BACKGROUND OF THE INVENTION 
     The present invention relates to Gifford McMahon (G-M) type multi-port pulse tube refrigerators. G-M type pulse tubes consist of a compressor, a valve mechanism, and an expander. The present invention improves on a previous rotary valve disc and valve seat mechanisms. The improvement lies in replacing the face seal ports for flow to and from the regenerator with a rotary spool valve having radial clearance, while retaining the face seal ports for flow to and from the pulse tubes. 
     It has been found that best performance at 4 K has been obtained with a pulse tube as shown in FIG. 9 of Gao, U.S. Pat. No. 6,256,998. This design has two valves controlling flow to the regenerator, and four valves controlling flow to the warm ends of the pulse tubes, which open and close in the sequence shown in FIG. 11 of U.S. Pat. No. 6,256,998. The single stage version of this pulse tube has four valves, two to the regenerator and two to the pulse tube, thus this control is commonly referred to as four-valve control. The main problem of multi-ported rotary disc valves is that the diameter of the disc is significantly increased, relative to the two ported valves that are commonly used in standard G-M refrigerators to control flow to and from the regenerator, to accommodate the valve ports that control flow to and from one or two pulse tubes. 
     Various solutions have been proposed to minimize the diameter of a rotary disc valve but the high to low pressure difference increases the sealing force substantially and thus requires a motor with larger torque to turn it. The sealing force of a larger diameter valve can be reduced by transferring some of the force to an axial bearing as described in Lobb, U.S. Pat. No. 4,987,743, and Heron, U.S. Pat. No. 6,694,749. 
     While these methods to reduce the sealing force, and thus the torque required to turn the valve, are effective, the wear dust from the plastic type material that is typically used for the valve disc against a hard valve seat generally collects in the orifices that control flow to and from the pulse tubes and changes the cooling performance over time. 
     The problems of reducing the torque required to turn the valve, eliminating wear dust, and extending the life of the valve, have been addressed in pending patent application U.S. Ser. No. 60/551,154, entitled “Wearless Valve”. This solution to these problems is based on supporting the rotating valve disc on a bearing and maintaining a small clearance between the valve disc and the valve face. This works well when the proper clearances are established. The small ports that control flow to and from the pulse tubes are very sensitive to leakage while the ports that control flow to and from the regenerator are less critical. 
     Yaron, U.S. Pat. No. 5,901,737 describes a rotating spool valve for a G-M type refrigerator that uses gas bearings to center the spool so that a small clearance can be maintained without contact. Sarcia, U.S. Pat. No. 4,333,755 describes a spool valve that shuttles back and forth to switch flow to the regenerator in a G-M type refrigerator 
     It is an object of the present invention to provide an alternate solution to these problems in a simpler construction by using a rotary spool valve. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a solution to these problems in a simple construction by using a rotary spool valve having radial clearance to control flow to and from the regenerator, and using face seal ports on the end of the spool to control flow to and from the pulse tubes. The diameter of the spool is much less than the diameter of a multi-ported disc valve so the torque required to turn it is reduced, even when ceramic materials with very low wear rate are used. In practice a ceramic material was used that was observed to show no signs of wear after about 15,000 hours of operation. 
     The present invention has been implemented with a ceramic material and a design that uses radial pressure balancing so that the friction forces between the spool and sleeve are minimized and a gas bearing is not needed. 
     The benefits of lowered sealing force, lowered torque required to turn the valve, and prevention of wear dust are obtained by using a rotary spool valve having radial clearance for the ports that control flow to and from the regenerator, while using axial face seal ports on the end of the spool for flow to and from the pulse tubes. 
     The spool, sleeve, and seat are all subject to wear. It is therefore important to use materials of construction that are highly resistant to wear. Ceramic materials are the preferred material of construction, with alumina ceramic being the most preferred material for the spool, sleeve, and seat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a four-valve two-stage pulse tube. 
         FIG. 2  is a timing chart for the valves shown in  FIG. 1 . 
         FIG. 3  is a cross section view of the present invention showing the porting of the valve, the relation to other components of the valve assembly, and the flow relation to the compressor and pulse tube. 
