A Reflex Electron Beam Discharge as a Plasma Source for Electron Beam Generation

A reflex electron beam glow discharge has been used as a plasma source for the generation of broad-area electron beams. An electron current of 120 A ( 12 A/cm2) was extracted from the plasma in 10 ps pulses and accelerated to energies greater than 1 keV in the gap between two grids. The scaling of the scheme for the generation of multikiloamp high-energy electron beams is discussed. E HAVE demonstrated that the plasma created by a reflex electron beam glow discharge constitutes an efficient source of thermal electrons for subsequent acceleration into intense broad-area electron beams. Electron currents of up to 120 A (12 A/cm2) were extracted from the plasma of the reflex discharge and accelerated in the gap between two grids at keV energies. Several types of electron sources, including thermionic cathodes [ 11, photoelectric emitters [2], vacuum plasmas [3]-[5], and hollow cathode discharges [6]-[8], have been used and are currently being studied for broad-area electron beam generation. Low-pressure, high-voltage glow discharges have been demonstrated to generate high current density ( > 10 A/cm2), broad-area electron beams [9]-[ 131. In these glow discharges almost the entire discharge voltage drops in the cathode sheath [13], where the emitted electrons are accelerated to form the electron beam. Electron emission occurs following the bombardment by energetic ions and fast neutrals. Cold cathode glow discharge electron guns do not suffer the phenomenon of diode closure and are of simple construction. They have been shown to produce electron beam current densities > 10 A/cm2 at energies of 50-90 keV [lo]. However, at energies > 100 keV the electron beam current density is limited by frequent arcs to values on the order of 1 A/cm2 [ 101. Here we present a novel electron beam generation scheme in which two glow discharge electron guns are used in a reflex configuration to create a dense and cold plasma in a large volume. The thermal electrons created mainly by electron beam ionization are subsequently accelerated by an externally applied electric field in the gap between two grids to produce a broad-area electron beam. The electron beam current density and energy are indeManuscript received September 16, 1987; revised May 31. 1988. This work was supported by the U.S. Air Force. B. Szapiro was supported by a fellowship from the Universidad Nacional de Buenos Airea. The authors are with the Department of Electrical Engineering, Colorado State University, Fort Collins. CO 80523. IEEE Log Number 8822661. pendently controlled by the voltage applied to the glow discharge guns and by the electric field sustained between the grids, respectively. Fig. l(a) is a schematic representation of the electron gun configuration used in the experimental demonstration of the concept reported here. Fig. l(b) shows the electron gun in the cross-shaped stainless steel vacuum chamber. Aluminum cathodes 3 cm in diameter surrounded by ceramic tubes constitute two glow discharge electron guns that are placed 15 cm apart in a reflex configuration. Both electron guns are maintained at the same potential. At an operating pressure of 0.2 torr of helium the multikilovolt beam electrons produced by the glow discharge guns have a reaching distance that is several times longer than the distance between the guns. Consequently, the electrons can travel back and forth between the cathodes, generating a dense negative glow plasma. This is as follows: the electrons emitted by one of the cathodes are accelerated in the corresponding cathode sheath region and travel towards the opposite cathode, where they are reflected by the potential barrier presented by the cathode sheath. The beam electrons maintain this oscillatory motion until they become thermalized after suffering a large number of ionizing and exciting collisions or are scattered outside the plasma. In their oscillatory motion the beam electrons ionize the gas, creating an almost electric field-free plasma in which the secondary electrons created by ionization thermalize to a very low electron temperature ( < 1 eV) [lo], [14]. The process is similar to the operation of hollow cathode discharges. Hollow cathode discharges have been used as electron sources for broad-area beams [6]-[SI. However, in the reflex electron beam discharge the electron production efficiency can be several times larger. This is a consequence of the much larger secondary electron emission yield of materials when bombarded by keV ions, as compared to ions with a few hundred eV in hollow cathode discharges [ 181. While in hollow cathode discharges the majority of the discharge current consists of ion flux bombarding the cathode, in high-voltage glow discharges with high electron yield cathode materials, it is not unusual for the electron beam current to be within a few percent of the total discharge current [ 101. The enhanced production of energetic primary electrons results in a more efficient creation of thermal electrons. Also, the reflex glow discharge might have fewer constraints to be scaled to large areas because at multikiloelectronvolt 0093-3813/88/1000-0570$01.00 O 1988 IEEE 57 1 MURRAY et al . : A REFLEX ELECTRON BEAM DISCHARGE I I I I I E B e a m U