Abstract:
A system and method of magnetically insulating the cathode of a cold-cathode electron gun is achieved. A strong magnetic field is applied in the vicinity of the cold cathode to deflect and constrain the flow of electrons emitted from structures within the electron gun. The magnetic field largely prevents re-reflected primary and secondary electrons from reaching the cathode, thereby improving the operation and increasing the life of the cold-cathode electron gun. In addition, the insulating magnetic field improves electron beam focusing and enables a reduction in the magnitude of static electric focusing fields employed in the vicinity of the cold cathode, further reducing the electron gun&#39;s susceptibility to destructive arcing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the design of cold cathode electron guns, and in particular, to structures and methods of magnetically insulating a cold cathode from damaging arc-inducing electron streams originating downstream of the cathode surface. 
     2. Description of Related Art 
     The use of an electron-emitting cathode as a source of electrons for focused-electron-beam devices is well known. Most such devices employ thermionic, or “hot” cathodes, but more recently, cold-cathode electron emitters have begun to emerge as higher performance electron sources that have the potential of producing high-density electron beams unachievable using thermionic cathodes. However, focusing and controlling the acceleration of an electron beam generated by a high-density cold-cathode electron emitter represents a significant technical challenge. The electron current density at the cold cathode emission surface is high due to the low electron-emission velocity and relatively small cathode size. This high current density produces a natural rapid Coulomb expansion of the electron beam as it flows away from the cathode surface. In addition, electron beams emitted from a cold cathode are generally characterized by a higher emittance, i.e., a higher inherent perpendicular velocity, than those emitted from an equivalent thermionic cathode. Both of these effects exacerbate the focusing and acceleration control challenges represented by such electron beams. 
     Certain techniques have been developed to address the focusing of cold-cathode electron beams, such as one invented by one of the inventors of the present application and described in U.S. Pat. No. 6,683,414. That patent describes a series of shaped electrostatic lenses located in front of a cold-cathode emission surface designed to focus the emitted electron beam and confine it within the magnetic field developed inside a travelling-wave tube. However, a difficulty with such a system and other similar approaches that employ electrostatic lenses is that they may increase the susceptibility of the system to destructive arcing between lens elements and the cathode surface. In particular, high voltages are required in order to focus and control the acceleration of the electron beam, and these voltages produce large electric fields in the beam region that can lead to breakdown and cathode arcing. 
     In addition, beam electrons may impinge upon the lens elements during operation, creating secondary electrons and high-energy re-reflected primary electrons. These electrons, emitted directly from the lens elements themselves, can flow to any  10  other structures within the electron gun that they have sufficient energy to reach, creating unwanted current flow, element heating, vacuum degradation, and potentially, arc initiation. Re-reflected primary electrons, in particular, may result in catastrophic current flow back to the cathode surface itself. It is, therefore, desirable to provide a structure and method for insulating the cold cathode region from the secondary electrons, re-reflected primary electrons, and arc electron streams that may be initiated within the electron gun structure in order to improve the operation and prolong the operating life of a cold-cathode electron emitter. 
     SUMMARY OF THE INVENTION 
     An apparatus and method for insulating the cold cathode of an electron gun includes an electron gun having a cold cathode configured to emit an electron beam. One or more electrostatic focusing lenses are provided along the beam path in front of the cold cathode. High voltage potentials are applied to the focusing lenses to focus and confine the electron beam within the region in the vicinity of the cold cathode and the electrostatic lenses and guide it into an optional transport region that may be downstream of the electron gun. A magnetic apparatus is located around the outside of the electron gun and is configured to apply a magnetic field that extends through the gun region, including the cold cathode, the electrostatic lenses, and the electron beam path through that region. The magnetic field is directed such that it is not substantially parallel to a direction from the cathode surface to a surface of any of the lens elements that may be emission sites for secondary electrons or primary re-reflected electrons. Oriented accordingly, the magnetic field acts to insulate the cathode from electrons emitted by the lens elements because they would have to cross many magnetic field lines to propagate from the lens surface to the cathode. 
