Abstract:
This invention provides an improved means of quickly warming a pulse tube ( 165 ) by shifting the phase relation of flow to the warm end of the pulse tube relative to flow to the warm end ( 117 ) of the pulse tube relative to flow to the warm end of the regenerator ( 160 ) using a “four valve” concept and the “active buffer” concept. Several different pulse tube configurations and valve timing relations that are effective at reversing the cycle from the normal mode, which produces cooling at the pulse tube heat station, to a reverse mode that produces heating are disclosed.

Description:
This application is the National Stage of International Application No. PCT/US03/06580, filed Mar. 5, 2003, which claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 60/361,651, filed Mar. 5, 2002. 

   BACKGROUND OF THE INVENTION 
   The Gifford-McMahon (G-M) type pulse tube refrigerator is a cryocooler, similar to G-M refrigerators, that derives cooling from the compression and expansion of gas. However, unlike the G-M systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in cooling cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers. 
   G-M type pulse tube refrigerators are characterized by having a compressor that is connected to a remote expander by high and low pressure gas lines. The pulse tube expander has a valve mechanism that alternately pressurizes and depressurizes the regenerators and pulse tubes to produce refrigeration at cryogenic temperatures. 
   A Cryopump cooled by a Pulse Tube refrigerator needs to be quickly regenerated to minimize the time it is out of service. At present heaters are being used with GM refrigerators to rapidly warm up the cryopanels. Heaters can also be used to warm up cryopumps that are cooled by pulse tubes e.g. as disclosed in Japanese patent 00283036. When using a pulse tube to cool the cryopanels, warm up can also be achieved without heaters by circulating gas through the pulse tube, such as described in U.S. Pat. No. 5,927,081. 
   It is the object of the present invention to provide an improved means of quickly warming a pulse tube. 
   SUMMARY 
   This invention provides an improved means of quickly warming a pulse tube by shifting the phase relation of flow to the warm end of the pulse tube relative to flow to the warm end of the regenerator. Not all pulse tube phase shifting mechanisms lend themselves to fast warm up by changing the valve timing. Surprisingly, there are several different pulse tube configurations and valve timing relations that are effective at reversing the cycle from the normal mode, which produces cooling at the pulse tube heat station, to a reverse mode that produces heating. 
   Two phasing mechanisms that lend themselves to fast warm up are the “four valve” concept and the “active buffer” concept. These were first described in the following papers, I] Y. Matsubara, J. L. Gao, K Tanida, Y. Hiresaki, and M. Kaneko, “An experimental and analytical investigation of 4 K pulse tube refrigerator”, Proc. 7 th  Intl Cryocooler Conf., Air Force Report PL-(P-93-1001 (1993) pp. 166–186; and 2] S. W. Zhu, Y Kakami, K Fujioka, and Y Matsubar, “Active-buffer pulse tube refrigerator”, Proceedings of the 16th Cryogenic Engineering Conference, T. Haruyama. T. Mitsui and K. Yamafriji. ed. Eisevier Science. Oxford (1997), pp. 291–294. 
   A split rotary valve is disclosed that illustrates a simple means of providing the desired change of phase when it is turned in reverse. Single stage pulse tubes are used to illustrate the invention but the principals can be applied equally well to multi-stage pulse tubes. 
   Cryopumps, which are cooled by two stage pulse tubes that use this invention, can be quickly warmed up without the need for heaters. 
   Disclosed are several different pulse tube configurations and valve timing relations that are effective at reversing the cycle from the normal mode, which produces cooling at the pulse tube heat station, to a reverse mode that produces heating. 
   In one embodiment of the invention, a split rotary valve illustrates a simple means of providing the desired change of phase when it is turned in reverse. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a single stage pulse tube that is known in the art as having “four valve” control and to which the present invention can be applied. 
       FIG. 2  is a schematic of a variation of the  FIG. 1  control scheme to which the present invention can be applied. 
       FIG. 3  is a schematic of a second variation of the  FIG. 1  control scheme to which the present invention can be applied. 
       FIG. 4  is a schematic of a single stage pulse tube that is known in the art as having “active buffer” control. 
       FIG. 5   a  is a pressure vs. volume (P-V) plot of the gas that enters the cold end of the  FIG. 1  pulse tube during the normal cooling mode. 
       FIG. 5   b  is a P-V plot of the gas that enters the cold end of the  FIG. 1  pulse tube during the heating mode per this invention. 
       FIG. 6  is a cross section of dual rotary valve that can implement the P-V plot shown in  FIG. 5 . 
       FIG. 6   a  is a top view of the valve plate. 
       FIGS. 6   b  and  6   c  show views from the back of each valve disc while it is rotating to produce cooling as shown in  FIG. 5   a.    