         FIG. 4  is a view of the faces of a valve seat and the end of the spool, showing the ports for flow to and from two four valve type pulse tubes, and having two cooling cycles per revolution of the spool. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic of a two-stage four-valve pulse tube refrigerator  10  that shows the gas flow paths through the system.  FIG. 1  illustrates refinements in a basic two-stage four-valve pulse tube refrigerator such as that illustrated in FIG. 9 of U.S. Pat. No. 6,256,998. High-pressure gas, Ph, flows from compressor  60  through gas line  57  to valves  11  (V 1 ),  13  (V 3 ), and  15  (V 5 ). Low-pressure gas, Pl, returns to compressor  60  from valves  12  (V 2 ),  14  (V 4 ), and  16  (V 6 ) through line  58 . Valves V 1  and V 2  control the flow to and from regenerator  21  (R 1 ) through line  50 . 
     Valve V 3  controls the flow to the first stage pulse tube  31  (PT 1 ) through line  53 , orifice  43  (O 3 ) and line  51 . Valve V 5  controls the flow to the second stage pulse tube  32  (PT 2 ) through line  55 , orifice  45  (O 5 ) and line  52 . Valve V 4  controls the flow from PT 1  through line  51 , orifice  44  (O 4 ) and line  54 . Valve V 6  controls the flow from PT 2  through line  52 , orifice  46  (O 6 ) and line  56 . 
     Some of the gas that flows in and out of the warm end of PT 1  flows through line  51 , orifice  41  (O 1 ), and buffer volume  33  (B 1 ). Similarly, some of the gas that flows in and out of the warm end of PT 2  flows through line  52 , orifice  42  (O 2 ), and buffer volume  34  (B 2 ). 
     The inlet ends of R 1 , PT 1 , and PT 2  are near ambient temperature while the other ends of PT 1  and PT 2  get cold as a result of the pulsing of gas into the cold ends after it flows through regenerator R 1 , regenerator  22  (R 2 ), and connecting tubes  23  and  24 . The gas that remains in the pulse tubes can be thought of as gas pistons. Gas flowing into the warm ends of PT 1  and PT 2  control the motion of the gas piston so that refrigeration is produced at the cold ends. A further description of the operation of a four-valve two-stage pulse tube is contained in U.S. Pat. No. 6,256,998. 
     The refinements shown in  FIG. 1  relative to FIG. 9 of U.S. Pat. No. 6,256,998 are orifices O 3 , O 4 , O 5 , O 6 , and the division of the buffer volume into two separate volumes, B 1  and B 2 . The orifices preferably are variable and can be adjusted to optimize the cooling during the manufacturing process. 
     Once the optimum size of the flow passages is determined, they can be incorporated into the ports in valves V 3 , V 4 , V 5 , and V 6 . Splitting the buffer volume into separate volumes for each pulse tube eliminates the possible circulation of gas from one pulse tube to the other through the buffer volume. 
       FIG. 2  is a timing chart for valves V 1  to V 6  showing the open periods that have been found to optimize the cooling. It is important to recognize the differences in timing for each of the valves and to emphasize the importance of having a fixed relationship of the opening and closing of each port. 
       FIG. 3  is a cross section view of valve assembly  100 , which also shows the flow relation to the compressor and pulse tube. Designations are the same as  FIG. 1 . Valve assembly  100  consists of spool  1 , sleeve  2 , valve motor  3  that turns shaft  4  which in turn holds drive pin  5  that engages and turns spool  1 . These components are contained in housing  6  which is sealed by valve plate  7 . Manifold  8 , which is machined into housing  6 , contains gas that cycles to and from regenerator  21  through line  50 . Manifold  9 , which is also machined into housing  6 , connects to the return side of compressor  60  through line  58  and contains gas at low pressure, Pl. 
     Manifolds  8  and  9  are sealed by “O” rings  17  on the outside of sleeve  2 . Gas at high pressure, Ph, flows from compressor  60  through line  57  into housing  1  where it fills the space around motor  3  and inside spool  1 . The difference between Ph acting on the top surface of spool  1 , and part of the bottom surface of spool being at Pl, provides the force needed to seal the ports in the bottom of spool  1  in sliding contact with the ports in valve plate  7 . 
     The cross section view of sleeve  2  shows circular ports  111  and  112  at a position 180° apart. This is the case for two cooling cycles per revolution of spool  1 . The right cross section view of spool  1  and sleeve  2  shows the porting for gas at Ph to flow to regenerator  21  through ports  11  (V 1 ) and  111 , and to pulse tubes  31  and  32  through V 3  and V 5 . 
