Cryopump

This invention relates to cryopumps and has particular application to crypumps cooled by two-stage closed-cycle coolers.

Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature surface, usually operating in the range of 4.degree. to 25.degree. K., is the primary pumping surface. This surface is surrounded by a higher temperature surface, usually operated in the temperature range of 70.degree. to 130.degree. K., which provides radiation shielding to the lower temperature surface. In addition, this higher temperature surface serves as a pumping site for higher boiling point gases such as water vapor. The radiation shielding generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and the chamber to be evacuated. In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shielding and condense on the primary pumping surface. A surface coated with an adsorbent such as charcoal or molecular sieve operating at or below the temperature of the primary pumping surface may also be provided in this volume to remove the very low boiling point gases. With the gases thus condensed and or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.

In systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the rear of the radiation shielding. The cold end of the second coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface or cryopanel which is connected to a heat sink at the coldest end of the second stage of the coldfinger may be a plain metal surface or an array of metal surfaces arranged around and connected to the second stage heat sink. The primary pumping surface contains the low temperature adsorbent. A radiation shield which is connected to a heat station at the coldest end of the first stage of the coldfinger surrounds the primary cryopumping panel in such a way as to protect it from radiant heat. The radiation shield must be sufficiently spaced therefrom to permit substantially unobstructed flow of low boiling temperature gas from the vacuum chamber to the primary pumping surface. The frontal radiation shield is cooled by the first stage heat sink through the side shield. Typically, the temperature differential across that long thermal path from the frontal array to the first stage heat sink is between 30.degree. and 50.degree. K. Thus, in order to hold the frontal array at a temperature sufficiently low to condense out water vapor, typically less than 130.degree. K., the first stage must operate at between 80.degree. and 100.degree. K.

The heat load which can be accepted by a cryocooler is strongly temperature dependent. At high operating temperatures conventional cryocoolers can accept higher heat loads. Thus, a reduction in the temperature differential between the frontal array and the first stage heat sink will allow an increase in the operating temperature of the first stage heat sink. This will allow the cryocooler to accept a higher heat load while maintaining the frontal array at an acceptable operating temperature. To accomplish this reduction in temperature differential, conventional cryopump designs utilize high conductivity materials such as copper in the radiation shields. The gradient can be further reduced by increasing the cross sectional area of the radiation shielding to thus increase the thermal conductance of that shielding. This increased mass of the shielding adds both weight and cost to the product and disadvantageously increases the cool down time and regeneration time of the cryopump.

An object of this invention is to provide a cryopump which minimizes the temperature differential between a cryopanel and associated heat sink without substantially increasing the mass of the system while at the same time allowing the cryocooler to operate at a higher loading level (higher temperature).

In the primary embodiment of this invention, high conductance thermal struts provide a relatively short thermal path from a first stage heat sink to a frontal cryopumping surface. By adding these thermal struts to the system, the surrounding radiation shield need no longer serve as the primary thermal path to the frontal shield. Due to their shorter length, the struts can provide a given conductance between the frontal cryopanel and its heat sink with a lesser mass than would be required by radiation shields serving the same purpose.

To minimize the length of the thermal struts and to minimize interference with gas flow to the primary pumping surface, the struts may extend through holes in the primary pumping surface. They must be isolated from that surface, as by a clearance, in order to prevent loading of the coldest heat sink by thermal short circuiting of the higher temperature surface and the primary pumping surface. With such a structure, the frontal cryopanel need not be connected to the side radiation shield. With the cryopanel thus supported only by the thermal struts, fabrication is simplified.

In one form of the invention, the high conductance thermal path between the frontal cryopanel and its corresponding heat sink is provided by a heat pipe. Due to the very high thermal conductivity of a heat pipe, the length of the pipe is not so critical. The mass of a heat pipe would also be less than that of a corresponding thermal strut further enhancing its application.

A heat pipe extending from a heat sink to its associated cryopanel should generally have a fluid therein which vaporizes and condenses in a temperature range which includes the operating temperature of the cryopanel. Additional heat pipes or thermal struts may be required to bring the cryopanels down to the operating temperature of the cryopanel.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 is a cross sectional view of a cryopump embodying this invention;

FIG. 2 is a top view of the frontal array of the cryopump of FIG. 1.

FIG. 3 is a partial cross-sectional view of an alternative arrangement in which a heat pipe serves as a thermal switch.

The cryopump of FIG. 1 comprises a main housing 12 which is mounted to the wall of a work chamber along a flange 14. A front opening 16 in that housing 12 communicates with a circular opening in the work chamber. Alternatively, the cryopump arrays may protrude into the chamber and a vacuum seal be made at a rear flange. A two stage cold finger 18 of a refrigerator protrudes into the housing 12 through an opening 20. In this case, the refrigerator is a Gifford-MacMahon refrigerator but others may be used. A two stage displacer in the cold finger 18 is driven by a motor 22. With each cycle, helium gas introduced into the cold finger under pressure through line 24 is expanded and thus cooled and then exhausted through line 26. Such a refrigerator is disclosed in U.S. Pat. No. 3,218,815 to Chellis et al. A first stage heat sink, or heat station, 28 is mounted at the cold end of the first stage 29 of the refrigerator. Similarly, a heat sink 30 is mounted to the cold end of the second stage 32. Suitable temperature sensor and vapor pressure sensor elements 30 and 34 are mounted to the rear of the heat sink 30.

The primary pumping surface is a panel mounted to the heat sink 30. This panel comprises a disc 38 and a set of circular chevrons 40 arranged in a vertical array and mounted to disc 38. The cylindrical surface 42 may hold a low temperature adsorbent. Access to this adsorbent by low boiling point gases would be through chevrons 40. The surfaces 38, 40 and 42 can be loosely termed the primary, low temperature cryopanel.

