Analysis of longitudinal plasma waves by ponderomotive forces and its applicability to plasma diagnostics

We report on the numerical analysis of Langmuir waves [1] excited by the cross-focusing of two intense laser beams within a plasma. Our findings show a resonance in the electron density perturbation as the frequency detuning approaches a value consistent with the Bohm-Gross dispersion relation [2]. We consider and propose the applicability of this resonance as a new tool for the characterization of electron temperature in a plasma; by scattering a third laser beam at the Bragg angle from the electron density perturbation, electronic temperature and density measurements can be made possible. The main advantage of this approach is that the resulting scattered signal is a coherent laser beam, which allows for the detector to be placed at a considerable distance from the plasma source, mitigating any noise from inherent plasma radiation. In contrast with non-coherent optical diagnostics, this is anticipated to enhance the signal-to-noise ratio of the measurement by several orders of magnitude. The excitation of electron waves is expected to be observable at high-pressure plasmas being the analogue of Brillouin peaks observed in atomic and molecular neutral gases. This study is performed in the hydrodynamic regime, which becomes applicable at relatively high-pressure plasmas with a grating period larger than the Debye length.

[1] Lewi Tonks and Irving Langmuir. Oscillations in ionized gases. Phys. Rev., 33:195–210, Feb 1929.

[2] D. Bohm and E. P. Gross. Theory of plasma oscillations. a. origin of medium-like behavior. Phys. Rev., 75:1851–1864, Jun 1949.