Abstract:
A component for use in pulse tube cryorefrigerators which integrates the reservoirs (buffer volumes) as well as the housing for the rotary valve and valve plate and drive motor into a convenient, unified assembly. Other components required by the pulse-tube refrigerators, such as the heat sink, orifices, phase shifting valves, connecting tubing, etc., may also be integrated into the buffer volume/valve/motor housing within the teachings of the invention.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to the field of cryorefrigeration. More particularly, the invention pertains to an integrated component for a pulse tube cryorefrigerator. 
     2. Description of the Related Art 
     Typical closed-cycle expansion cryogenic refrigerators include the Stirling, Gifford-McMahon and pulse tube types, all of which provide cooling through the alternating compression and expansion of a cryogen, with a consequent reduction of its temperature. Stirling and Gifford-McMahon regenerative refrigerators use displacers to move a cryogen (usually helium) through their regenerators, exhaust the heat in the return gas to the compressor package. The noise and vibration induced by the displacer creates problems, and the wear of the seals on the displacer require periodic maintenance and replacement. 
     Therefore, it is highly desirable to invent cryorefrigeration devices that generate less vibration and less acoustic noise than prior art cryorefrigerators. It is also desirable to decrease the number of moving parts used in cryorefrigeration devices and to significantly increase the required maintenance intervals. 
     Pulse tube refrigerators are a known alternative to the Stirling and Gifford-McMahon types, which do not use a mechanical displacer. 
     A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube refrigerator operates by cyclically compressing and expanding a cryogen in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the cryogen in the gas phase. 
     Prior art single-stage valved pulse tube cryorefrigerators generally include a pulse tube, a rotary valve to generate the oscillating compression-expansion cycle, a reservoir to contain the expanding cryogen gas, orifices for the movement and phasing of the gas between the reservoir or buffer volume and the rest of the system, and a regenerator for absorbing heat temporarily and reversibly. Single stage pulse tube cryorefrigerators are generally capable of reaching temperatures above 20° K., and achieving lower temperatures has in the past required staging of the pulse tubes. U.S. Pat. No. 3,237,421 to Gifford and other prior art publications disclose multistage pulse tube cryorefrigerators. 
     Prior art two-stage pulse tube cryorefrigerators generally include, in addition to the foregoing components, a first-stage pulse tube, a first-stage regenerator, a second-stage pulse tube, a second-stage regenerator and first and second cooling stages. 
     Although an improvement over mechanical displacement devices, prior art pulse tube cryorefrigerators were ungainly arrangements of separate components, which leads to inefficiency and difficulty in manufacture and maintenance. 
     Pulse tube coolers can be employed in a wide variety of applications from civilian to government to military. Most of the applications below are dependent on the availability of a cheap cryocooler with a long life—long life is a unique advantage of the pulse tube cooler. 
     Sensors: Infrared; atmospheric studies, thermal losses, pollution monitoring, process monitoring, night vision, missile guidance, missile surveillance, Gamma-ray, monitor nuclear activity 
     Semiconductors in computers: (large speed gain at small cost penalty, temperatures around 100—200 K.) 
     Hi-Tc superconductors: Cellular phone base stations (more channels, temperatures under 80 K.), High speed computers, SQUID magnetometers, heart and brain studies 
     Magnets: maglev trains, mine sweeping 
     Cryopumps for the semiconductor industry 
     Cryogenic catheters, Cryosurgery 
     Liquefaction of gases: Helium, Hydrogen, Neon, Nitrogen, Argon Oxygen, Natural Gas, etc.—remote wells or peak shaving (providing extra gas at peak loads to minimize steady pipeline capacity) or for fleet vehicles 
     Perhaps the application of cryorefrigeration which is most familiar to the public is its use in Magnetic Resonance Imaging (hereinafter “MR”). MRI is an imaging technique used widely within the medical field to produce high quality images of the inside of a human body. 
     Generally, the most expensive component of a MRI system is the imaging magnet, which is typically an electromagnet made from a superconducting material. When cooled to a temperature near absolute zero (i.e., −273.15° C. or 0° K.), the superconducting wire in the magnet&#39;s coil has an electrical resistance approaching zero. Therefore, MRI imaging magnets are usually maintained at a temperature of 4.2° K. using liquid helium. 
     Typically, the main superconducting coils of a MRI imaging magnet are enclosed in a pressure vessel contained within an evacuated vessel (i.e., Dewar vessel), and superconducting temperatures are obtained by boiling a liquid cryogen, such as liquid helium, within the pressure vessel. Because distribution, storage and handling of liquid helium is difficult and costly, mechanical displacement cryorefrigerators, such as the Gifford-McMahon type, typically are used to condense and recycle the helium gas generated by boiling the liquid cryogen. 