     In one embodiment in accordance with the present invention, the magnetic apparatus comprises one or more permanent magnets surrounding the gun region, which includes the region in the vicinity of the cold cathode and the electrostatic lenses. For example, the magnetic apparatus may comprise a single permanent magnet that is sized to fit over the electron gun to create a magnetic field within the gun region. This single permanent magnet may be annular in shape, although other configurations would also fall within the scope and spirit of the present invention. In another embodiment, the magnetic apparatus may comprise several permanent magnets arranged around the electron gun to create the required magnetic field. In still another embodiment, the magnetic apparatus may comprise an electromagnet including a number of windings around the gun region through which an electric current is applied to generate a magnetic field. 
     In addition to providing insulation of the cathode from secondary electrons and arcing events, the magnetic field also provides additional electron beam focusing, allowing the voltage potentials applied to the lens elements to be reduced. The lower electric field gradients further reduce the probability of arcing within the gun region. 
     In another embodiment, a transport region including an electron beam drift tube is located along the electron beam path and downstream from the electrostatic lenses in order to receive the electron beam. A transport-region magnetic apparatus may optionally be located in proximity to the drift tube in order to produce a magnetic field within the drift tube for further controlling and confining the electron beam. 
     Thus, certain benefits of an apparatus and method for magnetically insulating the cathode of a cold-cathode electron gun from secondary electrons and arcing events is achieved. Further advantages and applications of the invention will become clear to those skilled in the art by examination of the following detailed description of the preferred embodiment. Reference will be made to the attached sheets of drawing that will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual drawing of a cold-cathode electron gun employing electrostatic focusing lenses as is typical of the prior art; 
         FIG. 2  is a cross section of the cold-cathode electron gun depicted in  FIG. 1 , illustrating simulated trajectories of secondary electrons emitted from a focusing lens element; 
         FIG. 3  is a cross section of the cold-cathode electron gun depicted in  FIG. 1 , illustrating simulated trajectories of primary re-reflected electrons from a focusing lens element; 
         FIG. 4  is a conceptual drawing of a magnetically insulated cold-cathode electron gun in accordance with a preferred embodiment of the present invention; 
         FIG. 5  is a cross section of magnetically insulated cold-cathode electron gun employing a permanent magnet to create an insulating field according to the preferred embodiment of the present invention, illustrating simulated trajectories of secondary electrons emitted from a focusing lens element; 
         FIG. 6  is a cross section of the cold-cathode electron gun of  FIG. 5 , showing its increased immunity to the effects of primary re-reflected electrons; 
         FIG. 7  illustrates an alternative embodiment of a cold-cathode electron gun in accordance with the present invention that employs an electromagnet to create an insulating field; 
         FIG. 8  depicts the beam focusing performance of an electron gun that does not employ magnetic insulation; and 
         FIG. 9  depicts the beam focusing performance of an electron gun employing magnetic insulation in accordance with the present invention, illustrating that similar beam performance can be achieved with substantially lower focusing voltages. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the detailed description that follows, like element numerals are used to indicate like elements appearing in one or more of the figures. 
       FIG. 1  illustrates a cold-cathode electron gun  10  employing electrostatic focusing lenses typical of the prior art. Electrons are emitted from the surface of a cold cathode  110  and enter the gun region  120  of the vacuum chamber  102 . Shaped electrostatic lenses  104 ,  106 ,  108  focus the electron beam  116  and help to maintain a laminar profile as it propagates from the vicinity of the cold cathode and the electrostatic lenses into the transport region  122 . The transport region  122  may take on different configurations depending on the application to which the cold-cathode electron gun is applied. Such a transport region  122  often includes a drift tube  112  surrounded by one or several permanent magnets  114 , although such a structure is not required for use of the present invention. 
