Experimental study of compressional hydromagnetic waves

An experiment is described in which a compressional hydromagnetic wave is observed in a hydrogen plasma-filled waveguide. The theory of a cool, partially ionized, resistive plasma in a magnetic field is described briefly and expressions are derived for the dispersion relation and transfer function which include both the propagation and attenuation constants as a function of frequency. Measurements of the cutoff frequency are presented, which verify its linear dependence on the magnetic field, and they show good agreement with theory on the variation with the ion mass density. The impulse response of the plasma is studied, transformed into the frequency domain, and quantitative comparisons are made with the theoretical transfer function to determine the degree of ionization, the resistivity, and the ion-neutral collision frequency.Results indicate that the degree of ionization varies over a range from 75% to 45% when the initial density varies from 1.3 × 10²¹ to 1.4 × 10²² atoms/m³. The measured resistivity appears to increase with the magnetic field, with the mean value corresponding to a temperature of the order of 5 × 10³ °K. The average value of the product of the charge exchange cross section and the neutral thermal speed is found to be approximately (5.5 ± 1.3) × 10⁻¹⁵ m³/sec.