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A 3He/4He dilution refrigerator is a cryogenic device that provides continuous cooling to temperatures as low as 2 mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat of mixing of the Helium-3 and Helium-4 isotopes. It is the only continuous refrigeration method for reaching temperatures below 0.3 K. The dilution refrigerator was first proposed by Heinz London in the early 1950s, and was experimentally realized in 1964 in the Kamerlingh Onnes Laboratorium at Leiden University. == Theory of operation == The refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4. When cooled below approximately 870 millikelvin, the mixture undergoes spontaneous phase separation to form a 3He-rich phase (the concentrated phase) and a 3He-poor phase (the dilute phase). As shown in the phase diagram, at very low temperatures the concentrated phase is essentially pure 3He, while the dilute phase contains about 6.6% 3He and 93.4% 4He. The working fluid is 3He, which is circulated by vacuum pumps at room temperature. The 3He enters the cryostat at a pressure of a few hundred millibar. In the classic dilution refrigerator (known as a ''wet dilution refrigerator''), the 3He is precooled and purified by liquid nitrogen at 77 K and a 4He bath at 4.2 K. Next, the 3He enters a vacuum chamber where it is further cooled to a temperature of 1.2–1.5 K by the ''1 K bath'', a vacuum-pumped 4He bath (as decreasing the pressure of the helium reservoir depresses its boiling point). The 1 K bath liquefies the 3He gas and removes the heat of condensation. The 3He then enters the main impedance, a capillary with a large flow resistance. It is cooled by the still (described below) to a temperature 500–700 mK. Subsequently the 3He flows through a secondary impedance and one side of a set of counterflow heat exchangers where it is cooled by a cold flow of 3He. Finally, the pure 3He enters the mixing chamber, the coldest area of the device. In the mixing chamber, two phases of the 3He–4He mixture, the concentrated phase (practically 100% 3He) and the dilute phase (about 6.6% 3He and 93.4% 4He), are in equilibrium and separated by a phase boundary. Inside the chamber, the 3He is diluted as it flows from the concentrated phase through the phase boundary into the dilute phase. The heat necessary for the dilution is the useful cooling power of the refrigerator, as the process of moving the 3He through the phase boundary is endothermic and removes heat from the mixing chamber environment. The 3He then leaves the mixing chamber in the dilute phase. On its way up, the cold, dilute 3He cools the downward flowing 3He via the heat exchangers until it enters the still. In the still, the 3He flows through superfluid 4He which is at rest. The pressure in the still is kept low (about 10 Pa) by the pumps at room temperature. The vapor in the still is practically pure 3He, which has a much higher partial pressure than 4He at 500–700 mK. The pump therefore creates an osmotic pressure difference, which drives more 3He from the concentrated to dilute phases in the mixing chamber, and then up from the mixing chamber to the still. Heat is supplied to the still to maintain a steady flow of 3He. The pumps compress the 3He to a pressure of a few hundred millibar and feed it back into the cryostat, completing the cycle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dilution refrigerator」の詳細全文を読む スポンサード リンク
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