Abstract:
A method, system, and apparatus for providing reduced dark current in a linear accelerator includes a cavity having an input aperture and an output aperture, and a particle source coupled to the input aperture, the input aperture having a radius greater than a radius of the output aperture.

Description:
This application claims benefit to Prov. No. 60/310,612, filed Aug. 6, 2001. 
    
    
     BACKGROUND 
     The present invention relates generally to particle accelerators. More particularly, embodiments of the present invention relate to the reduction of dark current in particle accelerators. 
     Particle accelerators have been used for a number of years in various applications. For example, one common and important application is their use in medical radiation therapy devices. In this application, an electron gun is coupled to an input cavity of a linear accelerator. The electron gun provides a source of charged particles to the accelerator. The accelerator then accelerates the charged particles to produce an accelerated output beam of a desired energy for use in medical radiation therapy. 
     It is important to ensure that the beam output from a particle accelerator is generated efficiently and is of the desired energy. The energy and other characteristics of the beam are dependent upon the resonant frequency of the accelerator which in turn depends upon the shape and manufacture of the accelerator. The output characteristics of accelerators can be impaired as a result of the emission of unwanted electrons from the walls of the accelerator structure during operation. These unwanted electrons can be captured and accelerated by the accelerating fields in the device, resulting in the creation of so-called “dark current”. 
     Dark current can impair the operating efficiency of a particle accelerator such as a linear accelerator. It would be desirable to provide an accelerator structure which can reduce dark current. It would further be desirable to provide an accelerator structure which can reduce dark current and which can be readily manufactured with few design changes to existing accelerator designs. 
     SUMMARY 
     To alleviate the problems inherent in the prior art, embodiments of the present invention provide a method, system and apparatus providing reduced dark current in linear accelerators. According to some embodiments of the present invention, a method, system, and apparatus for providing reduced dark current in a linear accelerator includes a cavity having an input aperture and an output aperture, and a particle source coupled to the input aperture, the input aperture having a radius greater than a radius of the output aperture. 
     In some embodiments, the input aperture and the output aperture are substantially circular in shape. In some embodiments, the accelerator further includes an anode plate, coupled between the particle source and the input aperture, where the anode plate has an anode aperture and a thickness. In some embodiments, the size of the anode aperture and a thickness of the anode plate are sized to attain a resonant frequency of the linear accelerator. In some embodiments, the radius of the input aperture is selected to reduce the dark current beam generated from the anode plate. 
     According to some embodiments of the present invention, a cavity for a linear accelerator includes an input aperture having a first radius, and an output aperture having a second radius smaller than the first radius, where the input cavity receives particles from a particle source, and directs the particles to the output aperture. 
     The present invention is not limited to the disclosed embodiments, however, as those skilled in the art can readily adapt the teachings of the present invention to create other embodiments and applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exact nature of this invention, as well as its objects and advantages, will become readily apparent from consideration of the following specification as illustrated in the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: 
     FIG. 1 is cross-section of an accelerator according to some embodiments of the present invention; 
     FIG. 2 is a partial cross-section depicting cavities of the accelerator of FIG. 1; 
     FIG. 3 is a partial cross-section of the accelerator of FIG. 1; 
     FIG. 4 is a partial cross-section of a first half cavity of the accelerator of FIG. 1; and 
     FIG. 5 is a further partial cross-section of a first half cavity of the accelerator of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art. 
     Referring first to FIG. 1, a block diagram of a standing-wave linear particle accelerator  10  according to one embodiment of the present invention is shown. As depicted in FIG. 1, particle accelerator  10  is an elongated structure that includes both an input side and an output side. In operation, an electron gun  12  (or other particle injector) is typically coupled to the input side of accelerator  10 , while an accelerated particle beam is driven out of an output side, typically through a bending magnet structure  20  for delivery to a target or other device. 
     In a typical structure, as depicted in FIG. 1, electron gun  12  is coupled to a body  16  of accelerator  10  using a flange  14 . Accelerator  10  includes a number of accelerating cavities  18   a-n . Charged particles, input into accelerator  10  from electron gun  12  are bunched together in the first few accelerating cavities  18   a-n . The bunch of charged particles will pass through each successive cavity during a time interval when the electric field intensity in that cavity is a maximum. Preferably, each of the cavities is shaped and tuned such that its resonant frequency ensures that the bunched electrons pass at the peak of intensity of each cavity. 
     Referring now to FIG. 2, a partial cross-sectional view of cavities of a standing-wave linear particle accelerator  10  according to some embodiments of the present invention is shown. As depicted in FIG. 2, accelerator  10  includes a number of accelerating cavities  18   a-n . Bunches of electrons are accelerated through openings in each successive cavity along a beam axis  60 , toward an output end of accelerator  10 . The first cavity of accelerator  10  is a half cavity  18   a  which abuts a flange (not shown) and which receives input particles from an electron gun (not shown) via an input cavity  24 . Applicants have discovered that a significant portion of dark current which may be generated within accelerator  10  are generated in the first half cavity  18   a . Typical accelerators are formed such that each of the cavities along beam axis  60  are formed having approximately the same size (e.g., the same radius). 
     Referring now to FIG. 3, a partial cross-sectional view of one embodiment of a standing-wave linear particle accelerator  10  according some embodiments of the present invention is shown. In particular, FIG. 3 depicts an electron gun  12  coupled to a body  16  of accelerator  10  via a flange  14 . A first half cavity  18   a  of accelerator  10  is shown. First half cavity  18   a  has an input aperture  24  and an output aperture  26 . One side of first half cavity  18   a  is an anode plate  25  through which input aperture  24  is formed. Input aperture  24  is positioned to receive charged particles from electron gun  12 . Generation and focusing of electrons is assisted with a gun anode  22  having an anode aperture  23 . 
