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

The relative frequency spectrum ω^(2)/ω^(2)_(p) of plasma wave resonances in the positive column of a low pressure mercury discharge tube has been shown to depend upon the parameter r^(2)_(w)/λ^(2)_(D) where r_w is the radius of the column, λ^(2)_(D) is the Debye length defined in terms of the average electron density, and ω^(2)_(p) 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 r_w ranging from 0.30 to 0.87 cm. For these measurements r^(2)_(w)/λ^(2)_(D) varies from about 10^2 to 10^5, 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/ω_g ≈ .8 ± .1, while for the next higher order resonance Δω/ω_g ≈ .5 ± .2, where ω_g is the cyclotron frequency. These preliminary results are in good accord with calculations made by Parker, again using the moment equation approach