       FIGS. 6   bb  and  6   cc  show views from the back of each valve disc while it is rotating in reverse to produce heating as shown in  FIG. 5   b.    
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is applicable to G-M type pulse tubes that use valves to control the phase relationship of the flow to the warm end of the regenerator relative to the flow to the warm end of the pulse tube. By changing the phase relationship, the pulse tube can be made to shift from a cooling mode to a warming mode. 
   The single stage pulse tube shown in  FIG. 1  illustrates an embodiment of the invention.  FIG. 1  shows Pulse Tube Refrigerator  100 , which is comprised of Regenerator  160 , Pulse Tube  165 , Connecting Tube  115 . Gas Line  110 , Gas Line  111 , Valve  120 , Valve  125 , Valve  910 , Valve  915 , Cold Heat Station  116 , and Hot Heat Station  117 . 
   Gas Line  110  brings high-pressure gas from the compressor and Gas Line  111  returns gas at low pressure to the compressor. Valve  120  admits high-pressure gas to the warm end of Regenerator  160  and Valve  125  returns gas from the warm end of Regenerator  160  to the compressor. Valve  910  admits high-pressure gas to the warm end of Pulse Tube  165  and Valve  915  returns gas from the warm end of Pulse Tube  165  to the compressor. Connecting Tube  115  connects the cold end of Regenerator  160  with the cold end of Pulse Tube  165 . Heat is picked up at the cold end of Pulse Tube  165  in Cold Heat Station  116 . It may be transferred to ambient temperature from Hot Heat Station  117 , or returned to the compressor through Valve  915 . 
   Cooling is produced at the cold end of Pulse Tube Refrigerator  100  when the valve timing is approximately as shown in Table 1 under the heading “COOLING”. With this timing the P-V relation for the gas flowing in and out of the cold end of Pulse Tube  165  is approximately as shown in  FIG. 5   a . The phases when each of the valves is open are noted on  FIG. 5   a.    
   A P-V plot that follows a clockwise path is known to produce work. The work is equal to the cooling that is produced and can be measured from the area of the P-V plot. Energy in the form of work is transferred from a low temperature to ambient temperature. 
   When the timing of opening and closing Valves  910  and  915  relative to Valves  120  and  125  is changed as shown in Table 1 under the heading listed “WARM UP”, the P-V relation changes to approximately the plot shown in  FIG. 5   b . This plot follows a counter clockwise path that transfers work energy from ambient temperature to the cold end of Pulse Tube  165 . The heating is equal to the amount of work that is transferred and will cause the cold end of Pulse Tube  165  to warm up. 
     FIG. 2  shows Pulse Tube Refrigerator  200  as a variation of the  FIG. 1  control scheme in which Buffer Tank  180  is connected to the warm end of Pulse Tube  165 . Valve  205  controls the timing of flow in and out of Buffer Tank  180 . Valve timing for the normal cooling mode is shown in the upper part of Table 2 and timing for the warm up mode is shown in the lower part of Table 2. Adding Buffer Tank  180  and Valve  205  improves the efficiency of the pulse tube relative to  FIG. 1  by having some of the gas that flows to and from the warm end of the pulse tube come from Buffer Tank  180  rather than from the compressor. The P-V plots for cooling and heating are essentially the same as those shown in  FIGS. 5   a  and  5   b . In the cooling mode, this is accomplished by opening Valve  205  before opening Valve  190 . 
     FIG. 3  shows Pulse Tube Refrigerator  300  as a variation of Pulse Tube  200  in which Valve  205  is replaced with Fixed Restrictor  145 . It serves the same function as Valve  205  but is less efficient because gas flows in and out of Buffer Tank  180  during the entire cycle and some of the gas comes direct from the compressor. Valve timing is approximately the same as shown in Table 2 with Valve  205  deleted. The P-V plots for cooling and warm up are similar to  FIGS. 5   a  and  5   b.    
     FIG. 4  is a schematic of Pulse Tube Refrigerator  400 , which has “active buffer” control. Gas from the compressor flows through Gas Line  110  into the warm end of Regenerator  160  through Valve  120 . Gas returns to the compressor from Regenerator  160  through Valve  125  and Gas Line  111 . Gas flow to and from the warm end of Pulse Tube  165  comes through Valves  510  and  512 , which connect to Buffer Tank  184  and through Valves  520  and  522 . which connect to Buffer Tank  182 . Buffer Tank  184  is near high pressure, PH, and Buffer Tank  182  is near low pressure, PL. 
   Table 3 shows the valve timing for cooling in the upper part of the table and for warm up in the lower part of the table. The standard active buffer control system that is designed solely for cooling would have a single valve, Valve  510 , in place of Valves  510  and  512  and a single valve, Valve  520 , in place of Valves  520  and  522 . In order to have a counter clockwise path for the PV plot, so the pulse tube will quickly warm up, it is necessary to add Valves  512  and  522  and shift their timing relative to the other valves. 