     Output aperture  26  couples the first half cavity  18   a  with another cavity  18   b . First half cavity  18   a  is formed to direct and focus charged particles along a beam path  30  through subsequent cavities of accelerator  10 . 
     Applicants have discovered that disruptive amounts of dark current can be generated in the first half cavity of accelerator  10 . In particular, Applicants have discovered that anode plate  25  can become coated with oxides as a result of normal operation. In operation (particularly during high energy operation), electrons can be pulled from the surface of anode plate  25  and accelerated through accelerator  10  as dark current. This dark current can reduce the overall efficiency of accelerator  10 . 
     Applicants have discovered that dark current generated in the first half cavity can be substantially reduced by modifying the size of input aperture  24 . In particular, Applicants have discovered that dark current can be reduced by increasing the size of input aperture  24 . In some embodiments, a radius of input aperture  24  is greater than a radius of output aperture  26 . In some embodiments, a radius of input aperture  24  is selected to be greater than a radius of a dark current beam which is generated from electrons emitted from a surface of anode plate  25 . The radius of the dark current beam generated from the surface of anode plate  25  can be modeled, for example, using the so-called “PARMELA” code developed for the simulation of linear accelerator effects and described in L. M. Young. “PARMELA”, Los Alamos National Laboratory, LA-UR-96-1835, 1996, the contents of which are incorporated herein in their entirety. 
     In some embodiments, to compensate for the change in shape of first half cavity  18   a , dimensions of anode plate  25  are modified, thereby maintaining the ability to generate a focused and efficient beam without the need to modify the overall accelerator design. For example, in some embodiments, the size of aperture  23  of anode plate  25  is increased. In some embodiments, a thickness of anode plate  25  is increased (Applicants believe this prevents RF fields from fringing into the electron gun). For example, the thickness of anode plate  25  may be increased to cut off the RF field and to provide proper focusing during beam transport. In some embodiments, the inner dimensions of first half cavity  18   a  may also be modified to maintain the resonant frequency of the cavity. 
     In some embodiments, gun anode  22  of electron gun  12  is also modified (e.g., by reducing the thickness of gun anode  22  and by varying the size of anode aperture  23  to compensate for the modifications to anode plate  25 ). Each of these modifications are made to ensure accelerator  10  may continue to operate efficiently and with desired output while enjoying lowered amounts of dark current. 
     An example embodiment will now be described by referring to FIGS. 4 and 5. Referring first to FIG. 4, a sample first half cavity  100  is shown which may be used in a linear accelerator of the type suitable for use in medical radiation therapy applications. First half cavity  100  has an input aperture  102  and an output aperture  104 , each having a diameter “A” (that is, the size of input aperture  102  and the size of output aperture  104  are substantially similar). Sample first half cavity  100  is positioned between a flange (not shown, but similar to flange  14  of FIG. 3) and a second cavity (not shown, but similar to cavity  18   b  of FIG.  2 ). 
     An anode plate having an anode aperture is positioned to form a side of first half cavity  100  and to form input aperture  102 . In an example configuration, first half cavity  100  has the following general dimensions: internal height of first half cavity appx. 3.133″, an input cavity radius of appx. 0.197″ and an output cavity radius of appx. 0.197″. In the same example configuration, the gun anode has an aperture of appx. 0.2″ and the anode plate has a thickness of appx. 0.475″. 
     As depicted in FIG. 4, electric field characteristics are shown as modeled using PARMELA code and depicted as lines  110 . As shown, the example configuration results in a focused beam directed through output aperture  104 . Simulations indicated that a potentially disruptive amount of dark current was generated in this configuration. 
     Referring now to FIG. 5, a first half cavity  200  is shown which has been fabricated using techniques of the present invention. Pursuant to embodiments of the present invention, input aperture  202  is larger than output aperture  204 . First half input cavity  200  of FIG. 5 has been fabricated to produce similar beam output characteristics as first half cavity  100  of FIG. 4, but with reduced dark current. As a result, an accelerator using first half cavity  200  will enjoy greater efficiency and accuracy in operation. 
     First half cavity  200  is formed with the following dimensions: internal height of first half cavity is appx. 3.149″ (appx. 0.016″ greater than cavity  100 ), an input aperture  202  radius of appx. 0.276″ (appx. 0.079″ greater than input cavity  102 ), an output aperture  204  radius of appx. 0.197″ (appx. 0.079″ smaller than input cavity radius), and anode plate  25  has a thickness of approximately 0.450″. Additionally, characteristics of the gun anode  22  are modified to achieve desired beam characteristics, with dimensions including a gun anode aperture  23  of appx. 0.276″. Other dimensions of components of the accelerator may also change (for example, in some embodiments, it may be desirable to modify the size and position of one or more vacuum pumping holes, other characteristics of the anode flange, the thickness or shape of the gun anode, or the like). Simulations of first half cavity  200  indicate that the cavity enjoys reduced dark current as compared to first half cavity  100 . 
     In some embodiments, reduced dark current may be achieved by increasing the size of input aperture  202  as compared to the size of output aperture  204 . In some embodiments, input aperture  202  is greater than the size of output aperture  204 . 
     Although the present invention has been described with respect to a preferred embodiment thereof, those skilled in the art will note that various substitutions may be made to those embodiments described herein without departing from the spirit and scope of the present invention.