Abstract:
An electron gun includes a cathode, a cavity, and an energy input. The cavity resonates at a particular frequency, and the cathode generates an electron beam along its longitudinal axis when driven by resonant electromagnetic radiation of the particular frequency. The energy input introduces electromagnetic radiation of the particular frequency into the cavity along the longitudinal axis of the cathode.

Description:
TECHNICAL FIELD 
   This invention relates in general to RF electron guns, and more particularly to an axisymmetric emittance compensated electron gun. 
   BACKGROUND OF THE INVENTION 
   Electron guns generate and accelerate narrowly focused beams of electrons. In a radio frequency (RF) electron gun, the electrons are accelerated with RF energy. The transverse emittance of the electron beam refers to the electrons that travel in a direction other than along the axis of the beam. Because these electrons do not travel within the beam of the electron gun, the energy consumed in generating these electrons may be considered as wasted energy. Consequently, it is desirable to have an electron gun structure that minimizes transverse emittance. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method which substantially reduces or eliminates certain sources of emittance growth and allows more thorough optimization of emittance compenstation. In particular, certain embodiments of the electron gun of the present invention provides an architecture that permits easier placement of emittance-compensating solenoids and permits greater flexibility in cavity design. 
   In accordance with one embodiment of the present invention, an electron gun includes a cathode, a cavity and an energy input. The cavity resonates at a particular frequency, and the cathode generates an electron beam that is accelerated along the cavity&#39;s longitudinal axis when driven by resonant electromagnetic radiation of the particular frequency. The energy input introduces electromagnetic radiation of the particular frequency into the cavity. 
   In accordance with another embodiment of the present invention, an electron gun has a coaxial cable coupled to a cavity where the cavity resonates when electromagnetic radiation of a particular frequency is introduced into the cavity through the coaxial cable. A center conductor of the coaxial cable extends into the cavity and generates an electron beam along the longitudinal axis of the coaxial cable when driven by resonant electromagnetic radiation of the particular frequency. The coaxial cable introduces electromagnetic radiation of the particular frequency into the cavity along the longitudinal axis of the cable. 
   Important technical advantages of certain embodiments of the present invention include easier solenoid placement. The axisymmetric design of certain embodiments allows a cavity design without protrusions used to introduce RF energy. Thus, the solenoid&#39;s position may be adjusted without having to compensate for the presence of such protrusions. 
   Other important technical advantages of certain embodiments of the present invention include flexible cavity size. In certain embodiments, the cavity wall surrounding the electron filament may be adjusted to change the resonance of the cavity of the electron gun. These adjustments allow the electron gun to be tuned to a variety of frequencies depending on the intended use. Furthermore, because the position of the solenoid may also be adjusted in certain embodiments, the emittance compensation of the new configuration may be corrected for the new cavity size. 
   Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  shows an electron gun in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an electron gun  100  according to a particular embodiment of the present invention. Cavity  102  comprises one or more cells of conductive material enclosing volumes of space. In the particular embodiment depicted, cavity  102  is a one-and-a-half cell cavity that includes a half-cell  104  and one booster cell  106 . However, it should be understood by one skilled in the art that the size and number of cells in the cavity may be selected to produce any desired resonance for a selected wavelength and power of RF input, and the depicted embodiment is only one example of cavity  102 . Cavity  102  may be evacuated by vacuum pump  103  so that the electron beam is not scattered from air molecules within cavity  102 . 
   Exit channel  108  provides a means for the electron beam to exit the electron gun  100 . Exit channel  108  is typically circular with the radius of exit channel  108  aperture selected to minimize transverse non-linear fields. 
   Cathode  110 , with longitudinal axis  117 , may be a photocathode, thermionic cathode, or field emission cathode, or any other device that can emit electrons. The material of cathode  110  may be selected based on a wide variety of desired properties, including efficiency, durability, ease of replacement, or other suitable consideration. Cavity  102  is axisymmetric around longitudinal axis  117 . 
   RF source  111  represents any suitable source of radio frequency radiation. RF source  111  may be a klystron, magnetron, or any other suitable device, whether tunable or not. RF source  111  may be positioned in any suitable or convenient location allowing RF energy from source  111  to enter energy input  112 . 
   Energy input  112  represents any cable, conduit, waveguide, or other suitable medium for delivering RF energy from RF source  111  to cavity  102 . In a particular embodiment, energy input  112  is a waveguide transitioned to a coaxial line  122 , and cathode  110  represents an extension of the central conductor of coaxial line  122 . The RF energy from energy input  112  travels into the cavity, and the resonance of the RF energy in cavity  102  produces strong electric fields. The electric fields accelerate the electrons from cathode  110 , which then exit channel  108 . By using a sufficient power level and a frequency tuned to the resonance frequency of cavity  102 , the electrons may be accelerated relatively quickly, so that the repulsion produced by space charge effects is lessened. Because there is a finite acceleration time for the electrons, the electrons will phase lag slightly behind the resonant electric fields in the cavity. 
   A technical advantage of aligning energy input  112  longitudinally with cathode  110  is axial symmetry. Many conventional RF electron guns deliver RF energy using a rectangular waveguide directing the RF energy through a small hole in the wall of cavity  102 . This creates a field asymmetry in the resonant electric field, which produces or enhances transverse emittance growth. The asymmetry in structural design also makes it more difficult to fabricate cavity  102 , and may reduce the overall mechanical strength of cavity  102 . Furthermore, it may be more difficult to position other components of gun  100 , such as solenoid  114 , as well as brackets or other apparatus for positioning and securing gun  100 . Other advantages of axial symmetry include that computer models of gun  100  may be more accurate because the models do not need to take into account the asymmetric effects of protrusions, transverse RF radiation, and other complexities. 
   Solenoid  114  is a powered conductive coil that produces a magnetic focusing field within cavity  102 . The strength of the magnetic field and the position of solenoid  114  may be adjusted to maximize the focusing effects of solenoid  114 . The rapid acceleration produced by the electric field combined with the phase normalization of the magnetic field of solenoid  114  substantially reduces the amount of transverse emittance in the electron beam. Bucking coil  116  offsets the field of solenoid  114  at cathode  110  to allow the electrons to be emitted in zero magnetic field. 
   Particular embodiments of gun  100  allow cavity  102  to be tuned to different frequencies. In a particular embodiment, endwall  118  of cavity  102  may be deformed or otherwise adjusted in order to affect the size of booster cell  106 . Similarly, cathode  110  may be repositioned slightly within half-cell  104  to affect the overall resonance of cavity  102 . For example, if cathode  110  is the center conductor of a coaxial line  122 , a connector  120  couples coax  122  to cavity  102 . By moving the center conductor of coax  122  in or out using connector  120 , the position of cathode  110  is precisely adjusted. By performing such adjustments, the resonant frequency of cavity  102  may be changed to match other system components. 
   Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art. In particular, the described techniques are adaptable to a wide variety of cathode materials, cavity configurations, solenoid designs, energy inputs, energy sources, resonant frequencies, and various other design choices. It is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims. 
   Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present invention is not intended to be limited to the particular embodiments described in the specification. As one of ordinary skill in the art will readily appreciate from the foregoing disclosure, alternatives presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized in accordance with the present invention.