helium, or perhaps a little lower, will be required. Since there is no known means for maintaining such a temperature other than by compression and expansion of helium, the problem is reduced to the question of the feasibility of constructing a mechanical closedcycle helium refrigerator of sufficient reliability. In order to liquefy helium it is necessary to compress the gas to approximately 200-300 lb/in 2, cool it to approximately 15 °K or colder, and expand it through a Joule-Thomson cycle to one atmosphere or less. All liquefiers or refrigerators must necessarily have this feature in common. Refrigerators differ from one another according to the way in which they accomplish the initial cooling from 300 to 15°K. The classical method, and the one first used to liquefy helium, (1) uses liquid ammonia, sulphur dioxide or Freon, liquid nitrogen and liquid hydrogen to accomplish the necessary precooling. This of course leads to a complex, bulky machine having low efficiency, poor reliability and with very poor possibility of development in the direction of simplicity and self-sufficiency. A considerably improved method o f providing precooling was developed by COLLINS and has led to the standard ADLCollins laboratory liquefier (2,3). In this system, portions of compressed helium gas are expanded in expansion engines at intermediate temperatures to produce the refrigeration required for cooling the Joule-Thomson stream. It is possible that the COLLINS system could provide a basis for the ,development of a refrigerator having the required :high reliability, but it appears that there may be 273 formidable problems connected with the refinement of the expansion engines and valve mechanisms. Moreover, the COLLINS system carries the entire helium flow through countercurrent heat exchangers, which are necessarily more bulky and costly than reciprocating-flow thermal regenerators. A new engine-expansion system has been developed (4,5) which utilizes reciprocating flow through a thermal regenerator instead of counterflow heat exchangers. This feature permits the engine valves to be operated at room temperatures, thereby eliminating many of the mechanical problems associated with them. A distinctive feature of the new expansion engine is that enthalpy is ejected from the system in the form of heat rather than as mechanical work. This eliminates the necessity of any large mechanical forces and allows for simplicity of design and high reliability.