Performance evaluation of a Gifford-McMahon refrigerator for cryogenically cooled computer applications

As part of a study of the feasibility of developing a computer based on cryogenically cooled CMOS devices, a Gifford-McMahon (GM) cycle refrigerator was considered for the cooling technology. The GM refrigerator was selected in spite of being inefficient and large in size because it was the only reasonably priced, commercially available technology that could possibly meet the computer cooling needs. However, there were concerns regarding the mechanical reliability and performance degradation of the machine. The test program for investigating the GM refrigerator performance degradation is described. Two 55-W at 77-K single-stage refrigerators were tested over a cumulative period of approximately one year. The two cold heads were installed in a vacuum dewar maintaining a pressure of better than 10^-6 torr. The cold head was covered with superinsulation material to eliminate radiation effects from the cryostat. A programmable resistive load was attached to the cold head so that the temperature could be maintained at 77 K. Test data were taken weekly, and the power was continuously adjusted to obtain a 77-K cold head temperature. A third refrigerator was tested in a working demonstration computer for a period of 9900 h. The tests of the two cold heads and the data from the demonstration machine verified the existence of performance degradation in a continuous-operation mode at a constant temperature of 77 K. Two degradation modes were identified: permanent and recoverable. The permanent degradation of 8%-10% per year could be attributed to mechanical wear and contamination in the system. The preliminary findings indicated that without correction of the degradation problem, this refrigerator technology could not be used in cryogenic computers