Low-Voltage, High-Current-Density Electron Beam Sources.

摘要

Electron beams are attractive for materials processing applications because of the potential for low-cost, compact sources. Electron range considerations dictate sources that are much lower in voltage, but with correspondingly higher current density, than comparable ion beam sources. The lower voltage has many advantages but requires an orders-of-magnitude enhancement of current density over that achievable with simple space-charge-limited vacuum gaps. Therefore, any electron beam source that does not rely on high geometrical focusing will need a plasma fill to supply the positive space charge necessary to exceed the space charge limited current. In addition, any source will require both a cathode capable of emitting high current densities at low voltages, and a means of guiding the beam to the target. In the past we have studied plasma-filled diodes at the 100 kV-1 MV level for x-ray production and improved load coupling. We are now beginning to study plasma filled diode operation at low voltages to produce electron beams for materials applications. This paper presents our first work in this area. Appropriate electron-beam sources require a low-voltage cathode, plasma to neutralize the beam space charge, and a means of guiding the beam to the target. Candidate cathodes include surface flashover sources, highly-enhanced structures like graphite fibers, and hollow cathodes. Plasma can be produced by a variety of surface-discharge sources, or by ionization of a gas ambient, either by the electron beam precursor or an auxiliary Penning discharge. The beam can be guided either magnetically or electrostatically by a dielectric channel.