Non-resonant, seedless coherent Rayleigh-Brillouin scattering four-wave mixing laser diagnostics for low temperature plasmas

Coherent Rayleigh-Brillouin scattering (CRBS), a non-linear and non-resonant four wave mixing process, is a powerful diagnostic technique for the measurement of thermodynamic properties of neutral species in weakly ionized plasmas, particularly for the density, flow velocity and the translational temperature [1]. Based on the interaction of intense, fast moving optical lattices with the particles in a medium (atoms or molecules, neutral or charged) CRBS can provide with a single shot ($\sim 200 \mathrm{~ns}$) measurement of the velocity distribution function (VDF) of the medium [2, 3] and thus detect departures from Maxwellian VDFs. Here, the current state-of-the-art of CRBS will be presented, while directions towards the utilization of CRBS for the detection of the VDF of ions (Maxwellian or not) will be discussed. Apart from neutral species diagnostics in plasma discharges, we are currently working towards utilizing the optical lattice concept found in CRBS, for the measurement of ionic temperature; progress on this field will be reported. Additionally, we report on the development of novel laser equipment to perform these measurements. Finally, we have hypothesized that a similar four-wave mixing concept can be utilized for the measurement of electronic temperature, in the form of four-wave mixing Thomson scattering [4]. It is foreseen that, if successful, this method will largely improve the capabilities of Thomson scattering, enabling the characterization of electronic temperature at much lower densities than attainable today, while detecting departures from Maxwellian distributions, as is the case with single shot CRBS.