A simple electron gun by obstructed discharge and its discharge-sustaining mechanism

• Published in: Vacuum 2000-10, Vol.59 (1), p.358-372
• Main Authors: Fukao, Masayuki, Ishida, Masazumi, Ohtsuka, Yoshio, Matsuo, Hironobu
• Format: Article
• Language: English
• Subjects: Charge exchange; Concave cold cathode; Fast neutrals; Low-pressure obstructed discharge; Secondary electron emission by fast neutrals
• ISSN: 0042-207X; 1879-2715
• DOI: 10.1016/S0042-207X(00)00289-X

An electron gun with a very simple structure is described as being suitable for the energy range from 2 to 10 keV or more. The gun consists of a concave cold cathode and an anode with an extraction orifice, which are maintenance-free. Helium or hydrogen gas is filled with low pressure for operation in a condition called “obstructed discharge (OD)”. The concave structure of the cathode converges the electron beam well. The well-focussed electron beams are demonstrated by a shining spot on a thin plate, the current density is measured by a Faraday cup, and the energy deposit profile by a micro-calorimeter probe. The energy spectrum of electron the beam obtained was measured by an electrostatic analyzer of 90° deflection type to be sharply monochromatic with the energy of discharge voltage. This is owing to little interaction of electrons from the cathode with neutral gas. This fact requires that the discharge must be explained by mechanisms other than the well-known Townsend's theory for sustaining. It is proposed that the fast neutrals produced through charge exchange between the neutral gas and the ions accelerated toward the cathode play an essential role in the discharge-sustaining mechanism. This model is supported by a numerical calculation under several assumptions and is proved by comparison with experimental results.