Properties of the double mixing chamber dilution refrigerator

The use of two mixing chambers in cascade operation offers the simple possibility of lowering the temperature of a conventional dilution refrigerator below 5.5 mK. The parameter controlling the 3He dilution rate in each mixing chamber is the geometric flow impedance, located in the dilution exit line of the first mixing chamber. The optimum value of the geometric flow impedance is determined experimentally from the cooling power curve so that the temperature of the first mixing chamber is at least 10% higher than that of the second one. Using the experimental data, the total 3He circulation rate is calculated from Radebangh's expression for the cooling power, and the calculated value is 20% to 40% higher than the measured value. The lowest temperature of two mixing chambers obtained is 6.1 mK. The external heat leak is believed to be high, i.e. about 0.2 to 0.4 ergs/sec. The lowest temperature is limited by the maximum circulation rate, obtainable with a small mechanical vacuum pump. The 3He molar rate diluted in the first mixing chamber is expressed by the formula, and the expected lowest temperature is evaluated. The temperature below 5 mK can be obtained in the second mixing chamber by increasing 3He circulation rate at low still temperature, and the importance of high pumping speed at the still can not be overstressed. (Nakai, Y