Upper-hybrid and electron-cyclotron waves in a laboratory magnetoplasma: weak spatial dispersion and parametric effects

Abstract

Radiation of short-wavelength plasma waves is studied in a cold (Te=0.5eV), large (1.2m length, 70 cm diameter), uniform, Maxwellian laboratory magnetoplasma in the upper-hybrid frequency range. Although the characteristic parameter of spatial dispersion b2=VTe2/c2 is very small (b2<10-6), dispersion characteristics of the upper hybrid plasma waves is found to be strongly modified by the thermal motion of plasma electrons. Indeed, the resonance cone-like dispersion of UH plasma waves has been observed only if the parameter range satisfies simultaneously two inequalities: wc<w<2wc, wp<w<wuh. In this parameter range, the maximal refractive index in the resonant directions has been found to be limited by nmax~15; new UH mode with its dispersion determined by conical refraction has been found in the vicinity of the upper hybrid resonance (w=wuh). Wave dispersion at w<2wc, wp<w<wuh does not correspond to the cold plasma approximation and is determined by thermal plasma waves propagating along the ambient magnetic field. Special attention is given to the wave propagation characteristics in the vicinity of the electron cyclotron resonance. An interesting effect of reduced electron cyclotron damping has been observed when two electromagnetic waves (w1=wc, w2=wc+Dw) propagate simultaneously with Dw approximately equal to the lower hybrid frequency. This effect can be explained as a three-wave parametric process, analogous to the effect of electromagnetically induced transparency in three-level atomic system. Reduced electron cyclotron damping is extensively studied as a function of the beat frequency and the amplitude of the pump wave. Possible applications of the observed effect in plasma physics and electronics are discussed.