Abstract:
An improved dewar design that accelerates the manufacturing process of a dewar. In a preferred embodiment, the dewar includes an evacuation port that may be larger in size by a factor of ten over the size of evacuation ports of conventional dewars. The oversized evacuation port, however, does not result in an increase in the overall size or profile of the dewar. The dewar is evacuated and hermetically sealed using an re-usable evacuation tool.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to dewars for high temperature superconducting (HTS) filter systems for use in, for example, cellular PCS systems and, more particularly, an evacuation port and closure for such dewars. 
     BACKGROUND OF THE INVENTION 
     Recently, substantial attention has been devoted to the development of high temperature superconducting radio frequency (RF) filters for use in, for example, cellular telecommunications systems. Those skilled in the art will appreciate that, when multiple HTS filters are deployed, for example, within a dewar cooled by a cryocooler, on a telecommunications tower, substantial durability and reliability issues may arise. For example, when a system is to be mounted at the top of a tower, the system must be able to withstand significant changes in climate and weather, and the system must be reliable and require minimal maintenance. 
     In this regard, the final step in manufacturing a durable, long life dewar, i.e., a dewar having a life span greater than 10 years, is to vacuum bake the dewar at as high a temperature as possible to degas the dewar and its components, which include temperature sensors, HTSC RF filters, getters, etc., without damaging these components and impacting their functional capability. While the dewar is baked, it is attached to a vacuum pump via a tip-off tube and evacuated. The vacuum pump will reduce the pressure within the dewar to less than 10 −4  torr and typically to less than 10 −8  torr at the time the tip-off tube is pinched off to seal the dewar. At these low pressures, the gas molecules that are outgassing from the dewar and its components will move in straight lines until the gas molecules strike a wall of the dewar or component, or another gas molecule. The gas molecules will be removed or evacuated from the dewar as they find the inside of the tip-off tube. Because the tip-off tube typically has a relative small inside diameter to minimize the size or footprint of the dewar, the degassing process tends to be quite time consuming. Typically, the dewar is vacuum baked for several days until the outgassing decreases to an acceptable level. 
     With the increased demand from the cellular telecommunications industry for these dewar deployed HTS filters, dewar manufacturers must find ways to increase the supply of these dewars at lower costs. Because the vacuum baking of the dewars is the most time intensive step of the manufacturing process, one option to increase the output of dewars would be to invest in more automated vacuum bakeout equipment. However, automated vacuum bakeout equipment is very expensive and, thus, this option is not necessarily the most desirable. Another option would be to reduce the time required to vacuum bake the dewars by increasing the rate at which the gas molecules are evacuated from the dewar. Because the gas molecules are only evacuated as they find the inside of the tip-off tube, the rate at which the gas molecules were evacuated would increase if the size of the tip-off tube were increased. However, because the length of the tip-off tube, or distance from the dewar at which the tip-off tube is pinched off, is directly proportional to the diameter of the tip-off tube, this option would result in an undesirable increase in the overall size or profile of these dewars. 
     Thus, it would be desirable to increase the manufacturing output of these dewar deployed HTS filters without drastically increasing a manufacturers capital equipment investment or increasing the size of the dewar. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved dewar for high temperature superconducting RF filter systems and process for manufacturing the same. In a particularly innovative aspect, a dewar in accordance with the present invention includes an oversized evacuation port, which may be greater in size by about a factor of ten than the size of an evacuation port of a conventional dewar, without increasing its overall size or profile. The incorporation of an oversized evacuation port is particularly advantageous from a manufacturing standpoint in that the time it takes to vacuum bake the dewar is substantially reduced. Specifically, there is a greater probability that the gas molecules being outgassed from the dewar and its components will find the inside diameter of a larger evacuation port and, thus, will be more quickly evacuated from the dewar. Moreover, a dewar in accordance with the present invention comprises a low profile cap that seals the evacuation port. 
     Prior to vacuum baking the dewar, a re-usable evacuation tool is coupled to the evacuation port of the dewar. The tool includes a housing, a capping tool positioned in the housing, and a side arm extending from the housing, which is attachable to a vacuum pump. The tool is advantageously bakeable up to a temperature of 100° C. to 125° C. Once the vacuum bakeout process is completed, the capping tool is actuated to cold weld the low profile cap to the tip-off flange on the end of the evacuation port and hermetically seals the dewar. 
    
    
     Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a typical dewar of the prior art that has high temperature superconductor RF filter assemblies thermally attached to a heatsink. 
     FIG. 2A is a plan view of a tip-off tube of the prior art that has been pinched off. 