     FIG. 6  is a cross section of dual rotary valve Assembly  400  that can implement the P-V plot shown in  FIGS. 5   a  and  5   b . Assembly  400  is comprised of a fixed Valve Plate  430 . primary Valve Disc  410 , secondary Valve Disc  420  Drive Shaft  490 , Drive Pins  402 . Springs  406 , spring Retainer Pins  402 , and Enclosure  480 . 
   A top view of Valve Plate  430  is shown in  FIG. 6   a . Valve Plate  430  has a center hole  432 , through which Drive Shaft  490  and high-pressure gas pass, Port  430  which connects to low-pressure return line  111 , Port  436  which connects Valve Disc  410  to the warm end of Regenerator  160 , and Port  438  which connects Valve Disc  420  to the warm end of Pulse Tube  165 . 
     FIG. 6   b  shows a top view of Valve Disc  410  as it is rotating in a clockwise direction. Drive Pin  402   a , which is engaged in Slot  412 , drives Valve Disc  410 . A mechanism to center the valve discs on Shaft  490  without blocking the flow of high-pressure gas is not shown. The face of Valve Disc  410  that is in contact with Valve Plate  430  has slots that alternately connect the high-pressure supply and low-pressure return gas to Port  436 . 
   With reference to Table 1 “Cooling”, Valve Disc  410  is shown at 225° with Valve  125  ( FIG. 1 ) just opening. The slot shown in cross section A—A connects the warm end of Regenerator  160  to low-pressure Port  434  for about 125°.  FIG. 6   b  shows the slot that affects the open period for Valve  120 . This slot connects the high-pressure supply from Line  110  to the warm end of Regenerator  160  for about 125°. High-pressure gas  110  acting on the back side of Valve Disc  410  and low-pressure gas  111  in the slot on the face create a pressure difference during operation that results in a force that seats Valve Disc  410  against Valve Plate  430 . Prior to starting the compressor Spring  406   a , which is retained by Pin  404   a . holds Valve Disc  410  against Valve Plate  430  with sufficient force to get an initial seal. 
     FIG. 6   c  shows a view from the back of Valve Disc  420  as it is rotating in the same direction as Valve Disc  410 . Drive Pin  402   b  engages Faces  422  to drive Valve Disc  420 . The gap between Faces  422  and  424  can be thought of as a slot like  412  that has been enlarged. The face of Valve Disc  420  that is in contact with Valve Plate  430  has slots that alternately connect the high-pressure supply and low-pressure return gas to Port  438 . With reference to Table 1 “Cooling”, Valve Disc  420  is shown at 2250, with Valve  915  ( FIG. 1 ) open. The slot shown in cross section B–B connects the warm end of Pulse Tube  165  to low-pressure Port  434  for about 90°. 
     FIG. 6   c  shows the slot that affects the open period for Valve  910 . This slot connects the high-pressure supply from Line  110  to the warm end of Pulse Tube  165  for about 90°. High-pressure gas  110  acting on the back side of Valve Disc  420  and low-pressure gas  111  in the slot on the face create a pressure difference during operation that results in a force that seats Valve Disc  420  against Valve Plate  430 . Prior to starting the compressor Spring  406   b , which is retained by Pin  404   b , holds Valve Disc  420  against Valve Plate  430  with sufficient force to get an initial seal. 
   Rotation of Valve Discs  410  and  420  in the direction shown in  FIGS. 6   b  and  6   c  produces cooling as shown in  FIG. 5   a .  FIGS. 6   bb  and  6   cc  show Valve Discs  410  and  420  being turned in the opposite direction by having reversed the direction of rotation of Drive Shaft  412 . Valve Disc  410  is shown in the same position as in  FIG. 6   b  but Drive Pin  402   a  is acting on the other side of Slot  412 . Valve Disc  420 , on the other hand, is shown rotated about 90°, with Drive Pin  402   b  now acting on Faces  424 . The shift in angular position of Valve Disc  420  relative to Valve Disc  410  affects the valve timing shown in table 1 under “Warm Up” and results in the P-V plot shown in  FIG. 5   b.    
     FIG. 6  shows a valve assembly that executes one cycle of the pulse tube with one revolution of the valve. This was done to keep the drawing simple. In actual practice it is more common to have two cycles of the pulse tube for each revolution of the valve discs in order to have the valve face wear more uniformly. Valve assemblies similar to the one shown in  FIG. 6  can also be made to implement the valve timing in the cooling and warm up modes shown in Tables 2 and 3. Similar valve assemblies can be made for two stage pulse tubes that would provide cooling in one direction of rotation and heating in the other.