Editing Dilution refrigerator (section)

== Theory of operation ==
The refrigeration process uses a mixture of two [[isotope]]s of [[helium]]: [[helium-3]] and [[helium-4]]. When cooled below approximately [[Orders of magnitude (temperature)|870]] [[Kelvin|millikelvins]], the mixture undergoes spontaneous phase separation to form a <sup>3</sup>He-rich phase (the concentrated phase) and a <sup>3</sup>He-poor phase (the dilute phase). As shown in the phase diagram, at very low temperatures the concentrated phase is essentially pure <sup>3</sup>He, while the dilute phase contains about 6.6% <sup>3</sup>He and 93.4% <sup>4</sup>He. The [[working fluid]] is <sup>3</sup>He, which is circulated by vacuum pumps at room temperature.

The <sup>3</sup>He enters the cryostat at a pressure of a few hundred [[Bar (unit)|millibar]]. In the classic dilution refrigerator (known as a ''wet dilution refrigerator''), the <sup>3</sup>He is precooled and [[Cold trap|purified]] by [[liquid nitrogen]] at 77&nbsp;K and a <sup>4</sup>He bath at 4.2&nbsp;K. Next, the <sup>3</sup>He enters a vacuum chamber where it is further cooled to a temperature of 1.2–1.5&nbsp;K by the ''1&nbsp;K bath'', a vacuum-pumped <sup>4</sup>He bath (as decreasing the pressure of the helium reservoir depresses its boiling point). The 1&nbsp;K bath liquefies the <sup>3</sup>He gas and removes the [[heat of condensation]]. The <sup>3</sup>He 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&nbsp;mK. Subsequently, the <sup>3</sup>He flows through a secondary impedance and one side of a set of counterflow heat exchangers where it is cooled by a cold flow of <sup>3</sup>He. Finally, the pure <sup>3</sup>He enters the mixing chamber, the coldest area of the device.

In the mixing chamber, two phases of the <sup>3</sup>He–<sup>4</sup>He mixture, the concentrated phase (practically 100% <sup>3</sup>He) and the dilute phase (about 6.6% <sup>3</sup>He and 93.4% <sup>4</sup>He), are in equilibrium and separated by a phase boundary. Inside the chamber, the <sup>3</sup>He 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 <sup>3</sup>He through the phase boundary is endothermic and removes heat from the mixing chamber environment. The <sup>3</sup>He then leaves the mixing chamber in the dilute phase. On the dilute side and in the still the <sup>3</sup>He flows through [[superfluid]] <sup>4</sup>He which is at rest. The <sup>3</sup>He is driven through the dilute channel by a pressure gradient just like any other viscous fluid.<ref>{{cite book |last1=de Waele|first1=A.Th.A.M.|last2=Kuerten |first2=  J.G.M. |editor-first=D. F.|editor-last=Brewer|date= 1991|title= Progress in Low Temperature Physics, Volume 13 |chapter= Thermodynamics and hydrodynamics of <sup>3</sup>He–<sup>4</sup>He mixtures|pages=167–218|publisher= Elsevier |isbn= 978-0-08-087308-4}}</ref> On its way up, the cold, dilute <sup>3</sup>He cools the downward flowing concentrated <sup>3</sup>He via the heat exchangers and enters the still. 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 <sup>3</sup>He, which has a much higher partial pressure than <sup>4</sup>He at 500–700&nbsp;mK. Heat is supplied to the still to maintain a steady flow of <sup>3</sup>He. The pumps compress the <sup>3</sup>He to a pressure of a few hundred millibar and feed it back into the cryostat, completing the cycle.