An Experimental Study of the Hydromagnetic Waveguide

Abstract

The hydromagnetic waveguide consists of a cylindrical metal tube filled with a longitudinally magnetized plasma. Among the classes of waves which propagate in this system are the compressional hydromagnetic modes, characterized by a waveguide cutoff at low frequencies and by a resonance at the electron cyclotron frequency. This paper presents the results of observations of the propagation of such waves in a decaying hydrogen plasma at frequencies from 0.8 to 3.4 times the ion cyclotron frequency. The phase shift and attenuation of the waves are interpreted in terms of the ion density and the temperature by applying a theory based on a three-fluid description of the plasma. Spectroscopic measurements of the H_B line profile and absolute intensity are used to check the density and temperature inferred from the wave measurements. The results of this study indicate that a simple approximate relationship between the phase factor and density obtained by neglecting dissipation gives densities which agree well with the spectroscopic measurements. As a diagnostic tool this method may yield densities to within ± 25% over a range of two decades. In the case of amplitude measurements only semiquantitative agreement between the wave and spectroscopic measurements is found, but the amplitude curves do show evidence of interferences between modes and a sharp cutoff at a critical density, both effects predicted by the theory.

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