Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   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. 

   ORIGIN OF THE INVENTION 
   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 FIELD 
   This invention relates generally to refrigeration heat switches and more particularly to a heat switch used for adiabatic refrigeration devices. 
   BACKGROUND OF THE INVENTION 
   Heat 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 close 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 INVENTION 
   Accordingly, 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 DRAWINGS 
     The following detailed description of the invention will be more readily understood when considered together with the accompanying drawings wherein: 
       FIG. 1  is an isometric view generally illustrative of the passive gas-gap heat switch of the present invention; 
       FIG. 2  is a cross-sectional view of  FIG. 1 ; 
       FIG. 3  is illustrative of a conductor component of the passive gas-gap heat switch of the present invention; 
       FIG. 4  is illustrative of an axial view of  FIG. 3 ; 
       FIG. 5  is an exploded view of  FIG. 1 , and 
       FIGS. 6   a - 6   e  are illustrative of the heating and cooling stages of a continuous adiabatic demagnetization refrigeration process. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
   Referring now to the drawings and more particularly to  FIGS. 1 and 2  which show passive gas-gap heat switch  10 . Heat switches that use a gas to complete a thermal circuit are called gas-gap switches. The concept is to place two conductive surfaces close to each other and introduce a gas (not shown) between them to turn the switch On, and remove the gas to turn the switch Off. Prior art switches use a getter to move the gas into and out of the switch. A getter is a material that will strongly absorb the gas below a threshold temperature. Common getter materials include zeolite and charcoal. The present invention, heat switch  10 , consist of fewer parts and its operation is simpler than the getter actuated switched. Heat switch  10  includes thermal/mechanical interfaces  11  and  12 , copper conductors  40  and  50  which include multiple fins  20  and  30  respectively, a small gap  60  ( FIGS. 3 and 4 ) located between each of the fins  20  and  30  of conductors  40  and  50  respectively. Thermal/mechanical interfaces  11  and  12  include bolt holes 14  and  16  which facilitate the physical and thermal connection of heat switch  10  with components (not shown) to be heated or cooled. Conductors  40  and  50  are contained within containment tube  70  that physically supports and aligns the conductors  40  and  50 . 
   Now referring to  FIGS. 3 and 4  of the present invention.  FIG. 3  shows conductor  50  which includes fins  30  and thermal/mechanical interface  12 . Fins  20  and  30  are the means by which heat is transferred from the gas to thermal/mechanical interfaces  11  and  12  respectively. The thermal conductance in the On state is proportional to the surface area  35  of the fins  30  and the thermal conductivity of the gas which is approximately proportional to the gas pressure. Therefore, any means of increasing the conducting surface area  35  of the fins  30  and the conducting surface area  25  of fins  20  will increase the effectiveness of the transfer of heat to and from conductors  40  and  50 . A wire electric discharge machining (EDM) technique is used to machine conductors  40  and  50  so as to achieve a very large fin surface area and uniform spacing of gap  60  which is located between fins  20  and  30 . The conductors  40  and  50  are cut from a single cylinder of material using the EDM technique. Copper may be used as it provides properties that are desirable for conducting heat. After machining, fins  20  and  30  remain attached to conductors  40  and  50  respectively. The number of fins  20  and  30  and their dimensions are a consideration in the design of heat switch  10 . Increasing the number of fins  20  and  30  and their surface areas  25  and  35  respectively will, in general, increase the On conductance of switch  10 . But if fins  20  and  30  become too thin, their reduced ability to carry heat may actually reduce the On conductance. The thermal properties of copper and the helium gas used will dictate the optimal number of fins  30  and  40  for the width of gap  60 . The width of gap  60  is determined by two factors. First, the ability to reliably assemble heat switch  10  without any of the interleaved fins  20  and  30  contacting each other. As fins  20  and  30  are formed by the wire EDM process, stresses within the material will cause them to bend along their length by an amount that depends on the cube of their thickness. Gap  60  therefore should be larger than the maximum bending that will occur. Second, it is important for heat switch  10  to operate in a regime where the thermal conductance of the gas is pressure dependent. This regime is known as the molecular region and is characterized by the mean free path of the gas being larger than the physical dimensions of the space it occupies. In a passive gas-gap switch, that dimension is the gap width. Since the gas pressure in the switch is engineered to be strongly temperature dependent, operating in the molecular region results in the thermal conductance of the switch being strongly temperature dependent. For the low and higher temperature versions of the passive gas-gap switch, the strong temperature dependence of gas pressure is derived from two different mechanisms. For the low temperature switch, the attractive force between helium-3 atoms causes them to condense rapidly out of the gas phase as the temperature drops below about 0.3° K. This happens regardless of the amount of helium present. For the higher temperature heat switches, the attractive force between helium atoms and the internal components of the switch causes the helium to bind (i.e., adsorb) to those surfaces as the temperature drops. As long as the amount of helium present is less than the amount that would form a single atomic layer, there will be a threshold temperature in the 1-5° K range where the vapor pressure of the helium is strongly dependent on temperature. This threshold depends on the amount of helium present and the type of surface used. Thus this switch can be tailored to passively turn on and off over a range of temperatures. 
