Abstract:
A conduction cooling system for linear accelerator cavities. The system conducts heat from the cavities to a refrigeration unit using at least one cavity cooler interconnected with a cooling connector. The cavity cooler and cooling connector are both made from solid material having a very high thermal conductivity of approximately 1×10 4  W m −1  K −1  at temperatures of approximately 4 degrees K. This allows for very simple and effective conduction of waste heat from the linear accelerator cavities to the cavity cooler, along the cooling connector, and thence to the refrigeration unit.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention described herein was made by an employee of the United States Government and may be manufactured and used by the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to the field of electric lamp and discharge devices and more specifically to linear accelerators (linacs). 
     2. Description of Related Art 
     Linear accelerator devices use intense radio frequency electromagnetic fields to accelerate the speed of particles to create beams used for a variety of applications. These applications include driving industrial processes, security &amp; imaging applications, food and medical sterilization, medical treatments, isotope creation and physics research. Superconducting radio frequency (SRF) technology allows the construction of linear accelerators that are both compact and efficient at using “wall plug” electrical power to create a particle beam. The cavity of an SRF linear accelerator must operate at an extremely low temperature. Excitation with the radio frequency power required for particle acceleration requires constant removal of waste heat generated in the SRF cavity. 
     Currently, cooling SRF cavities uses large quantities of cryogens such as liquid helium. These cryogens are pressurized fluids having an extremely low temperature. To provide the needed cryogens, the cryogenic systems themselves require complex integration of expansion engines or turbines, heat exchangers, cryogen storage units, gaseous inventory systems, compressors, piping, purification systems, control systems, and safety relief and venting systems. These systems require substantial space, energy, labor and money for operation and maintenance. Use of cryogens also requires cavity tuners to compensate for radio frequency resonance changes in SRF cavities due to pressure changes. Presently these issues limit the utility of SRF linear accelerators. 
     There is an unmet need for more efficient and less complex cooling systems for SRF based linear accelerators. 
     BRIEF SUMMARY OF THE INVENTION 
     A conduction cooling system for at least one linear accelerator cavity includes at least one cavity cooler operatively interconnecting the at least one linear accelerator cavity and a cooling connector, and a refrigeration source operatively connected to the cooling connector. The at least one cavity cooler and the cooling connector are made from a material having a thermal conductivity no lower than approximately 1×10 4  W m −1  K −1  at temperatures of approximately 4 degrees K. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  illustrates an exemplary embodiment of a system for conduction cooling linear accelerator cavities. 
         FIGS. 2-4  illustrate alternate embodiments of systems for conduction cooling linear accelerator cavities. 
         FIG. 5  illustrates a flowchart of an exemplary embodiment of a method of making a system for conduction cooling linear accelerator cavities. 
     
    
    
     TERMS OF ART 
     As used herein, the term “quality factor” is the ratio of the stored energy of the linear accelerator cavity to the energy lost as heat into the cavity walls per radio frequency oscillation cycle. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an exemplary embodiment of a system  100  for conduction cooling linear accelerator cavities. System  100  includes at least one linear accelerator cavity  10 , at least one cavity cooler  20 , a cooling connector  30 , an optional mechanical support system  40  and a refrigeration source  50 . The average cross-section A of cavity cooler  20  and cooling connector  30  is determined using the equation 
             A   =       Q   *   L       Δ   ⁢           ⁢   T   *   C             
wherein Q is an average heat load of linear accelerator cavity  10 , L is an average distance between linear accelerator cavity  10  and refrigeration source  50 , ΔT is a maximum allowable temperature rise from linear accelerator cavity  10  to refrigeration source  50  and C is a thermal conductivity of cavity cooler  20  and cooling connector  30 .
 
