Chapter 3: Thermodynamics and Hydrodynamics of 3He-4He Mixtures

Abstract

Partly supported by the "Stichting FOM" (Foundation for Fundamental Research on Matter), which in turn is sponsored by NWO (Dutch Organisation for the Advancement of Research).

Phase boundary detection for dilution refrigerators

“…The device facilitates the correct adjustment of the 3 Dilution refrigerators (DR) have made the temperature region below 0.5 K widely available to the research community. We show how a simple modification of a standard capacitive level gauge (segmentation of one of the electrodes) permits a direct calibration of the capacitance versus phase boundary position.…”

“…For small DR's with large 3 He circulation pumps (to get high flow rates and cooling powers) the large fraction of dead volume behind the pumps makes the 3 He/ 4 He fraction in the DR depend on the operating conditions. If the sample is mounted in the mixing chamber, the mixing chamber volume will depend on the sample size, which will lead to different optimal quantities of 3 He and 4 He.…”

“…3 We measured the value of the level gauge in vacuum, C v = 28.832 pF, and the value with only diluted 3 He phase, C͑1͒ = 30.061 pF. If it is not feasible to measure full and empty values under operating conditions, one can calibrate the gauge with phases of known dielectric constants.…”

“…Leaks to vacuum, blockages and thermal shorts usually lead to catastrophic failure, and are easy to diagnose, but difficult to solve. In contrast, not having the right amounts of 3 He and 4 He in the system is easily solved, but may not be readily recognized.…”

“…For optimal performance of the DR, the total amount of mixture, and the 3 He/ 4 He fraction of the mixture must be tuned, which means putting the phase boundaries in the still (between the diluted liquid and the vapour phase) and in the mixing chamber (between the concentrated 3 He and the 3 He diluted in 4 He phases) at the correct positions. The still level is determined by the total quantity of mixture in the DR.…”