Nonlinear Plasma Absorption of High-Frequency Radiation

The interaction of electromagnetic radiation with a plasma has been studied. The relationship between the field strength and the induced current density becomes nonlinear when the electrons are accelerated to velocities in excess of the phase velocities of longitudinal electron waves in the plasma. The rate of energy absorption from a radiation field by a plasma has been calculated, in the limit of high frequencies, as a function of power input. The results are applicable when the radiation frequency exceeds the electron plasma frequency as well as the electron-ion collision frequency. It is found that the usual linear dependence of energy absorption on power input is directly applicable when all wavelengths of electron plasma waves, emitted by a particle which oscillates in the radiation field, exceed the excursion distance of the particle from its equilibrium position. When this condition is violated, the deviations from the linearized results are logarithmic, as long as the directed velocities attained by electrons in the radiation field are small compared to the thermal electron speed. Finally, when electrons are accelerated to velocities much in excess of thermal speeds, the deviations from the linearized results become very important.