Experimental Study of Plasma Wave Resonances in a Hot Nonuniform Plasma Column

Abstract

The relative frequency spectrum ω2/ω2p of plasma wave resonances in the positive column of a low pressure mercury discharge tube has been shown to depend upon the parameter r2w/λ2D where rw is the radius of the column, λ2D is the Debye length defined in terms of the average electron density, and ω2p is the square of the average plasma frequency. This paper presents observations of both dipole and quadrupole resonance spectra made on several discharge tubes with rw ranging from 0.30 to 0.87 cm. For these measurements r2w/λ2D varies from about 102 to 105, and the best fit electron temperatures are found to be of the order of 3 ev. The average electron densities are directly measured using a cavity perturbation technique. The results of these observations are found to be in good agreement with the theory based upon the first two moments of the correlationless Boltzmann equation in conjunction with Parker's electron density profile for a low density positive column. The results of a preliminary investigation of the effects of an axial, static magnetic field on the dipole resonance spectrum are also presented. These results indicate that in the presence of an axial magnetic field not only does the lowest resonance (approximately predicted by the cold plasma theory) split, but the next higher order resonance also splits. For the lowest resonance, it is found that Δω/ωg ≈ .8 ± .1, while for the next higher order resonance Δω/ωg ≈ .5 ± .2, where ωg is the cyclotron frequency. These preliminary results are in good accord with calculations made by Parker, again using the moment equation approach.