     Valve V 1  is open when port  11  in spool  1  is aligned with port  111  in sleeve  2 . Valve V 3  supplies gas at Ph to the warm end of PT 1 ,  31 , through line  53  orifice  43  and line  51 . Valve V 5  supplies gas at Ph to the warm end of PT 2 ,  32 , through line  55  orifice  45  and line  52 . 
     The left cross section view of spool  1  shows the porting when spool  1  is rotated 90° with respect to the right hand view. This view shows the porting for gas at Pl to flow from regenerator  21  through port  111 ,  12  (V 2 ) and  112 , and from pulse tubes  31  and  32  through V 4  and V 6 . 
     Valve V 2  is open when port  12  in spool  1  is aligned with ports  111  and  112  in sleeve  2 . Valve V 4  returns gas at Pl to compressor  60  from the warm end of PT 1 ,  31 , through line  51  orifice  44  line  54  and line  58 . Valve V 6  returns gas at Pl to compressor  60  from the warn end of PT 2 ,  32 , through line  52  orifice  46  line  56  and line  58 . The design of the ports for V 3 , V 4 , V 5 , and V 6  are described in connection with  FIG. 4 . 
     The phase chart in  FIG. 2  represents one cycle as 360°; however, a spool valve that only had one cycle per revolution would have very unbalanced lateral pressure forces that would result in high friction forces. A spool valve with radial ports should have at least two cooling cycles per revolution of the valve to balance the lateral pressure forces. 
     A two-cycle valve would have two ports  111 , and ports  112 , in sleeve  2 , 180° apart, as shown in  FIG. 3 . These are typically round holes. There would also be two ports  11 , and ports  12 , in spool  1 , 180° apart. These are typically slotted to provide the duration of time that is desired for the port to be open. 
       FIG. 4  is a view of the face of a valve seat  7  and the end of spool  1 , showing the ports for flow to and from two four valve type pulse tubes, and having two cooling cycles per revolution of the spool. Ports  17  are symmetrical, as are ports  18 , in spool  1 . Ports  17  are in communication with gas at Ph in the center of spool  1 . Ports  18  are in communication with gas at Pl, through hole  119  and circumferential groove  118  on the outside of spool  1 , as seen in  FIG. 3 . Port  113  in valve seat  7  is the inlet to line  53  as seen in  FIG. 3 . Valve  3  is open when port  17  slides over port  113 . Port  115  in valve seat  7  is the inlet to line  55  as seen in  FIG. 3 . 
     Valve  5  is open when port  17  slides over port  115 . Valve  5  is open longer than valve  3  by virtue of port  115  being slotted relative to port  113 . Valve  4  is open when port  18  slides over port  114 . Port  114  in valve seat  7  is the inlet to line  54  as seen in  FIG. 3 . Valve  6  is open when port  18  slides over port  116 . Valve  6  is open longer than valve  4  by virtue of port  116  being slotted relative to port  114 . 
     Having valves V 3  and V 5 , which both control flow at Ph, at the same radius, and V 4  and V 6 , which both control flow at Pl, at a second radius, minimizes the difference between the inside and outside diameters of the spool. 
     If three cooling cycles per revolution of spool  1  are desired then the angle between the right and left cross sections in  FIG. 3  is 60° instead of 90° and there would be three pairs of radial ports at 120° that connect to regenerator  21 . The end view of spool  1  shown in  FIG. 4  would have three pairs of ports  17  and  18  at 120°. Valve seat  7  in  FIG. 4  would have the same ports as shown, but ports  116  and  114  in the outer track would be 120° rather than 180° apart, and ports  113  and  115  in the inner track would be spaced 60° from them. 
     It is recognized that variations of the preferred design that has been described can be used, such as wear resistant materials other than ceramics, axial loading by mechanical rather than pneumatic means, flow in the reverse direction, or ports on more tracks. 
     The foregoing describes the invention in terms of embodiments foreseen by the inventors for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.
The problems of reducing the torque required to turn the valve, eliminating wear dust, and extending the life of the valves in Gifford McMahon (G-M) type multi-port pulse tube refrigerators are solved by using a rotary spool valve having radial clearance to control flow to and from the regenerator, and using face seal ports on the end of the spool to control flow to and from the pulse tubes.