     In the exemplary embodiment, linear accelerator cavity  10  is an SRF cavity with a minimum quality factor of approximately 1*10 8 . Linear accelerator cavity  10  comprises a metallic or ceramic material that is superconducting at a cavity operating temperature. This material may constitute the entire cavity or be a coating on an inner surface of linear accelerator cavity  10 . In the exemplary embodiment, linear accelerator cavity  10  comprises pure niobium. In other embodiments, linear accelerator cavity  10  may be, but is not limited to, a niobium, aluminum or copper cavity coated in niobium-tin (Nb 3 Sn) or other superconducting materials. 
     In the exemplary embodiment, cavity cooler  20  at least partially encircles linear accelerator cavity  10 , making thermal contact to remove heat from linear accelerator cavity  10 . Materials used for cavity cooler  20  must have a minimum thermal conductivity of approximately 1×10 4  W m −1  K −1  at temperatures of approximately 4 degrees K. Such materials with high thermal conductivity include, but are not limited to, high-purity aluminum, diamond or carbon nanotubes. In certain embodiments, cavity cooler  20  includes multiple cavity coolers  20 . 
     Cavity cooler  20  may also include an intermediate conduction layer  25  between cavity cooler  20  and linear accelerator cavity  10  to improve thermal conductivity. Intermediate conduction layer  25  is a ductile material, such as, but not limited to, indium or lead. The thermal conductivity of intermediate conduction layer  25  results in a thermal resistance between linear accelerator cavity  10  and cavity cooler  20  of no more than approximately 10% of the thermal conductivity of cavity cooler  20 . 
     In the exemplary embodiment, cooling connector  30  connects each cavity cooler  20  to refrigeration source  50 . Materials used for cooling connector  30  must have a minimum thermal conductivity of approximately 1×10 4  W m −1  K −1  at temperatures of approximately 4 K. Such materials with high thermal conductivity, include, but are not limited to, high-purity aluminum, diamond or carbon nanotubes. In certain embodiments, multiple cooling connectors  30  connect cavity cooler  20  to refrigeration source  50 . In certain embodiments, cooling connectors  30  are flexible. 
     Optional mechanical support system  40  stabilizes linear accelerator cavity  10 . In the exemplary embodiment, mechanical support system  40  is a plurality of support rods. In another embodiment, mechanical support system  40  is a solid cylinder. Mechanical support system  40  connects to linear accelerator cavity  10  via endplates  45 . Mechanical support system  40  and endplates  45  are made of a material having an identical or substantially similar thermal expansion coefficient as linear accelerator cavity  10 . 
     In the exemplary embodiment, refrigeration source  50  is a commercially available cryocooler having a power requirement of approximately 1 W to approximately 100 W. In another embodiment, refrigeration source  50  is a vessel containing cryogenic fluid. A cold tip  55  of refrigeration source  50  clamps to cooling connector  30 . The clamping connection results in a thermal resistance between cooling connector  30  and cold tip  55  of no more than approximately 10% of the thermal resistance of cooling connector  30 , allowing efficient conduction of heat from cooling connector  30  to refrigeration source  50 . 
       FIG. 2  illustrates an alternate embodiment of a system  200  for conduction cooling linear accelerator cavities  10 . In system  200 , cavity cooler  20  is a cooling ring  220  and cooling connector  30  is a plurality of cooling strips  230   a  connected to a cooling bar  230   b.  Cooling ring  220  may be applied to linear accelerator cavity  10  through direct casting, diffusion bonding, mechanical clamping or any other fabrication method resulting in a low thermal conductivity connection. 
       FIG. 3  illustrates an alternate embodiment of a system  300  for conduction cooling linear accelerator cavities  10 . In the embodiment of system  300 , cavity cooler  20  forms an integral cooling block  320  around multiple linear accelerator cavities  10  and cooling connector  30  is a flexible cooling braid  330 . In this embodiment, mechanical support system  40  is unnecessary. Cooling block  320  may be applied to linear accelerator cavity  10  through direct casting, mechanical clamping or any other fabrication method resulting in a low thermal conductivity connection. 
       FIG. 4  illustrates an alternate embodiment of a system  400  for conduction cooling linear accelerator cavities  10 . In the embodiment of system  400 , cavity cooler  20  is a coating  420   a  and a cooling ring  420   b  around a portion of linear accelerator cavity  10 , while cooling connector  30  is a plurality of cooling strips  430   a  connected to a cooling cylinder  430   b.  Coating  420  may be applied to linear accelerator cavity  10  through direct casting, diffusion bonding, mechanical clamping or any other fabrication method resulting in a low thermal conductivity connection. 
       FIG. 5  illustrates a flowchart of an exemplary embodiment of a method  500  of making a system  100  for conduction cooling linear accelerator cavities  10 . 
     In step  502 , method  500  creates at least one linear accelerator cavity  10 . 
     In optional step  504 , method  500  forms intermediate conduction layer  25  around at least part of linear accelerator cavity  10 . 
     In step  506 , method  500  forms at least one cavity cooler  20  around at least part of linear accelerator cavity  10 . This formation may be through casting, fabrication, or deposition. 
     In step  508 , method  500  forms at least one cooling connector  30  in contact with at least one cavity cooler  20 . This formation may be through casting, fabrication, or deposition. In certain embodiments, method  500  may perform steps  506  and  508  simultaneously. 
     In step  510 , method  500  attaches cooling connector  30  to refrigeration source  50 . In one embodiment, cold tip  55  of refrigeration source  50  clamps to cooling connector  30 . 
     It will be understood that many additional changes in the details, materials, procedures and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 
     It should be further understood that the drawings are not necessarily to scale; instead, emphasis has been placed upon illustrating the principles of the invention. Moreover, the terms “substantially” or “approximately” as used herein may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related.