Pulse tube refrigerator

A pulse tube cooler or pulse tube cooler ( also pulse tube cooler) is based on the principle of the Stirling engine chiller. The advantage over the Stirling engine is that near the cold heat exchange point no mechanically moving parts are needed. This very compact cooling heads are possible and the attainable minimum temperature is not limited by the mechanical friction heat of these parts. Now can be achieved with pure pulse tube coolers temperatures of 1.3 K ( = -272 ° C).

Applications

The areas extend over a wide range of industrial, research, medical and military applications where extremely low temperatures are required: Liquefaction of gases, cooling of sensors, cooling superconducting magnetic field coils, superconducting circuits in mobile receiving stations, low-temperature experiments and space experiments. Thanks to its independence of cryogenic liquids own it very suitable for outdoor applications: in the military for cooling of infrared sensors for cooling optical sensors in space telescopes or for a future manned mission to Mars, to liquefy the oxygen in the Martian atmosphere before the arrival of astronauts. In research, they can make cryostats independent of the supply of expensive cryogenic fluids such as liquid helium or nitrogen. They are used here as a direct precursor for further cooling stages, such as 3He evaporative cooler, 3He - 4He mixing cooler or paramagnetic Entmagnetisierungsstufen. With a dilution refrigerator with pulse -tube preamp a temperature of 4.3 mK was achieved without a supply of cryogenic liquids (2003), Walther- Meissner - Institute, Garching.

History

With the 1963 by WE Gifford and RC Longworth presented basic principle, the BPTR (English: Basic Pulse Tube Refrigerator ), minimum temperatures of 124 K ( = -149 ° C) could be achieved. Over the years, several research groups published variations with more improved efficiency and lower the minimum temperature. A 1984 published version of the type OPTR (English: Orifice Pulse Tube Refrigerator ) reached a temperature of 60 K. With a further variant, from 1990, of the type DIPTR (English: Double Inlet Pulse Tube Refrigerator ), as well as strings of two or three pulse tube cooler, could finally be the boiling point of helium below ( less than 4 K). A research group from casting reached 1.3 K (2004) and together with a group from Eindhoven 1.2 K with an additional cooling stage ( 2005).

Operation

Stirling engine

The pulse tube refrigerator operates on the principle of the Stirling engine with a regenerator. In operation as a heat pump compresses and expands the piston of the Stirling engine in a cylinder periodically the gas therein, which initially causes a uniform periodic variation in temperature of the gas. So that a spatially -directed heat transfer occurs, the gas is periodically reversed on the one hand superimposed with a displacement piston, so that the compression takes place at a different location than expansion. On the other hand passes through the most Stirling engines, the gas is at a so-called regenerator, a gas-permeable material having a large heat capacity. This cools the gas in the compressed phase on the way to the cold end, takes a matter of heat, and thus warm the gas in the expanded phase on the way to the hot end. Both strategies ensure that one end of the timing means is colder than the other. If the warmer on average end kept at ambient temperature, the colder end can be used for cooling.

Pulse tube cooler

The pulse tube cooler avoids any moving parts, with the exception of the piston in often distant compressor that forces a periodic pressure fluctuation. The incoming and outgoing gas passes through a regenerator, and opens into a so-called pulse tube, which is a substitute for the other moving parts of the Stirling engine. At the other end of the pulse tube, the air can not escape or only slowly. Regarding a volume element of the gas in the middle of the pulse tube during periodic compression, as it moves relative to the stationary regenerator back and forth. The pulse tube acts like a piston and replaced the necessary in the second movable piston Stirling engine or a moving regenerator and displacer. A heat transfer only takes place when a time dephasing of the gas movement is provided to the pressure or the temperature. What is solved in the Stirling engine mechanical, succeeds when BPTR type in that the wall of the pulse tube absorbs heat and naturally gives off a slight delay. A much greater delay of the movement of gas reaches the OPTR, wherein the pulse tube is connected to a decelerating to a buffer volume which is filled with a certain inertia and emptied. In this way the strategies of the Stirling engine to be implemented, without the drawback of mechanical parts which counteract the frictional heat due to the cooling. Is the coldest point that can be used for cooling when the hot heat exchange points are held by water or air cooling to ambient temperature between the regenerator and the pulse tube.