Passive gas-gap heat switch for adiabatic demagnitization refrigerator
A passive gas-gap heat switch for use with a multi-stage continuous adiabatic demagnetization refrigerator (ADR). The passive gas-gap heat switch turns on automatically when the temperature of either side of the switch rises above a threshold value and turns off when the temperature on either side of the switch falls below this threshold value. One of the heat switches in this multistage process must be conductive in the 0.25° K to 0.3° K range. All of the heat switches must be capable of switching off in a short period of time (1-2 minutes), and when off to have a very low thermal conductance. This arrangement allows cyclic cooling cycles to be used without the need for separate heat switch controls.
Priority is claimed under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application No. 60/303,797 filed on Jul. 10, 2001.
The invention described herein was made by employees of the United States Government, and may be manufactured and used by or for the Government for government purposes without the payment of any royalty thereon or therefore.
TECHNICAL FIELDThis invention relates generally to refrigeration heat switches and more particularly to a heat switch used for adiabatic refrigeration devices.
BACKGROUND OF THE INVENTIONHeat switches are needed to control the heat flow between adjacent stages in an Adiabatic Demagnetization Refrigeration (ADR) process. Heat switches are typically one of two types: those that use a metallic switching element and those that use a gas or fluid switching element. The heat switches that use a metallic switching element are typically mechanical, super conducting or magnetoresistive switches. Heat switches that use a gas as the switching element are called gas-gap switches. The concept is to place two conductive metal plates dose to each other and introduce a gas between them to turn the switch On, and remove the gas to turn the switch Off. One of the disadvantages of all of the aforementioned switches is the need for some sort of actuator. Not only does this require ancillary control electronics, software and sensors, these requirements may have a significant thermal impact on the system. Actuators require wiring or drive shafts which conduct heat and dissipate heat when used. This could be a concern when operating in a cryogenic environment.
An ADR stage produces cooling (or heating) by the interaction of a magnetic field with the magnetic spins in a paramagnetic salt. Magnetizing the salt produces heating, and demagnetizing the salt produces cooling. A conventional “single-shot” ADR consists of a “salt pill” containing the magnetic salt, a superconducting magnet, and a heat switch. The salt pill is located in the bore of the magnet, and the heat switch links it to a heat sink. Regardless of the initial conditions, the refrigeration cycle consist of the following steps: First, the salt pill is magnetized, causing it to warm up. Second, when its temperature exceeds that of the heat sink, the heat switch is powered into the on state. Third, the salt continues to be magnetized, generating heat which flows to the sink. This continues until full field is reached, which necessarily is strong enough to significantly align the spins and suppress the entropy of the salt. Fourth, at full magnetic field, the heat switch is deactivated to thermally isolate the salt from the heat sink. Fifth, the salt is demagnetized to cool it to the desired operating temperature. In general, the salt will then be receiving heat from components parts. The heat is absorbed and operating temperature maintained by slowly demagnetizing the salt at just the right rate. Heat can continue to be absorbed until the magnetic field is reduced to zero, at which point the ADR has run out of cooling capacity.
Over the last few years there has been a growing need for more advanced ADR cooling technology. The space industry has been a pioneer in this technology because ADRs are the only low temperature (below 0.2°K) refrigeration technology that does not use any fluids, and therefore does not have the design constraints imposed by gravity. Recently ADRs have been developed for commercial use particularly in the high resolution, high efficiency, x-ray spectrometer industry. The trend in developing ADRs is toward using multiple cooling stages as this arrangement allows for greater efficiency, by reducing parasitic heat flows within the refrigerator, and greater operating temperature range. In this process each stage is thermally connected to the next via a heat switch. Thus, for low temperature ADR systems there exists a need for a heat switch that is capable of conducting heat at sub-Kelvin temperatures (down to approximately 200 mK) and is capable of being turned off at those temperatures. Existing gas-gap switches that use a getter to remove the conductive gas from the switch body cannot meet the latter requirement. As the benefits of using ADRs become more widely known it is anticipated that a wider array of industries will take advantage of this efficient cooling process.
The present invention, in a more general application, provides an easy way to cool something below room temperature and then automatically thermally isolate it at a low temperature. This is made possible by providing a heat switch that is capable of conducting heat at temperatures ranging from 0.25° K to above room temperature. One of the heat switches in this multistage process must be conductive in the 0.25° K to 0.3° K range. All of the heat switches must be capable of switching Off in a short period of time (1-2 minutes), and when Off to have a very low thermal conductance. Currently, no heat switches are capable of meeting these requirements. Superconducting switches have too much conductance in the Off state. Mechanical heat switches have too little conductance in the On state. Finally, traditional getter-activated gas-gap heat switches have long turn-off times. Thus there is a need in the industry for a passive gas-gap heat switch that can facilitate the heating/cooling of a device in the 0.25° K to above 1° K range in a manner such that the heat switch does not require long turn-off times.
