Abstract:
In an electron accelerator, a conductive housing defines a cavity. Photoelectrons are emitted from a photocathode into the cavity when light is applied to the photocathode. Via an opening formed in a wall of the conductive housing, the photoelectrons are output to the outside of the cavity. Coolant is flowed through a flow path formed in the wall of the conductive housing, to suppress a temperature rise of the conductive housing. The wall of the conductive housing is made by a metal additive manufacturing technique in such a way as to produce a flow path that has a gentle trajectory without discontinuities in gradient.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a particle accelerating structure, and more particularly to a structure suitable for both increasing the energy of and raising a repetition frequency of a particle beam periodically emitted from the structure. The particle stream emitting from an RF accelerating structure may be used for multiple purposes, for example medical treatment, non-destructive investigation of solid objects, and the like. 
   An example of a particle accelerator suitable for application of the present invention is a radio-frequency (“RF”) accelerator using a photocathode which typically comprises a conductive housing defining a cavity, a photocathode for emitting photoelectron into the cavity, and a wave guide for generating an RF electric field in the cavity. As light is periodically applied to the photocathode, photoelectrons are emitted into the cavity intermittently. These photoelectrons are converged and accelerated by an RF electric field generated in the cavity. The RF electric field is applied synchronously with application of light to the photocathode. A typical RF accelerator is described in U.S. Pat. No. 6,094,010 to Washio, which is incorporated herein by reference. 
   Such accelerating structures generally include a housing made from a conducting material such as copper. The housing defines a cavity. A photocathode is mounted on an inner surface of the housing. Into the cavity is fed light (laser) via a window, and illuminates the surface of the photocathode. Photoelectrons are emitted from the photocathode into the cavity. Such a housing may include one or more cells, dividing the cavity into a plurality of sub-cavities which are separated from each other by toroidal shaped discs (known in the art as “irises”). The sub-cavities are sized and configured to resonate in a particular harmonic mode which corresponds to the frequency of a particular electromagnetic field induced in the irises, with the result that a strong longitudinal electric field is generated along a longitudinal central axis of the housing. 
   Once a longitudinal electric field has been established in this way, the photoelectrons are accelerated along the longitudinal axis to emerge from an exit port. The resulting stream of photoelectrons may used for any of the multiple purposes known in the art. 
   In general, it is desirable to operate an RF accelerating structure at the highest power possible. Very high duty factor, high gradient photo-injectors and RF cavities in general are a critical component of the next generation of applications in high energy electron beam-based physics. Today, there is a compelling need for these applications, which include linear colliders, x-ray free-electron lasers, inverse Compton scattering sources, as well as associated imaging or analysis applications of interest to homeland security. The key issue for high average power, normal conducting, photo-injectors and RF accelerating structures is to effectively cool the housing structure. 
   Thermal management of very high duty factor, high gradient RF structures is crucial to their performance. A significant percentage of the applied RF power is deposited on the walls of the housing in specific locations depending on the magnitude and the direction of the electromagnetic fields in that location. This heating presents significant thermal engineering problems. The large amount of power dissipated in the structure can cause “hot spots” and local thermo-mechanical distortions which may lead to detrimental changes in RF properties and beam quality. One of the most challenging parts of an accelerator housing to be cooled are the “irises” which protrude into the cavity of the housing. Another challenging aspect to be cooled is the so-called RF coupler, which is a thin walled interface between the waveguide and the cavity. 
   Accordingly, the problem of thermal gain has been approached by providing channels within the housing structure, and forcing water to flow through the channels in combination with cooling the water on the outside of the housing by conventional heat dissipation means such as by radiator. However, the prior art is limited in the method for creating, and configuring the channels used for cooling the housing structure. 
   One method currently used to incorporate cooling channels into RF structures is achieved by drilling elongate cylindrical holes into the structure for example, as described in U.S. Pat. No. 6,094,010 to Washio, which is incorporated herein by reference, and where it is specifically described how cylindrical holes are provided to cool the irises of an RF accelerator. It will be readily understood that because these channels are drilled, they are limited to linear configurations, and are connected to each other at sharp angles. It will be understood that this kind of configuration greatly limits the cooling uniformity and rate of cooling in that fluid flow is dramatically slowed by the sharp changes in direction (discontinuities in flow gradient), thereby reducing the rate at which heat can be extracted. 
   Another method that has been used to introduce cooling channels into RF structure is to braze sections of the structure together with pre-machined, curve shaped channels cut out in each section. However, brazing multiple components to form high gradient RF structures is a delicate and expensive step, and many braze cycles are needed to build an effective cooling structure. Moreover, the resulting structure is not uniform or homogeneous, which adversely affects the efficiency at which heat can be extracted from the housing by water in the channels. 
   Thus, there is a need for a method and structure for fabricating an RF housing having a channel system with gentle changes in direction, suitable for cooling the RF housing structure. There is a further need for an RF housing structure having such channel system, that has a uniform and homogeneous configuration, that is not a collection of components, with sections cut out, brazen together. The present invention addresses these and other needs. 
