Propagation of Strong-Field Modulated Electromagnetic Waves in Plasmas

The distortion of the wave form of a modulated plane electromagnetic wave propagating in an anisotropic plasma has been experimentally investigated over a range of field strengths of the wave and plasma properties. By using right-hand circularly polarized waves, the effective frequency is [Formula: see text] (where ω is the r.f. radian frequency and ωb the cyclotron frequency) and hence the results are also applicable to the propagation of low-frequency waves in an isotropic plasma. Severe "overmodulation" of the wave form transmitted through the plasma is found in the regime [Formula: see text] where ν is the effective collision frequency for momentum transfer. The distortion of the wave form is found to increase with depth of modulation of the incident wave and decrease with increasing modulation frequency.The "demodulation" is in qualitative agreement with theory for an unmodulated wave with a non-Maxwellian (Druyvesteyn) velocity distribution for the electrons. Many of the effects of the modulation frequencies can also be qualitatively predicted by considering the variation of electron temperature in the presence of the strong-field modulated wave. A theory is developed for large changes in electron temperature induced by the incident field which shows that marked distortion of the modulation is possible.