     One problem associated with cryorefrigerators using displacers is that the motion of the displacer creates a series of repetitive knocking sounds and mechanical vibrations, which become especially rapid as the magnet in the MRI is cycled on and off to generate the magnetic field gradients that are used to collect information regarding the molecular structure of a patient&#39;s body. The MRI equipment thus generates high acoustic noise levels, and also vibrates. Because of the volume of this noise, it is recommended that patients undergoing MRI use hearing protection devices. In fact, some MRI imaging sites even go to such lengths as to provide an airplane-like audio headphone system for their patients, in order to protect their hearing and mask the acoustic noise, which may agitate or frighten the patient. 
     SUMMARY OF THE INVENTION 
     The present invention is a component for use in pulse tube cryorefrigerators which integrates one or more of the reservoirs (buffer volumes) as well as the housing for the rotary valve and valve plate and drive motor into a convenient, unified assembly. Other components required by the pulse-tube refrigerators, such as the heat sink, orifices, phase shifting valves, connecting tubing, etc., may also be integrated into the buffer volume/valve/motor housing within the teachings of the invention. 
     Cryorefrigerators using the novel component have increased efficiency, reduced manufacturing cost, and increased compatibility with varied cryostats due to the compactness of the component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a single-stage cryorefrigeration apparatus of the present invention. 
     FIG. 2 shows a block diagram of a two-stage cryorefrigeration apparatus of the present invention. 
     FIG. 3 shows a perspective view of a cryorefrigerator of the invention. 
     FIGS. 4 a  and  4   b  show side cut-away and top drawings, respectively, of an integrated pressure control housing and buffer volume, for use in a single-stage cryorefrigerator. 
     FIGS. 5 a  and  5   b  show side cut-away and top drawings, respectively, of an integrated pressure control housing and buffer volume with an integrated secondary buffer volume, for a two-stage cryorefrigerator. 
     FIGS. 6 a  and  6   b  show side cut-away and top drawings, respectively, of an integrated pressure control housing and buffer volume, with an external secondary buffer volume, for use in a two-stage cryorefrigerator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is component for a single, double or multiple-stage pulse tube cryorefrigeration apparatus which integrates a number of the parts into a single housing, and cryorefrigerators using the component. The overall system using the component may be used as a stand alone cryorefrigerator or in a cryostat in conjunction with a larger cryorefrigeration system. 
     Referring to FIG. 1, a single-stage cryorefrigeration apparatus of the invention includes a rotary valve  20  or other devices for controlling pressure oscillation, the actuator for the valve  20 , shown here as motor  30 , and a reservoir or buffer volume  50 , all integrated within housing  80 , thereby forming a single integrated unit. Flow channels  105  connect the orifices  40 , valve  14 , reservoir  50  and external components  60  and  70 . 
     The integrated motor housing and reservoir is made from any suitable material capable of withstanding high pressure (i.e., greater than 300 psig), such as machined aluminum, copper, bronze, brass or stainless steel. In a preferred embodiment, the housing is machined from a single block of 6061-T6 aluminum. 
     The single stage pulse tube cryorefrigerator is a simple heat pump that pumps heat from a cooling load (not shown) to a heat sink  110 , and thus to the ambient environment. Compressor  10 , typically a piston type compressor, delivers cryogen under pressure to the pressure oscillation controller, such as, for example, a rotary valve  20  rotated by motor  30 . Housing  80 , which optionally serves as a heat sink  110 , integrates pressure oscillation controlling means  20 , its power source  30 , and reservoir  50  into a single unit, with the parts connected by flow channels  105 . Preferably, the orifices  40  are also integrated into the housing  80   
     In operation, compressor  10  delivers cryogen (usually helium) under pressure to pressure oscillation controlling means  20 , thereby causing an alternating mass flow throughout the pulse tube refrigerator. As shown in FIG. 1, as the rotary valve  20  rotates, the bores  14  through the body  15  of the rotary valve  20  alternately connect the regenerator inlet connection  12  to the pressurized cryogen inlet  11  connected to the compressor  10  output line  17  (the valve is shown in this position), and to the lower pressure cryogen outlet  13  connected to the compressor  10  return line  16 . The alternating pressure and mass flow produced by compressor  10  and pressure oscillator  20  constitutes pressure/volume (PV) work, causing regenerator  60  to pump heat from the cooling load to the heat sink, where the heat is ultimately rejected. The result of this heat pumping action is to lower the temperature of the cooling load. Meanwhile, the PV work travels down pulse tube  70 , where it is rejected as heat to the heat sink. 
     Regenerator  60  is typically filled with a stack of screens which acts as a thermal sponge, alternately absorbing heat from the cryogen and rejecting the absorbed heat back to the cryogen as the pressure oscillates. Pulse tube  70  is a thin-walled tube of a lower thermal conductivity material, such as stainless steel. Pulse tube  70  has screen regions, preferably of copper, at both the hot and cold ends. The two screen regions typically are thermally connected to copper blocks to form the cold and hot end heat exchangers of the pulse tube. 