     FIG. 2B is a partial cross-sectional view of the tip-off tube shown in FIG. 2A taken along line  2 B— 2 B. 
     FIG. 3 is a partial plan view of a cap port and evacuation tool of the present invention, wherein the evacuation tool is attached to the tip-off flange of a dewar. 
     FIG. 4 is a partial plan view of the cap port captured by the evacuation tool. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 1 provides across-sectional view of a typical dewar  10  of the prior art. The dewar  10  includes a hermetically sealed cylindrical drum-like housing  11  preferably formed from stainless steel. A heatsink  12 , to which high temperature superconductor (HTS) RF filter assemblies (not shown) are thermally attached, is fixed in place within the housing  11  via a series of struts  13  which attach to a series of supports  19  embedded in the housing  11 . The heatsink  12  is cooled by a closed cycle cryogenic cooler (not shown) that thermally interfaces to a dewar coldfinger  14  through a supply tube  15 . The supply tube  15 , which extends through the base of the housing  11 , includes a flange  16  that mates to a cryo-cooler flange (not shown). The dewar  10  also typically includes a series of DC power connectors  18 , a series of RF connectors  17 , and a getter  20 . Lastly, a tip-off tube  24 , which is typically formed from annealed copper tubing, is brazed to mate with an evacuation port  22 . 
     A final step in the process of manufacturing a durable dewar  10  with a life expectancy of 10 years or more, is to vacuum bake the dewar  10  at as high a temperature as possible to degas the dewar  10  and its components, which include temperature sensors, HTSC RF filters, getters, etc., without damaging these components and impacting their functional capability. While the dewar  10  is baked, the tip-off tube  24  is attached to a vacuum pump (not shown) to evacuate the dewar  10 . The vacuum pump will reduce the pressure within the dewar  10  to less than 10 −4  torr and typically to less than 10−8 torr at the time the tip-off tube  24  is pinched off, i.e. squeezed between two rollers that cause the copper tubing of the tip-off tube  24  to cold weld to itself, to create a hermetic seal (see FIGS.  2 A and  2 B). At these low pressures, the gas molecules that are outgassing from the dewar  10  and its components will move in straight lines until the gas molecules strike a wall of the dewar  10  or component, or another gas molecule. The gas molecules will be removed or evacuated from the dewar  10  as they find the inside of the tip-off tube  24 . The larger the inside diameter of the tip-off tube  24 , the easier it is for the molecules to be removed by the vacuum pump. However, because the distance from the dewar  10  at which the tip-off tube  24  can be pinched-off is directly proportional to the diameter of the tip-off tube  24 , and because it is desirable to minimize the dewar&#39;s  10  profile, the tip-off tube  24  typically has a relative small inside diameter. As a result, the degassing process tends to be quite time consuming as the gas molecules slowly find the inside of the small diameter tip-off tube  24 . Typically, the dewar  10  is vacuum baked for several days until the outgassing decreases to an acceptable level. 
     To accelerate the vacuum baking step of the manufacturing process, the evacuation port of a dewar of the present invention has a cross-sectional area that is significantly larger than the cross-sectional area of the tip-off tube of a conventional dewar. Moreover, a dewar evacuation port according to the present invention can be increased in size by a factor of ten over the conventional dewar evacuation port without increasing the overall size or profile of the dewar. Increasing the cross-sectional area of the evacuation port significantly increases the probability that a gas molecule will be removed by the vacuum pump and, thus, shortens the time the dewar must be vacuum baked. 
     Turning to FIG. 3, the dewar  110  of the present invention includes a large diameter evacuation port  122  that extends from the housing  111  of the dewar  110 . A tip-off flange  126  is formed on the end of the evacuation port  122 . A reusable evacuation tool  130 , which is used to evacuate the dewar  110  and seal its large diameter evacuation port  122 , is coupled to the dewar  110 . The evacuation tool  130  is advantageously bakeable at a temperature of up to 125° C. and comprises metallic surfaces that are low outgassing. 
     The evacuation tool  130  includes an elongated cylindrical housing  132  and a cylindrical side arm or vacuum port  138  that opens into the housing  132  and extends from the housing  132  to a vacuum pump (not shown). A flange  134  is formed on the end of the housing  132  adjacent the dewar  110  and is coupled to the tip-off flange  126  of the dewar  110  with a clamp (not shown). A vacuum seal is maintained between the tip-off flange  126  and the flange  134  of the evacuation tool by a low outgassing o-ring  136  such as a Viton® or Kal Rez™ (Dupont trademarks) o-ring. The other end of the housing  132  is sealed with a cover  131 . 
