Coherent Thomson scattering: a four-wave mixing approach to plasma diagnostics

The proposed novel four-wave mixing technique of coherent Thomson scattering (CTS) makes use of induced optical lattices within a plasma, to scatter off a third beam from them, giving rise to the coherent CTS signal beam. The technique builds on an established and demonstrated single shot diagnostic method, termed coherent Rayleigh-Brillouin scattering (CRBS), which has already been successfully applied to neutral flows and neutral species in glow discharges. Owing to its four-wave mixing nature, CTS will enable higher spatial resolution and lower detectable number densities for the electrons than conventional Thomson scattering, as applied to low and high temperature plasmas. Towards this goal, the theoretical framework for CTS has been developed, as well as the specification of the appropriate operational experimental parameters for successful CTS implementation in e.g. a low temperature plasma. The one-dimensional nonstationary Boltzmann equation was solved for electrons in the BGK approximation with allowance for the ponderomotive force, together with the Poisson equation for the potential distribution. As an example, nonequilibrium weakly ionized Argon plasma was considered. Importantly, this has led to the simulation of the anticipated CTS spectra for the case when the wavelength of the induced optical lattices is much smaller than the Debye length. Ultimately, successful experimental demonstration of CTS can find application in a multitude of areas in plasma physics, since it will allow for detailed, non-perturbative measurements of electron density and temperature, hard to be attained by other measurement techniques.