     FIGS. 4 a  and  4   b  show a side cut-away and top view, respectively, of the integrated buffer volume and pressure control housing  80  of the single-stage embodiment of the invention. The motor  30  mounts within a motor chamber, and the valve  20  is connected to the motor shaft. A valve disk  21  is attached to the valve  20 , with gas passage holes  22  allowing gas to pass through the valve body  20  and disk  21 . The section above the housing  80  is not shown, but would be mounted above the housing  80  in this figure, so that gas from the valve disk  21  holes  22  would pass to and from that section. 
     Referring to FIG. 2, a two-stage cryorefrigeration apparatus of the invention includes a first cooling stage  90  and a second cooling stage  100 , the first cooling stage  90  having a first stage temperature which is higher than a second stage temperature of the second cooling stage  100 . The two-stage cryorefrigerator includes a rotary valve  20  and motor  30  for controlling pressure oscillation, and a primary reservoir  50  and a secondary reservoir  51 , all integrated within housing  80 . Thus, housing  80  integrates the housing for pressure oscillation means  20  and its power source  30  with the reservoirs  50  and  51 , thereby forming a single integrated unit. Optionally, the secondary reservoir  51  is externally located, as will be discussed below. 
     The integrated motor housing and reservoir is made from any suitable material capable of withstanding high pressure (i.e., greater than 300 psig). As in the single-stage embodiment, these materials include copper, brass, bronze, stainless steel or aluminum, and in a preferred embodiment preferably 6061-T6 aluminum. 
     In addition to the components of the single-stage pulse tube cryorefrigerator, the two-stage cryorefrigeration apparatus of the invention includes first and second stage regenerators  61  and  62 , and first and second stage pulse tubes  71  and  72 . 
     The lower-temperature second stage pulse tube  72  is connected in series or parallel with the cold end of first stage pulse tube  71 . In operation, compressor  10  supplies a continuous pressure wave to first stage regenerator  61 . After providing cooling in the first stage regenerator  61 , the pressure wave provides further cooling in second stage regenerator  62 , with the cold end of second stage second stage pulse tube  72  being in thermal contact with the cooling load (not shown). The pressure wave continues through the two pulse tubes  71  and  72 , and the PV work is rejected as heat to the heat sink. 
     FIG. 3 shows a perspective view of the cryorefrigerator of the invention, in a two-stage embodiment corresponding to the cut-away and top drawings of FIGS. 5 a  and  5   b . At the top is the secondary buffer volume  51  housing, which is mounted to the integrated pressure control/buffer volume housing  80 . The inlet  11  for refrigerant gas and electrical connector  82  for the pressure control extend from the side of housing  80 . A lower section  81  provides connection to the gas outlet  13 . The pulse tube  71  and  72  and regenerators  61  and  62  extend below the lower section  81 . A flange  83  allows the lower part of the cryorefrigerator (pulse tubes and regenerators) to be suspended within a vacuum tank for insulation (not shown). 
     Referring to FIGS. 5 a  and  5   b , the cutaway view of FIG. 5 a  is shown inverted relative to the complete view of FIG. 3, so that the secondary buffer volume  51  is on the bottom. It can be seen that the secondary reservoir or buffer volume  51  is mounted to the housing  80 , with a gas-tight joint. The motor  30  mounts within a motor chamber, and the valve  20  is connected to the motor shaft. A valve disk  21  is attached to the valve  20 , with gas passage holes  22  allowing gas to pass through the valve body  20  and disk  21 . The lower section  81  is not shown, but would be mounted above the housing  80  in this figure, so that gas from the valve disk  21  holes  22  would pass to and from the lower section  81 . 
     FIG. 5 b  also shows that additional buffer volumes  52  and  53  could be optionally be incorporated into the primary housing  80 , for use in two-stage, three-stage or higher embodiments. Also, although the buffer volumes  50 ,  52  and  53  are shown as round bores in the housing  80 , it will be understood by one skilled in the art that the buffer volumes could be oval, rectangular, or any regular or irregular shape desired, depending on the volume needed and the configuration of the motor/valve chambers and other elements incorporated into the housing. 
     FIGS. 6 a  and  6   b  show an alternate housing for a two-stage cryorefrigerator. The parts are all as described for the embodiment of FIGS. 3,  5   a  and  5   b , with the exception that the secondary reservoir  51  is contained in a separate housing  54  mounted to the side of the main housing  80 , instead of to the end of the housing  80  as shown in the embodiment of FIGS. 5 a  and  5   b . In FIG. 6 b , it is seen that primary buffer volume  50  can be rectangular in shape, although as discussed above, the shape might vary widely within the teachings of the invention, depending on the specific needs of the embodiment. 
     Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.