     The evacuation tool  130  includes a capping tool  140  used to cap the evacuation port  122  on the dewar  110 . The capping tool  140  includes a clamping knob  141  connected to an elongated threaded shaft  142  that slidably extends through the threaded section of cover  131  of the evacuation tool  130 . The shaft  142 , which includes a tooling ball  146  attached to its end, is mechanically coupled to a tooling head  148  and a diaphragm bellows  144 . The tooling ball  146  is rotatably captured in a tooling seat  150  of the tooling head  148 . Rotation of the clamping knob  141  and, hence, the shaft  142 , of the capping tool  140  causes the bellows  144  to linearly expand or contract without rotating. Expansion of the bellows  144  causes the shaft  142  to extend into the housing  132  and forces the tooling head  148  toward the flange  134  end of the evacuation tool  130 . Rotation of the clamping knob  141 in the opposite direction causes the bellows to linearly contract, which causes the shaft  142  to withdraw from the housing  132  and the tooling head  148  to withdraw toward the cover  131 end of the evacuation tool,  130 . 
     A preferably low profile port cap  158  (see, in detail, FIG. 4) is releasably captured by the tooling head  148 . The tooling head  148  is substantially cup shaped having a base  147  and sidewall  149  defining a holding area  145 . Hardened CRES balls  154  are mounted in retaining cavities  157  formed in the side wall  149  of the tooling head  148 , such that only a portion of the CRES balls  154  extend into the holding area  145  of the tooling head  148  to engage a recess  153  formed in the perimeter of a head portion  155  of the port cap  158 . The CRES balls  154  are lightly loaded with disc or coil springs  152  to releasably retain the port cap  158 . Spring covers  156  hold the disc springs  152  in the retaining cavities  157 . 
     The surface  160  of the port cap  158  that makes contact with the tip-off flange  126  is preferably electroplated with a layer  161  of indium metal. The layer  161  of indium metal is preferably 0.002 to 0.010 inches thick. Alternatively, the indium metal may be in the form of an o-ring or washer attached to the surface  160  of the port cap  158 . Because indium is a very soft, compliant metal and because the mating surfaces of the indium layer  161  and the tip-off flange  126  are very clean after being vacuum baked over several days at a temperature of about 100° C. to 125° C., the indium layer  161  and tip-off flange  126  are easily cold welded when pressure is applied. 
     In operation, the evacuation tool  130  is connected to the dewar  110  by clamping the flange  134  of the evacuation tool  130  to the tip-off flange  126  of the dewar  110 . The evacuation tool  130  is placed in an open position, as shown in FIG. 3, with the tooling head  148  and port cap  158  withdrawn toward the cover  131 end of the housing  132 . The vacuum port  138  is attached to a vacuum pump (not shown). While the dewar  110  and tool  130  are baked at a temperature of about 100° C. to 125° C., the vacuum pump is operated to evacuate the gas molecules through the opening of evacuation port  122  and tip-off flange  126  and create a vacuum “V” within the dewar  110 . The opening in the evacuation port  122  and tip-off flange  126  is preferably about 1.57 inches in diameter. Such a large opening will tend to reduce the vacuum baking time necessary to sufficiently evacuate the gas molecules being outgassed from the dewar  110  and its components. 
     When the vacuum baking process is completed, the evacuation tool  130  is used to hermetically seal the opening of the tip-off flange  126  of the dewar  110 . The clamping knob  141  of the capping tool  140  is rotated to expand the bellows  144 . The bellows  144  is expanded until the evacuation tool  130  is effectively closed and the evacuation port  122  of the dewar  110  is sealed by cold welding the indium layer  161  of the port cap  158  to the tip-off flange  126 . 
     With the evacuation tool  130  closed and the evacuation port  122  sealed, atmospheric pressure enters the housing  132  of the tool  130  through vacuum port  138  by opening a valve at the vacuum pump to atmosphere. As a result, atmospheric pressure is asserted on the port cap  158  to hold it in place. With the cap  158  of the preferred embodiment at atmospheric pressure, i.e., 14.7 pounds per square inch, more than 28.4 pounds of force is applied to the cap  158  which has a diameter greater than the 1.57 inch diameter opening of the tip-off flange  126 . As a result, when the clamping knob  141  is rotated to open the evacuation tool  130  by contracting the bellows  144 , the atmospheric pressure exerted on the port cap  158  overcomes the pressure exerted by the CRES balls  154  and disk springs  152 , and causes the port cap  158  to disconnect from the tooling head  148  and remain connected to the dewar  110 . With the port cap  158  hermetically sealed to the dewar  110 , the clamp physically holding the evacuation tool  130  to the tip-off flange  126  is removed to remove the evacuation tool  130 . 
     While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.