       FIG. 2  is an R-Z plot of an electron gun having a cold-cathode electron emitter, with radial distance shown along the vertical axis, and z-length along the horizontal axis. Electrons are emitted from the cold cathode surface  202  and proceed along the beam path  204 . Large voltages are applied to electrostatic lens elements  206 ,  208 ,  210  in order to control the electron beam as it moves through the gun region. Magnets  220  surrounding a drift tube in the transport region produce a strong magnetic field, indicated by flux lines  216 , that confines the electron beam in this region. However, there is little magnetic field in the gun region. Thus, when secondary electrons are emitted from one of the lens elements due to beam interception, they can flow relatively freely to other structures within the electron gun, creating unwanted current flow, element heating, vacuum degradation, and potentially, arc initiation. For example, simulated trajectories of electrons emitted from surface  212  of lens element  210  are depicted at  214 . These electrons are produced as the result of an impact by either a primary cathode electron or another secondary electron originating elsewhere within the electron gun, including those created due to ionization of background gas. In reality, such electrons are emitted from all surfaces of lens elements  206 ,  208 , and  210 , but emission only from surface  212  is shown for clarity. In this simulation, the electrons  214  are slow secondary electrons with zero initial energy. It can be observed from the figure that the emitted electrons fill the electron gun region, crossing the axis where the main beam propagates. Current also passes to the adjacent lens element  208 , which, in an operating device, would generate still more secondary and re-reflected primary electrons. This would cause heating and contamination of the background vacuum. 
       FIG. 3  is an R-Z plot of a similar electron gun having a cold-cathode electron emitter. In this figure, the effect of re-reflected primary electrons is illustrated. Such electrons are in reality emitted from all surfaces of the lens elements  206 ,  208 ,  210 , but emission from a single point  312  is shown for clarity. Primary re-reflected electrons are emitted with an energy equal to the electron charge multiplied by the voltage potential difference between the surface of the cathode and that of the lens element. In other words, E=q e *(|V K −V L |), where E is the electron energy, q e  is the electron charge, V K  is the cathode potential, and V L  is the lens potential. Such electrons have a broad range of injection angles, as shown by simulated trajectories  314 . Again, it can be seen that such electrons fill the electron gun region, and in this case, even achieve a direct path back to the cathode surface. Such direct cathode interception dramatically increases the probability of life-limiting cathode arcing. 
     In the preferred embodiment of the present invention, the cold cathode is magnetically insulated from these damaging electrons by imposing a magnetic field within the gun region to prevent the secondary and re-reflected primary electrons from reaching the cathode.  FIG. 4  is a conceptual drawing of a cold-cathode electron gun including a protective magnetic field imposed by a permanent magnet  418  in accordance with the preferred embodiment of the present invention. Electron beam  416  is emitted from cold cathode  410 . Electrostatic lenses  404 ,  406 , and  408  are used to focus and confine the electron beam within the gun region  420  and pass it to the transport region  422 . In  FIG. 4 , a transport region  422  is depicted surrounded by one or more permanent magnets  414 , although such a structure is not required for use of this invention. Rather, this is merely indicative of one application in which a cold cathode gun in accordance with the present invention may be used. In this embodiment, a strong permanent magnet  418  is situated outside the region including the cathode and lens elements in order to apply a magnetic field within the gun region  420  to insulate the cathode  410  from secondary and primary re-reflected electrons. It should be appreciated that the permanent magnet  418  may be a single magnet having an annular geometry, as shown, or may be comprised of a plurality of magnetic pieces configured and arranged around the gun region to create a magnetic field within the gun region  420 . Other configurations of the magnet  418 , suitable for applying a magnetic field within the gun region  420 , could also be used and would fall within the scope and spirit of the present invention. 
       FIG. 5  is an R-Z plot of a cold-cathode electron gun in accordance with the preferred embodiment. Permanent magnet  418  creates a magnetic field in the vicinity of the cold cathode and the electrostatic lenses and is indicated by flux lines  522 . Electron beam  416  is emitted from cold cathode  410  into the gun region, where it is focused by electrostatic lens elements  404 ,  406 , and  408 . However, when secondary electrons are emitted from surface  510  of a lens element, they interact with the magnetic field indicated by flux lines  522 , and their trajectories  524  are strongly deflected to minimize the gun volume affected by the emitted electrons. The slow electron trajectories  524  are now confined to large radial distances far from the main beam path. They do not extend into the beam path or to elements far upstream of the emission point. 
