Editing Cryocooler (section)

== GM-refrigerators ==
[[File:GM cooler schematic02.jpg|class=skin-invert-image|300px|thumb|Fig.5 Schematic diagram of a GM-cooler. V<sub>l</sub> and V<sub>h</sub> are buffer volumes of the compressor. The compression heat is removed by the cooling water of the compressor via a heat exchanger. The rotary valves alternatingly connect the cooler to the high- and the low-pressure sides of the compressor and runs synchronous with the displacer.]] Gifford-[[Howard O. McMahon|McMahon]] (GM) coolers<ref>W.E. Gifford and R.C. Longsworth, Advances in Cryogenic Engineering 11, 171 (1966)</ref> have found widespread application in many low-temperature systems e.g. in MRI and cryopumps. Fig.5 is a schematic diagram. Helium at pressures in the {{convert|10-30|bar|psi}} range is the working fluid. The cold head contains a compression and expansion space, a regenerator, and a displacer. Usually the regenerator and the displacer are combined in one body. The pressure variations in the cold head are obtained by connecting it periodically to the high- and low-pressure sides of a compressor by a rotating valve. Its position is synchronized with the motion of the displacer. During the opening and closing of the valves irreversible processes take place, so GM-coolers have intrinsic losses. This is a clear disadvantage of this type of cooler. The advantage is that the cycle frequencies of the compressor and the displacer are uncoupled so that the compressor can run at power-line frequency (50 or 60&nbsp;Hz) while the cycle of the cold head is 1&nbsp;Hz. In this way the swept volume of the compressor can be 50 or 60 times smaller than of the cooler. Basically (cheap) compressors of domestic refrigerators can be used, but one must prevent overheating of the compressor as it is not designed for helium. One must also prevent oil vapor from entering the regenerator by high-quality purification traps.

===Cooling cycle===
[[File:GM Cycle Cryocooler02.jpg|class=skin-invert-image|300px|thumb|Fig. 6 The four stages in the cooling cycle of the GM cooler.]]The cycle can be divided in four steps, with Fig.6, as follows:
The cycle starts with the low-pressure (LP) valve closed, the high-pressure (HP) valve open, and the displacer all the way to the right (so in the cold region). All the gas is at room temperature.
* From a to b. The displacer moves to the left while the cold head is connected to the HP side of the compressor. The gas passes the regenerator entering the regenerator at ambient temperature ''T''<sub>a</sub> and leaving it with temperature ''T''<sub>L</sub>. Heat is released by the gas to the regenerator material.
* From b to c. The HP valve is closed and the LP valve opened with fixed position of the displacer. Part of the gas flows through the regenerator to the LP side of the compressor. The gas expands. The expansion is isothermal so heat is taken up from the application. This is where the useful cooling power is produced.
* From c to d. The displacer moves to the right with the cold head connected to the LP side of the compressor forcing the cold gas to pass the regenerator, while taking up heat from the regenerator.
* From d to a. The LP valve is closed and the HP valve opened with fixed position of the displacer. The gas, now in the hot end of the cold head, is compressed and heat is released to the surroundings. In the end of this step we are back in position a.
{{clear}}