The performance of the pumped pot in terms of cooling power has been analyzed, and the results are presented below. A comparison of theoretically expected and experimentally measured performance is included.

The theoretical value of cooling power is 4.5 mW/10^-4mol/sec, given by de Long et al. (1971). The displacement of the mechanical pump at the end of the pumped pot line is 16.5 m ³/h, which gives a molar flow rate of 4*10^-4 mol/sec at a pressure of 1.5 Torr. This in turn yields an expected critical power of 18 mW.

An experimentally measured cooling power of 0.2 mW at a pumped pot pressure of 60 mT was obtained in October of 1998. This would imply a 5 mW cooling power for pressure of 1.5 T. There is roughly a factor of four discrepancy in the theoretical and experimental cooling powers. A combination of two effects can account for this discrepancy -- a slight offset in the zero-reading of the pressure gauge in the pumped pot line and a small parasitic heat load introduced by the pumping line. If we take the pressure reading to be accurate, this would imply a parasitic heat load of about 0.5 mW. If we instead take the measurement of cooling power to be accurate, this leads us to the conclusion that the zero in the pressure gauge is off by 40-45 mT. It is also possible that both pressure and cooling power were underestimated, by virtue of the zero in the pressure gauge being below the true zero pressure. In such a case, using our 40 mT error as the figure of merit, a maximum parasitic heat load of 1 mW is possible.

A second measurement was made in November of 1998, the results of which showed a cooling power of 1.8 mW for a pressure of 160 mT. This is much more in line with the theoretical prediction. If we assume a pressure gauge which reads 40 mT too low, we arrive at a maximum parasitic heat load of 0.6 mW. The results of the Novemeber measurement will be taken as typical, as this measurement was less sensitive to errors in the zero of the pressure gauge and SPARO is often run with pumped pot pressures in the 150-200 mT range.

The ³He gas temperature during the cycle was also checked. Taking the November measurement of cooling power to be correct, the gas is at about 15 K when it comes into contact with the pumped pot on its way to the ³He pot. If we instead take the maximum possible cooling power to be correct and attribute the empirical value to a 0.6 mW parasitic heat laod, the gas temperature cannot be higher than 20 K. These estimates were made using the specific heat and latent heat of He-3 (see E-mail on specific heats ) and also using information on how long it takes to condense the He-3 in SPARO for various values of the pumped pot pressure. This information is found in logbook 6, page 8 (feb 3, 2000).