       FIG. 6  is an R-Z plot of the same cold-cathode electron gun of  FIG. 5  in accordance with the preferred embodiment. Here, the effect of re-reflected primary electrons is illustrated. The re-reflected electrons are shown as being emitted from a single point  610  with the characteristics described previously with respect to  FIG. 3 . However, with the magnetic insulation provided by magnet  418  in place, the electron trajectories  624  are much more confined. They are limited to radial regions far from the beam axis, and their extent in the Z direction is also constrained. Most importantly, it is seen that the electrons that would strike the cathode in the absence of magnetic insulation are now kept far from the cathode surface. The shape of the magnetic flux lines  522 , as indicated in  FIGS. 5 and 6 , is important because they are preferably oriented such that an electron originating from any of the high-voltage elements, such as lenses  404 ,  406 , and  408 , must cross many magnetic flux lines to reach the cathode surface. In other words, the magnetic field should be oriented in a direction that is not substantially parallel to a line of sight from the cold cathode to a re-reflected primary or secondary electron emission site. In one embodiment, this may be achieved by orienting the magnetic field to be substantially parallel to the electron beam axis, as shown in  FIGS. 5 and 6 . However, aligning the field along the electron beam path is not required, and other configurations requiring re-reflected primary and secondary electrons to cross multiple flux lines to reach the cathode are similarly effective in providing insulation, and such configurations of the magnetic field would fall within the scope and spirit of the present invention. 
     In an alternative embodiment in accordance with the present invention, the magnetic field in the gun region can be generated by an electromagnet rather than a permanent magnet.  FIG. 7  illustrates this structure schematically in an R-Z plot. Electromagnetic assembly  702  is comprised of electrical windings, shown schematically as element  704 , situated around the gun region. When electric current is applied to the windings  704 , a magnetic field indicated by flux lines  522  is created in the gun region. It should be noted that in either embodiment, while some magnetic leakage fields created by the transport-region magnets  414  may extend into the gun region, they are not of sufficient magnitude to provide adequate magnetic insulation. It should also be noted that the invention applies equally well to cold-cathode electron guns that employ an ion shielding potential profile in the acceleration region and those that do not. 
       FIGS. 8 and 9  depict a simulated electron beam profile within a cold-cathode electron gun without and with the magnetic insulation of the present invention, respectively. In  FIG. 8 , electron beam  820  is emitted from cold cathode  802  into a gun region that does not include an insulating magnetic field. The electron beam in this simulation has a beam current of 0.100 A and the cathode voltage is 3500 V. Electrostatic lenses  804 ,  806 , and  808  are set at high voltage potentials to focus and confine the electron beam  820  to the region near the central axis. 
       FIG. 9  shows a beam profile within a cold-cathode electron gun that includes magnetic insulation in accordance with an embodiment of the present invention. Electron beam  920  is emitted from cold cathode  902  into a gun region that now contains a substantial magnetic field indicated by flux lines  924  provided by a permanent magnet  922  in accordance with an embodiment of the present invention. It can be seen that the system of  FIG. 9  produces an electron beam with confinement characteristics very similar to that produced by the system of  FIG. 8 . However, only the system of  FIG. 9  provides significant insulation of the cold cathode from secondary and primary re-reflected electrons. In addition, the magnetic field indicated by flux lines  924  in the gun region has the added benefit of allowing much lower voltages to be applied to the lens elements  904 ,  906 , and  908 . Indeed, the voltages applied in the simulation shown in  FIG. 9  are between 24 and 67 percent below those applied in  FIG. 8  and yet achieve similar focusing and confinement. This greatly reduces the arc-inducing electric fields within the gun region, further protecting the cathode from destructive arcing events. 
     In conclusion, the invention provides a novel apparatus and method of magnetically insulating a cold cathode of an electron gun to reduce its susceptibility to damage from an electric arcing event and to reduce leakage currents and parasitic heating within the electron gun. Those skilled in the art will likely recognize further advantages of the present invention, and it should be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.