Pulse tube refrigerator

Pulse tube cryocoolers are used in niche industrial applications such as semiconductor fabrication and superconducting radio-frequency circuits.

[6] Figure 1 represents the Stirling-type single-orifice pulse-tube refrigerator (PTR), which is filled with a gas, typically helium at a pressure varying from 10 to 30 bar.

From left to right the components are: The part in between X1 and X3 is thermally insulated from the surroundings, usually by vacuum.

As a result, the gas also moves from left to right and back while the pressure within the system increases and decreases.

At the cold end of the tube, the gas enters the tube via X2 when the pressure is high with temperature TL and returns when the pressure is low with a temperature below TL, hence taking up heat from X2: this gives the desired cooling effect at X2.

is given by Carnot's theorem: However, a pulse-tube refrigerator is not perfectly reversible due to the presence of the orifice, which has flow resistance.

[9][10][11][12] The modern PTR was invented in 1984 by Mikulin who introduced an orifice to the basic pulse tube.

[14][15][16][17][18] This is shown in figure 4, where the lowest temperature for PTRs is plotted as a function of time.

PTRs for temperatures below 20 K usually operate at frequencies of 1 to 2 Hz and with pressure variations from 10 to 25 bar.

The gas flows through the valves are accompanied by losses which are absent in the Stirling-type PTR.

Both hot ends can be mounted on the flange of the vacuum chamber at room temperature.

In that case the PTR can be constructed in a coaxial way so that the regenerator becomes a ring-shaped space surrounding the tube.

In this clever way it is avoided that the heat, released at the hot end of the second tube, is a load on the first stage.

In applications the first stage also operates as a temperature-anchoring platform for e.g. shield cooling of superconducting-magnet cryostats.

[22] The coefficient of performance of PTRs at room temperature is low, so it is not likely that they will play a role in domestic cooling.

PTRs are commercially available for temperatures in the region of 70 K and 4 K. They are applied in infrared detection systems, for reduction of thermal noise in devices based on (high-Tc) superconductivity such as SQUIDs, and filters for telecommunication.

PTRs are also suitable for cooling MRI-systems and energy-related systems using superconducting magnets.

In so-called dry magnets, coolers are used so that no cryoliquid is needed at all or for the recondensation of the evaporated helium.

Use of an exchange gas above the vibration sensitive scanning head enabled the first PTR based low temperature STMs.

Figure 1: Schematic drawing of a Stirling-type single-orifice PTR. From left to right: a compressor, a heat exchanger (X 1 ), a regenerator, a heat exchanger (X 2 ), a tube (often called the pulse tube ), a heat exchanger (X 3 ), a flow resistance (orifice), and a buffer volume. The cooling is generated at the low temperature T L . Room temperature is T H .
Figure 2: Left : (near X 2 ): a gas element enters the tube with temperature T L and leaves it with a lower temperature. Right : (near X 3 ): a gas element enters the tube with temperature T H and leaves it with a higher temperature.
Figure 3: Coaxial pulse tube with a displacer
Figure 4: The temperature of PTRs over the years. The temperature of 1.2 K was reached in a collaboration between the groups of Giessen and Eindhoven. They used a superfluid vortex cooler as an additional cooling stage to the PTR.