Parametrically Excited Plasma Fluctuations

The enhancement of the spectrum of density fluctuations in a parametrically excited plasma is studied using the linearized theory. In the particular case studied, the plasma is "pumped" by a monochromatic electromagnetic field of either transverse or longitudinal polarization with frequency ${\omega }_0\simeq \left(\mathrm{electron}\mathrm{plasma}\mathrm{frequency}\right)+\left(\mathrm{ion}\mathrm{acoustic}\mathrm{frequency}\right)$. A strong enhancement of the fluctuation power at the two individual frequencies is predicted as the previously calculated threshold for instability or oscillation of the linearized theory is approached. The experiments of Stern and Tzoar and other recent theories are discussed. Approximate expressions are derived for the strength of the enhanced resonances and the range of k vectors which actively participate. The cross section for inelastic scattering of radiation from the excited plasma is derived and compared with a result due to Berk, who did not consider the important regenerative process of parametric amplification. The saturation of the noise level for pump powers above the linear threshold is treated.