   Now referring to  FIG. 5 , fins  20  of conductor  40  are inserted into end  72  of containment tube  70  and fins  30  of conductor  50  are inserted into end  74 , the opposite end of containment tube  70 . Containment tube  70  is rigid and holds conductors  40  and  50  together such that fins  20  and  30  interleave but do not touch. Containment tube  70  must be capable of providing low thermal conductivity, high strength and it must be impermeable to the gas (not shown) that is used as the conducting agent. The low thermal conductivity is required because containment tube  70  still conducts heat between the two ends  72  and  74  which contact thermal/mechanical interfaces  11  and  12  of conductors  40  and  50  respectively, when the switch is in the Off state. This heat flow is undesirable and should be as low as possible. The high strength is necessary because containment tube  70  physically supports conductors  40  and  50 . It is important that fins  20  and  30  of conductors  40  and  50  not touch and that the gap  60  remain intact. Since some small mechanical loads may result from how the heat switch  10  is attached to other components (not shown), containment tube  70  must have enough strength to absorb these loads without distorting fins  20  and  30 . Impermeability is required because the gas (not shown) must not leak over time or the heat switch  10  will be inoperable. There are several options one can consider for meetings these requirements. In the present invention two different materials are used to meet these requirements. One material provides structural support and the other provides a surface that is impermeable by the gas (not shown). Both materials must have low thermal conductance. Containment tube  70  is attached to conductors  40  and  50  so as to create a hermetic seal. The hermetic seal may be achieved via an epoxy or an indium seal. A small access tube  80  is soldered into end cap  90  to allow heat switch  10  to be evacuated and filled with the appropriate amount of gas (not shown). End cap is sealed onto thermal/mechanical interface  11  in such a way as to be removable and replaceable. Helium 3 gas is used in the present invention as this gas is most suitable at temperatures below 0.7° K and without regard to orientation (presence of gravity). The total volume of liquid helium required for the switch is small because the saturated vapor pressure in liquid helium is a few atomic layers thickness on most substrates. This is also important because a thin film on the substrate will not sag or drip in the presence of gravity. Helium 3 on a copper substrate has a binding energy of 0.4 Pa at about 57° K and a binding energy of only 2.7×10 −4  Pa at 3° K. Different materials have different binding energies and may be used according to need. 
   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). 
   Now referring to  FIGS. 6   a - 6   e , in  FIG. 6   a , the first stage  101  absorbs heat from the apparatus (not shown) and is slowly demagnetized to maintain constant temperature. When the magnetic field drops to some lower threshold, rather than magnetize it to warm it up (as is done for single-shot ADRs), the second stage  102  is demagnetized to a slightly lower temperature and then the heat switch  120  between the first and second stages  101  and  102  is activated. The system is designed so that the flow of heat from stage  101  to  102  exceeds the heat load from the apparatus, so there is a net flow of heat out of the first stage. Conceptually there is no problem with this: the first stage  101  merely needs to be magnetized now to maintain constant temperature. 
   In  FIG. 6   b  the second stage  102  is receiving this heat, so it is demagnetized while the first stage  101  is magnetized. When the magnetic field of the first stage  101  reaches an upper threshold, the heat switch  120  is deactivated and the first stage  101  is demagnetized because the heat flow out has ceased and there is a net heat flow into the stage. The important feature here is that the temperature control system  130  automatically magnetizes or demagnetizes the first stage  101 , so this process of recycling the first stage  102  “on the fly” does not involve a loss of temperature control. 
   In  FIG. 6   c , after recycling the first stage  101 , the second stage  102  is magnetized to a higher temperature and the third stage  103  is demagnetized to a slightly lower temperature. The temperatures are chosen based on a number of factors, including the thermal conductance of the heat switch  10  and parasitic heat flow to the lower stages. In the present invention, the passive gas-gap heat switch  10  is used to connect the second and third stages  102  and  103  respectively, so the temperatures are dictated almost entirely by the dependence of the thermal conductance on temperature. Passive operation turns out to be possible because the thermal conductance of the switch drops off extremely rapidly below 0.2 K. (This is due to the particular properties of helium-3 which is the gas used inside the switch). The recycling process consists of magnetizing the second stage  102  to about 0.275° K and the third stage to about 0.25° K. Heat will flow from the second stage  102  to the third stage  103 , requiring the second stage  102  to be magnetized and the third stage  103  to be demagnetized to maintain these temperatures. Magnetization of the second stage  102  is stopped once the second stage  102  hits its upper field threshold. The second stage  102  is then allowed to cool down some before demagnetization begins. As it cools below the third stage  103  some heat flows from stage  103  to stage  102  which is not beneficial, but this quickly diminishes as the second stage cools below the 0.2° K mark and the heat switch passively turns off (i.e. stops conducting heat). Once it cools below 0.15° K, the passive gas-gap heat switch  10  is completely off, and then the third stage  103  is warmed and recycled. 
   In  FIG. 6   d  the process is essentially the same: The third stage  103  is magnetized to a higher temperature and the fourth stage  104  is demagnetized to a slightly lower temperature and the heat switch between the two stages is activated. The third stage  103  will continue to be magnetized and the fourth stage  104  is demagnetized as heat flows from stage  103  to  104 . Again, once the stage  103  hits its upper field threshold, the heat switch  10  is deactivated. Stage  103  can be cooled and stage  104  warmed up to dump its heat to the heat sink  140  via heat switch. This whole process is then repeated to keep transferring heat from the first stage  101 . The time required is on the order of 1 hour. The faster it can be done, the more heat we can absorb at the first stage. In practice we can speed things up by having two recycling events occurring at the same time (i.e. stage  101  transfers heat to stage  102  while stage  103  transfers heat to stage  104 ). 
   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.