SUMMARY OF THE INVENTIONAccordingly, it is an object of the present invention to provide an improvement in a passive gas-gap heat switch for use in an ADR environment.
It is another object of the invention to, provide a passive gas-gap heat switch that can cool a device below room temperature.
It is a further object of this invention to provide a passive gas-gap heat switch that is capable of conducting heat at temperatures ranging from 0.25°K to above 1°K.
It is yet a further object of the present invention to provide a passive gas-gap heat switch that does not require a long turn-off time.
It is still a further object of the present invention to provide a passive gas-gap heat switch that has very low thermal conductance when turned off.
It is yet another object of the present invention to provide a passive gas-gap heat switch that can automatically thermally isolate a device once it is cooled down to liquid helium temperature.
The foregoing and other objects of the present invention are achieved by providing a passive gas-gap heat switch in an adiabatic refrigeration environment wherein the heat switch becomes thermally conductive (turns on) when the temperature of either side of the switch rises above a threshold value, and rapidly turns off if either side of the switch falls below the threshold value. This arrangement allows cyclic cooling cycles to be used without the need for separate heat switch controls.
BRIEF DESCRIPTION OF DRAWINGSThe following detailed description of the invention will be more readily understood when considered together with the accompanying drawings wherein:
To those skilled in the art, many modifications and variations of the present invention are possible in light of the teachings contained herein. It is therefore to be understood that the present invention can be practiced otherwise than as specifically described by these teachings and still be within the spirit and scope of the claims.
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In operation the system consists of multiple stages connected in series between the cold end and the heat sink. Each stage consists of a magnet, salt, and heat switch to thermally connect it to the next higher stage (or the heat sink). The number of stages needed depends on how warm the heat sink is, and on the thermal characteristics of the heat switches. In the present invention four stages are used because the desired heat sink temperature is 4-10° K (absolute temperature).
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Helium-3 refrigerators typically cool down to about 0.3°K, while better designs can cool to 0.25° K or a little lower. This is precisely the temperature required for the third stage during the recycling operation. The steep drop in the vapor pressure of helium-3 below 0.2° K prevents helium-3 refrigerators from cooling much below 0.25° K, and it causes the thermal conductance of the passive gas-gap heat switch to drop off precipitously. Therefore an alternate use for the passive gas-gap heat switch is to connect two ADR stages to a helium-3 refrigerator. For laboratory systems, helium-3 refrigerators can be quite inexpensive, so a hybrid system could be very attractive for its low cost.
To those skilled in the art, many modifications and variations of the present invention are possible in light of the teachings contained herein. It is therefore to be understood that the present invention can be practiced otherwise than as specifically describe by these teachings and still be within the spirit and scope of the claims.
Claims
1. A heat switch for an adiabatic refrigeration process wherein said heat switch comprises;
- a first conductor element;
- a first set of fins integrally formed with said first conductor element;
- a second conductor element;
- a second set of fins integrally formed with said second conductor element; and,
- a containment tube for physically supporting said first and second conductor elements.
2. The device according to claim 1 wherein said first and second sets of fins are interleaved.
3. The device according to claim 1 wherein a gap exists between said first and second sets of fins.
4. The device according to claim 3 wherein said first and second conductor elements each include a thermal interface.
5. The device according to claim 4 wherein said first and second conductor elements each include a mechanical interface.
6. The device according to claim 5 wherein said thermal interface and said mechanical interface are one in the same.
7. The device according to claim 1 wherein said containment tube is impermeable to helium gas.
8. The device according to claim 7 wherein said containment tube is hermetically sealed with said first and second conductor elements.
9. The device according to claim 1 wherein said adiabatic refrigeration process is continuous.
10. The device according to claim 9 wherein said heat switch is a passive gas-gap heat switch.
11. The device according to claim 1 wherein said heat switch is capable of conducting heat from 0.25° K to above 1° K.
12. A passive gas-gap heat switch for a continuous adiabatic refrigeration process wherein said heat switch comprises;
- a first conductor element;
- a first set of fins integrally formed with said first conductor element;
- a second conductor element;
- a second set of fins integrally formed with said second conductor element wherein said first and second sets of fins are interleaved and do not touch; and
- a containment tube for physically supporting said first and second conductor elements wherein said containment tube hermetically seals said first and second sets of fins.
Type: Application
Filed: May 10, 2005
Publication Date: Oct 20, 2005
Inventors: Peter Shirron (Hyattsville, MD), Michael Di Pirro (Silver Spring, MD)
Application Number: 11/126,516