Brush Cathode Plasma—A Well-Behaved Plasma

Abstract

Results of probe, spectroscopic, and microwave studies are used to describe properties of a novel cold-cathode discharge. The use of a brush cathode allows stable operation in the abnormal glow region where some of the usual cold-cathode mechanisms are greatly exaggerated and determine the features of the produced plasma. The cathode fall is about an order of magnitude larger than for the corresponding normal cold cathode. The brush cathode generates a uniform electron beam in the energy range 1 to 10 kV and a corresponding negative glow with a longitudinal dimension (reaching distance of the beam) one to two orders of magnitude larger than for the normal cathode glow. The large dimensions of the negative glow and the fact that it is well-behaved (no instabilities and no striations) make it ideal for a whole series of investigations in plasma physics and spectroscopy. The electron density in the helium plasma is in the range 1010 to 1014 cm−3, the electron temperature in the range 0.05 to 0.10 eV. The negative glow is beam generated (essentially field free) and recombination dominated making it a practically uniform plasma ideal for the study of rate and transport coefficients. Preliminary measurements of the particle balance give a recombination rate of 5×10−10 cm3 sec−1 at 1100°K and an electron density of 3×1012 cm−3 in agreement with the collisional-radiative recombination theory developed by Bates, Kingston, and McWhirter. The recombination light emitted by the plasma is sufficiently strong to make spectroscopic methods useful for measurements of the electron density and the electron temperature. Lines in the helium series 2s3S—np3P0 are observed up to the quantum level n=30.