DC glow discharge electron guns for the excitation of rare gases / R.J. Carman.

Abstract

Glow discharge electron guns are used to generate continuous electron beams at 0.5keV-3.0keV in the intermediate range of gas pressures (0.1mb-10.0mb). Cathodes incorporating internal cavities are used to generate distinct electron beam filaments in both Helium and Argon. The formation of such beam filaments has been investigated using a number of different cathode types, and criteria for the production of stable electron beams are established. The production of an electron beam in a glow discharge is largely determined by the motion of electrons in the Cathode dark space sheath region next to the cathode, and other discharge processes in this region. A theoretical model has been developed to simulate electron motion in the sheath region, and in the Negative glow plasma region, of a Helium discharge with a Cathode fall of between 150V and 1000V. It is shown that the electron flux at the 'sheath/Negative glow boundary becomes increasingly monoenergetic as the Cathode fall rises to 1000V. The results are also compared with experimental spatial emission profiles of the glow in the Cathode dark space and Negative glow regions of a helium discharge. In particular, properties of the Cathode glow region in the sheath are discussed. Aspects of the theoretical model and results from the experimental measurements are also used to discuss discharge processes in the sheath region of cathodes incorporating internal cavities, and mechanisms leading to the formation of the electron beam filaments. The production of fast electrons in a glow discharge has a number of applications, including the excitation of gases leading to laser action. Aspects relating to the excitation of high lying energy states in gases, corresponding to known laser transitions, are discussed. It is shown that the production of helium ions, which are responsible for the excitation of metal atoms via asymmetric charge transfer in metal ion lasers, is theoretically more efficient in an electron beam discharge. The results are compared with the theoretical ion production rates in Hollow cathode discharges, and high-voltage Hollow cathode devices. Several electrode geometries using multiple arrays of electron gun cathodes have been developed. Investigations of an electron beam excited argon plasma suggest that Ar II excited states are pumped directly by single electron impacts, even at very low current densities (~10<super>-3</super> A cm<super>-2</super>). From previous calculations using the 'sudden perturbation' approximation, those ion states known to have large cross-sections for direct electron impact excitation (3p44p2P) appear to be favourably pumped in the electron beam plasma.