   SUMMARY OF THE INVENTION 
   According to a preferred embodiment of the invention, there is described an RF accelerator that has improved cooling characteristics over the prior art. In a preferred embodiment, the RF accelerator has a conductive housing defining a cavity. The housing comprises cells which have a substantially homogeneous composition, wherein assembly of the housing is not achieved by brazing together sheets of metal having cutout shapes for forming an internal flow path. A cathode for emitting particles is provided. A wave guide is provided, coupled to the cavity for guiding a micro wave into the cavity. An opening is disposed in a wall of the conductive housing for guiding the photoelectrons emitted into the cavity out from the cavity to form an electron beam. 
   A flow path through which coolant flows to forcibly cool the conductive housing is configured to extend through the housing. The conductive housing comprises a cylinder having an inner circumferential surface of a cylindrical shape and a protrusion having a through hole in which the protrusion comprises a toroid like extension from the inner circumferential surface defined by the cylinder toward a center axis of the conductive housing. The flow path enters an external wall of the conductive housing from an outer circumferential surface, and circulates around the through hole defined in the central area of the protrusion, and then returns to the outer circumferential surface of the conductive housing. The flow path comprises a curved portion that in a preferred embodiment comprises at least a semi circle. 
   In a preferred embodiment, the flow path is configured to conform to the external geometry of the housing, whereby a substantially constant thickness of housing material separates the flow path from the exterior of the housing over a length of the flow path. In another aspect, the flow path has no sharp turns. In yet another aspect, the flow path is not circular in cross section, but may have an elongate cross section. 
   In a further aspect, the walls of the flow path have a surface roughness (Ra) of between 800 to 1200 micro inches in order to cause turbulent flow of the cooling fluid in the channel, thereby increasing the heat conduction from the housing. 
   Preferably, the housing is fabricated by a metal additive manufacturing technique, wherein a metal is deposited in layers using a directed material fabrication process controlled by a computer. The metal additive manufacturing technique uses an electron beam to melt metal feedstock in order to build up the layers of material, preferably copper, or an alloy of copper. 
   In a final aspect, the surface that is exposed to the radiofrequency field is subjected to processing after fabrication in order to achieve surface roughness of less than 63 micro inches. Such a surface is advantageous in order to prevent breakdown of the microwave field. 
   These and other advantages of the invention will become more apparent from the following detailed description thereof and the accompanying exemplary drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an RF accelerator having features of the present invention. 
       FIG. 2  is a perspective sectional view of the RF accelerator of  FIG. 1 . 
       FIG. 3  is a sectional view of portion of the RF accelerator of  FIGS. 1 and 2 , taken substantially along the line A-A in  FIG. 2 . 
       FIG. 4  is a perspective view of a full cell of the RF accelerator of  FIG. 1 , showing internal channels for fluid flow. 
       FIG. 5  is a sectional view of portion of the RF accelerator of the previous figures, taken substantially along the line B-B in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the drawings, which are provided by way of exemplification and not limitation, preferred embodiments of the invention are described below. Prior to describing the embodiments of the invention, however, known technology used in effectuating the invention will be described. 
   Turning now to a method of effectuating the present invention, a method of constructing a housing for an RF accelerator will be described that is capable of manufacturing structure capable of overcoming the shortcomings of the prior art. Metal additive fabrication technologies, such as Electron Beam Melting (EBM), have been described for example in U.S. Pat. No. 5,786,562 (Larson), U.S. Pat. No. 6,112,804 (Sachs et al.), U.S. Pat. No. 6,391,251 (Keicher et al.), U.S. Pat. No. 6,401,001 (Jang et al.). The contents of each of these patents is incorporated herein by reference. These technologies employ rapid prototyping layer methods to allow for virtually any three dimensional geometry to be physically constructed, including the provision of channels and openings. The metal additive fabrication techniques used in carrying out the present invention are capable of producing structure in the form of fully dense metal components that are homogenous in metallurgical structure, having no seams or joints, with properties similar to or better than that of conventionally machined materials, and which include a curved channel system that does not include sharp changes of direction or gradient discontinuities. 
   Using the above described metal additive fabrication techniques, the present invention describes preferred cooling channel configurations for improving the thermal cooling characteristics of an RF accelerator housing structure. These cooling channels have enhanced cooling uniformity, in that they provide gentle changes in flow direction, thus reducing hot spots. The cooling channels allow increased flow rate for a given pressure in a homogeneous metal structure by using smooth bends in the channels, as apposed to the intersections with sharp changes in direction (gradient discontinuities) used previously. Specifically, with reference to  FIGS. 1-5  a preferred embodiment of a housing structure for an RF accelerator is described. 
