Whistler-mode propagation in a laboratory plasma

Abstract

Propagation of 9280 Mc/s electromagnetic radiation through an argon afterglow plasma has been studied. A superimposed uniform magnetic field, parallel to the direction of propagation, was set at values between 1800 and 6800 G. The electron density and temperature throughout the plasma were measured with a Langmuir double probe. The propagation was observed for electron densities exceeding the critical value for 9280 Mc/s by more than 50 times, but only when the applied magnetic field exceeded the electron cyclotron resonance value. The amplitudes of the right-hand and left-hand circularly polarized components of the propagated signal were measured separately, and changes of plasma refractive index were recorded interferometrically. Both the polarization and refractive index observations were found to agree with the predictions for the `whistler mode' from Appleton's equation, excluding the Lorentz term. The collisional absorption predicted by this equation accounts quantitatively for the failure to obtain propagation when the electron density is very high.