A Novel RF Excited Plasma Cathode Electron Beam Gun Design

Abstract

This paper presents a new radio frequency (RF) excited plasma-cathode electron beam (EB) gun design and experimental results at a frequency of 84 MHz. The design offers the following benefits over thermionic-cathode triode EB guns: unlimited cathode lifetime and as a result reduced maintenance costs; no requirement for a grid electrode, avoiding beam aberration; and rapid beam pulsing. The construction of the diode gun was completed and the results of this work demonstrate that the electron beam can be switched on in 200 ns and off within 800 ns. Electrons were extracted from a 2-3 mm plasma chamber and then accelerated by an electric field applied in a vacuum chamber at 10-5 to 10-6 mbar producing a collimated electron beam. The ionized gas used was nitrogen at a 0.5 to 1 bar pressure. The EB gun has been operated at –60 kV accelerating potential and has produced beams of up to 3.2 kW power continuous wave (CW). Modulation of the RF signal was used to control the beam power. Details of the design features of the plasma device are given and evidence of the advantages over conventional EB guns is provided through empirical results.