     FIGS. 1-5  shows a preferred embodiment of the present invention. An RF injector is generally identified by the numeral  20 . A housing  22  is positioned at the center of the injector to define a cavity  25 , and may comprise a half cell  24  and a full cell  26 . More cells may be added, depending on requirements for acceleration. The cells  24 ,  26  may be joined to each other by brazing. In an alternative embodiment, using the method of fabrication described above, the cells may be fabricated as a single unitary structure. A wave guide  23  provides an entry point for RF wave introduction to the housing. The half cell  24  defines a half cell sub-cavity  36 , and the full cell  26  defines a full cell sub-cavity  38 . As described above, the resonant frequency of these sub-cavities is utilized to accelerate photon particles which are directed by an external magnetic field set up in the cell structures to exit from the downstream port  34 . Accessory ports  28 ,  30 ,  32  are provided as tuning ports or vacuum pump out ports, as needed. At the rear of the housing, a cathode  44  is provided as the source of photoelectrons for acceleration through the exit port  34 . 
   Each cell,  24 ,  26  includes a constriction or “iris” having a narrowed opening  46 ,  48 . Each “iris” may be imagined to be formed as a toroid that protrudes into an otherwise cylindrical interior of the housing, and adjoined continuously to the housing. Thus, the internal radius of the housing reduces to a local minimum at the openings  46 ,  48  of each toroid. This configuration permits enhanced intensification of the electric field for accelerating photoelectrons through the cavities  36 ,  38  and finally out of the exit port  34 . Of importance to the invention is the presence of cooling channels  40 ,  42  which are utilized to circulate water around the cells  24 ,  26  for cooling during operation. 
   As exemplified in  FIG. 2 , the channels  40 ,  42  may be positioned substantially in the toroidal portions of the housing, where the heating effect induced by the magnetic field is greatest. The shape of each channel, made according to the method of fabrication of the present invention described above, is configured to provide a superior cooling effect to the cells  24 ,  26  and the housing generally. The cross sectional shape of a channel is not limited to being circular, but in a preferred embodiment may be elongate, with an elongate axis extending radially outwardly from the center of the housing. This aspect provides for a greater surface area contact between fluid and metal cell. Furthermore, as seen in  FIG. 2  a channel may be positioned to extend over the majority of the radius of the solid portion of the toroid in which it is positioned, having one point of entry for fluid flow, and one point of exit. With these characteristics, as seen in  FIG. 2 , the cross sectional configuration of the channel may conform to the cross sectional geometry of the toroidal portion of the cell  26 , allowing a substantially constant thickness of metal to surround the channel, at least on the radially interior portion of the housing  22 . 
   Moreover, as exemplified in  FIG. 3 , the path of a channel through a cell may be substantially curved, preferably with no discontinuity in the gradient of the walls defining the direction of fluid flow.  FIG. 3  exemplifies the path of channel  42  through the cell  26 , and in a preferred embodiment has only continuously and evenly curved walls without rapid changes in direction, or discontinuities in flow gradient, allowing fluid to flow through the cell  26  at an enhanced speed, allowing for improved heat extraction during operation. In this plan sectional view of the channel  42 , the channel is seen to substantially conform to the circumferential exterior surfaces (internal and external) of the cell  26  over a substantial portion of the length of the channel within the cell, allowing a substantially constant thickness of metal to separate the channel  42  from an exterior circumferential surface. These combined characteristics provide for an improved heat extraction capability of the channel  42 . In a preferred embodiment, the cell is formed from copper, or an alloy of copper. However, any metal suitable for manufacturing in the described way may also be used. 
   In another aspect of the invention, exemplified in  FIGS. 4-5 , a cooling channel  60  is provided around the waveguide coupling hole  62 , which receives a heavy thermal load in a small area. The coupling inlet hole  62  is an opening in the housing that provides a connection between the cavity  25  and the wave guide  23  for admission of microwaves to the cavity. The coupling inlet hole  62  may have an oval shape to maximize the coupling of the RF power into the RF structure. In a preferred aspect, the cooling circuit  60  conforms to the shape of the coupling hole  62  to enhance the efficiency of cooling this area. In a preferred embodiment, the cooling channel  60  has no discontinuities in flow gradient, and surrounds the coupling hole  62 . 
   In a preferred aspect, the invention is directed to the surface roughness of the cooling channels. Because high surface roughness causes turbulent flow which is superior for cooling at a given flow rate, the present invention may have cooling channels in which dimples are introduced during manufacture, such that the resulting surface has a roughness of at least 1000 micro inches Ra, and preferably in the range of 800 to 1200 micro inches Ra. 
   In a final aspect of the present invention, the interior surface walls of the cells that are exposed to the radiofrequency field are subjected to processing after fabrication in order to achieve surface roughness of preferably less than 63 micro inches. This may be achieved by a simple machining process. Without such treatment, the radiofrequency field may cause local heating on the surface and reduce the efficiency of the accelerator. 
   The foregoing method and structure address certain shortcomings in the prior art. By fabricating the housing and the channels according to the method of the present invention, channels having a novel configuration are introduced that provides enhanced cooling, and therefore enhanced operation of the RF accelerator. 
   Thus, it will be apparent from the foregoing that, while particular forms of the invention have been illustrated and described, various modifications can be made without parting from the spirit and scope of the invention.