A cup shaped radiation shield 44 is mounted to the first stage, high temperature heat sink 28. The second stage of the cold finger extends through an opening 45 in that radiation shield. This radiation shield 44 surrounds the primary cryopanel to the rear and sides to minimize heating of the primary cryopanel by radiation. The temperature of this radiation shield ranges from about 100.degree. K. at the heat sink 28 to about 130.degree. K. adjacent the opening 16.

A frontal cryopanel 46 serves as both a radiation shield for the primary cryopanel and as a cryopumping surface for higher boiling temperature gases such as water vapor. This panel comprises a circular array of concentric louvers and chevrons 48 joined by spoke-like plates 50. The configuration of this cryopanel 46 need not be confined to circular concentric components; but it should be so arranged as to act as a radiant heat shield and a higher temperature cryopumping panel while providing a path for lower boiling temperature gases to the primary cryopanel.

In conventional cryopumps, the frontal array 46 is mounted to the radiation shield 44, and the shield both supports the frontal array and serves as the thermal path from the heat sink 28 to that array. The shield 44 must be sufficiently large to permit unobstructed flow of gases to the primary cryopanel. As a result, the thermal path length of that shield from the heat sink 28 to the frontal array is long. To minimize the temperature differential between the frontal array and the heat sink 28, massive radiation shields have been required.

In accordance with this invention, thermal members 54 extend between a plate 56 mounted to the heat sink 28 and the frontal array. Those struts may extend through clearance openings in the primary panel 38 and are thus isolated from that panel, or they may pass outside of the primary pumping surfaces 38, 42. The struts 54 need not serve as radiation shields and are thus able to have a very short length between the heat sink 28 and the cryopanel 46. As a result, a thermal path having a given conductance can be obtained with a much lesser cross sectional area than would be required of the radiation shield if it served as the sole heat flow path. The heat flow path from the heat sink 28 to the center of the cryopanel 46 can be reduced to less than one-half the conventional path length through the radiation shield 44. This permits a reduction of 20 to 25 percent in overall mass of the entire array of elements connected to the heat sink 28.

Even greater reduction in mass can be obtained by using heat pipes as the thermal struts 54. Heat pipes are metallic tubes, sealed at each end and filled with a quantity of low boiling temperature liquid and its vapors. Liquid is carried to the warm end of each heat pipe at the frontal array by a wick. Heat input to the heat pipe there causes the liquid to vaporize. That heated vapor is quickly dissipated throughout the heat pipe and thus rapidly carries the heat to the cold end of the heat pipe at the plate 56. There, the vapor condenses, giving off its heat to the heat sink 28. The condensed liquid is then returned to the warm end by the wick. If the cryopump were oriented above the work chamber the condensed liquid would flow to the warm end without need for a wick within the pipes. Even without such a wick, the pipe can be loosely termed a heat pipe.

There is virtually no temperature differential along the length of a heat pipe. Thus, the cryopanel 46 operates at a temperature very close to the operating temperature of the first stage 29 of the refrigerator. As a result, a refrigerator having a first stage operating at near 130.degree. K. can be used. Because the thermal load capability of a refrigerator increases with its operating temperature, such a cryopump has a much increased load handling capability.

With a heat pipe, the length of the thermal strut is not so critical. Thus, the heat pipe need not extend through the primary pumping surface 38 and may actually run close to the radiation shield 44. For economic reasons, however, the straight, short heat pipe is preferred. Further, a heat pipe positioned within the primary cryopanel does not obstruct gas flow from the vacuum chamber to that cryopanel. Thus, even where the thermal struts are heat pipes they preferably extend through the surface 38 with a clearance for isolation from that surface.

FIG. 3 shows an alternative use of a heat pipe in the cryopump. In the system of FIG. 3, heat pipes 60 extend between the first and second stage heat sinks 28 and 39. As the cryopump is cooling down, the primary cryopanel is cooled by both stages of the refrigerator. The heat pipe is designed, however, so that as the temperature of the heat sink 30 approaches its operating temperature, the vapor condenses out of the heat pipe completely. With no vapor to transfer heat along the length of the pipe, the pipe then acts as an open thermal circuit.

As noted above, a heat pipe operates by the condensation and vaporization of a gas within the pipe at a heat sink and a heat source. A given heat pipe may operate in a specific temperature range. At temperatures above that range, all or most of the gas vaporizes and thereby greatly reduces the conductance of the pipe. At temperatures below the range, the medium within the heat pipe condenses out and freezes. Using a heat pipe designed such that it is operable in the operating temperature range of the refrigerator, the load which can be accepted by the cryopump during continuous operation can be increased. It is unlikely, however, that a single heat pipe can be operable throughout the entire cooldown temperature range of the cryopump as well as at the operating temperature of the cryopump. Thus, it can be expected that the primary heat pipe which is operable at the operating temperature range will not operate properly at higher temperatures during cooldown.

To provide for rapid cooldown of the system, a parallel thermal path between the frontal cryopanel and its associated heat sink 28 must be provided. In one form, that auxiliary thermal path is one or more parallel heat pipes, designed to operate at higher, overlapping temperature ranges. Alternatively, the parallel thermal paths may be solid thermal struts. In either case, it is preferred that the primary operating heat pipe and the parallel thermal paths be in the form of thermal struts 54.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changed in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, a closed cycle, two stage refrigerator is shown. A cryopump cooled by an open cycle refrigerant such as liquid nitrogen, hydrogen or helium may also be used. Also combinations of single and two stage closed cycle refrigerators may be used to provide the cooling. Also, a low temperature adsorber may be provided to take out gases which are not condensed at the operating temperature of the primary cryopanel.