Non resonant, four wave mixing diagnostics for the thermodynamic characterization of neutral, charged species and nanoparticles in plasma discharges.

We report on the development and utilization of non-resonant, four-wave-mixing (FWM) optical diagnostics in the form of coherent Rayleigh-Brillouin scattering (CRBS), for neutral and charged species diagnostics as well as nanoparticle synthesis monitoring in plasma discharges. In particular, we report working progress towards directly probing the formation and decay of nanosecond pulsed discharges, which are characterized by a high degree of ionization and reactivity, as well as transient non-trivial dynamics that are not fully understood to date. By directly measuring the velocity distribution function of the probed species, their translational temperatures can be extracted during the discharge without having to rely on the assumption of local thermal equilibrium and/or Maxwellian distribution, thereby enabling testing the validity of recent theoretical models. Additionally, CRBS can be used to probe in situ the spatio-temporal evolution of nanoparticle properties—especially their size—during discharge. This enables a fundamental understanding of the physical mechanisms behind nanoparticle formation, which is essential for controlling and optimizing their synthesis. Finally, we report on the numerical analysis of electron and ion-acoustic waves excited by the cross-focusing of two intense laser beams within a plasma. Our findings show a resonance in the electron and ion density perturbation as the frequency detuning approaches a value consistent with the Bohm-Gross and ion-acoustic dispersion relations. We thus consider and propose the applicability of these resonances as a new tool for plasma characterization; by scattering a third laser beam at the Bragg angle from the plasma lattice, electron and ion parameters can be estimated. The main advantage of our FWM approach to plasma diagnostics 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 enhances the signal-to-noise ratio of the measurement by several orders of magnitude.