Abstract:
The Circular Hollow Anode Ion Electron Plasma Source is a hollow anode ion electron plasma source presenting the limited area of the inner surface only of an anode exit aperture, leading to high brightness and high efficiency in a simple robust plasma device.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is a plasma source. More specifically it is an ion-electron source of the Hollow Anode type. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention attempts to provide a unique hollow anode ion electron source type of plasma source that is capable of delivering plasma, ions and/or electrons, in a geometric shape other than that of a point source or of a linear slit. Specifically the present source is capable of providing charged particles in the form of a cylindrical sheath, an inwardly directed curtain or a diverging or converging cone of charged particles at any angle between axial and radial relative to the axis of symmetry of the device. Additionally, the source can have any convoluted shape desired, having any symmetry or lack thereof. Such devices might find extensive application as pre-ionization sources for Hall effect ion thrusters, as well as other types of accelerators that require plasma streams other than as a point source. A large market has developed over the last few decades for the need of plasma sources for material processing and charged particle beam applications. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    The present invention is directed generally towards an ion electron source of the hollow anode type. More specifically towards a Hollow Anode plasma source that is capable of producing an annular ring of plasma ions or electrons for injection into a solenoidal magnetic field. Such geometrical designs find useful application in the production of ion and electron beam sources. Specifically they may be utilized as pre ionization sources for the production of beam injection into solenoidal magnetic fields as found in Morehouse U.S. Pat. No 7,825,601 and Morehouse U.S. Pat. No 8,138,677, incorporated herein by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  represents the Prior Art in the field of Hollow Anode Ion Electron Source technology, being a point source. 
           [0005]      FIG. 2  represents the Prior Art in the field of Hollow Anode Ion Electron Source technology, being a linear slit source. 
           [0006]      FIG. 3  reveals a side cross sectional view of an axial annular ion electron plasma source. The working neutral gas  10  is introduced into the vacuum discharge structure  20 . Cathode  30  and Anode  40  are energized by electrical means capable of providing voltages and currents proper to the breakdown of the gas into plasma. The exit Anode  40  is comprised of an inner surface only conductive aperture, which means that the only conductive aspect of the anode is in the exit slit  40  itself, all other anode surfaces are insulated from electrical conductivity to the plasma within the discharge chamber  20 . 
           [0007]      FIG. 4  reveals a side cross sectional view of a circumferential radial ion electron plasma source. The working neutral gas  10  is introduced into the vacuum discharge structure  20 . Cathode  30  and Anode  40  are energized by electrical means capable of providing voltages and currents proper to the breakdown of the gas into plasma. The exit Anode  40  is comprised of an inner surface only conductive aperture, which means that the only conductive aspect of the anode is in the exit slit  40  itself, all other anode surfaces are insulated from electrical conductivity to the plasma within the discharge chamber  20 . 
           [0008]      FIG. 5  reveals a side cross sectional view of a conically converging ion electron plasma source. The working neutral gas  10  is introduced into the vacuum discharge structure  20 . Cathode  30  and Anode  40  are energized by electrical means capable of providing voltages and currents proper to the breakdown of the gas into plasma. The exit Anode  40  is comprised of an inner surface only conductive aperture, which means that the only conductive aspect of the anode is in the exit slit  40  itself, all other anode surfaces are insulated from electrical conductivity to the plasma within the discharge chamber  20 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    Miljevic, in U.S. Pat. No. 4,871,918, incorporated herein by reference, reveals a Hollow Anode Ion Electron source. It operates on the principle of an anode with a constricted gas exit from a discharge plasma generator. The patent reveals two outlet geometries. One is simply a single hole point source. This is the most basic design geometry. Another shape is revealed and claimed, that is a linear slit outlet. No other geometric design is claimed or revealed. The designs claimed by Miljevic fail to reveal and specify an annular or circumferential slit. 
         [0010]      FIG. 1  shows a point source hollow anode plasma source of the Prior Art. Hollow anode electrode  11 , cathode  12 , housing  13 , permanent of electromagnet  14 , extraction electrode  15  and gas source inlet  19 . Elements  191 ,  192  and  193  are the cathode, anode and extraction electrode leads, respectively. The lower side of the hollow anode  11  is the exit aperture of the source  18  and together with the extraction electrode  15  represents the modified Pierce geometry. The upper side of the anode  11  is insulated by a thin ceramic layer. 
         [0011]      FIG. 2  reveals a linear slit design of the Prior Art hollow anode plasma source. A hemispherical cathode  22  with the hollow anode aperture  26  in the center of curvature is shown. Gas source inlet  23 , hollow anode electrode  21 , magnet  24 , thin ceramic layer  27  and Pierce extraction system  25 - 28  are the same as in the previous drawing. 
         [0012]    The present invention operates on the same hollow anode discharge principle but introduces a number of novel geometric configurations, providing new and useful streaming charged particle configurations. One preferred embodiment entails a geometrical design that is annular as apposed to a point source or linear slit, as provided by the prior art. In a preferred embodiment the annular exit anode is aligned axially, such that charged particles exit the source in a cylindrical sheath encircling. Another preferred embodiment introduces a circumferential design that directs the exiting charged particle source through the constricted anode in a radially inward direction, producing a generally radial curtain of charged particles perpendicular to the axis of the overall device. As these designs are mutually perpendicular, another preferred embodiment is provided capable of producing any angular configuration in between the two extremes. The charged particle stream generated by intermediate angles between axial and radial would generally produce a converging or a diverging cone of plasma. In each case the principle of operation is simple and follows closely that of Miljevic. Gas in introduced  10  generally in the vicinity of the cathode  30  or at the end of the device associated with the cathode  30 . The gas travels from the cathode  30  through the body of the ionization chamber  20  towards the exit anode  40 . The exit anode  40  has a restricted exit throat. The plasma  40  exit is comprised of an inner surface only conductive hollow anode exit aperture, which means that the only conductive aspect of the anode is in the exit slit itself, all other anode surfaces are insulated from electrical conductivity. This design requirement places a close tolerance requirement on the exit throat as the operation is dependent upon the anode exit being a constriction, which is comprised of an inner only conductive surface where electrons which are sourced at the cathode  30 , become concentrated forming a high density of ionizing electrons at the area of the anode exit  40 , producing a stream of charged particles  50 . Another unique feature of the present invention applicable to all of the possible embodiments is that the ionization channel may be of any appropriate extended length to achieve the objective of the invention. The desirability of having a long ionization channel is that in order to produce a low density gas discharge, the ionization path needs to be long, to keep the discharge voltage at a minimum. The restriction on ionization is that set forth by Paschen and known widely as the Paschen curve, which equates the breakdown voltage to the gas pressure and the length of the discharge path. A further novel element to the present invention is that the channel walls  20 , typically made of insulators, can be comprised of a conducting material or of insulators. By this means the range of possible Paschen breakdown parameters (specifically; path length as well as electric field strength) can be anything from the shortest distance between the anode and cathode across the insulator between the two and the farthest cathode  30  end of the ionization channel and the exit anode  40 . 
         [0013]    The present embodiments provide charged particle sources of greater flexibility and applicability than those that presently exist. The hollow anode design is superior to other sources such as hollow cathodes because of the concentration of electrons at the exit from the device. It is electrons that primarily serve to ionize the neutral gas in a plasma source and the constriction provided by the hollow anode source optimizes the ionization capability of the electrons.