Atulya Kumar

Direct measurement of the particle velocity distribution function (VDF) of neutral gases and plasmas allows for their thermodynamic characterization (temperature, density, and velocity vector) and is of crucial importance for fields such as aeronautics, aerospace, microelectronics, etc. In recent years, the mechanical probes that have formed the state of the art — Pitot tubes, hot wires, Langmuir probes, etc. — are steadily being replaced by laser-based diagnostic techniques. Among the latter, single-shot coherent Rayleigh-Brillouin scattering (CRBS), a four-wave mixing technique, has been demonstrated to perform VDF characterization of atomic and molecular species within a single laser shot (∼ 100 ns). CRBS requires the use of highly specialized laser systems capable of generating laser beams with controllable chirps, variable pulse duration, controllable pulse shaping, and high pulse energies. However, the characterization of environments exhibiting fast flows, such as gas boundary layers, or low particle number densities, such as low-temperature glow discharges, have proven extremely challenging with current state-of-the-art laser systems mainly due to the need for pulse energies beyond 1 J. In this work, we report on the improvement of a custom nanosecond-pulsed laser system tailored for optical particle diagnostics and manipulation, and which is capable of pulse energies exceeding 2 Joule/pulse. In addition to the notable output energy, the laser system is capable of generating laser pulses with customizable temporal profiles and variable durations (10 ns to 1 μs), with a chirping range of several GHz over the pulse duration. To the best of the authors’ knowledge, this is the highest reported energy per pulse generated by a lab-scale frequency-chirped laser source at nanosecond timescales. The expanded output energy range is anticipated to greatly broaden the laser system’s application potential via CRBS for thermodynamic diagnostics of both supersonic/hypersonic gas flows in wind tunnel and shock-tube facilities, as well as plasma species in space-like conditions, e.g. ion thrusters.

A novel, to the best of our knowledge, laser diagnostic technique enabling simultaneous velocity measurements at different spatial locations is presented. Using single-shot coherent Rayleigh–Brillouin scattering (CRBS) and spatially splitting the signal beam, we show that distinct regions within the probe volume can be measured independently and simultaneously, thus enabling localized flow diagnostics at the micrometer scale. To demonstrate the new technique, measurements are performed in the flow of an under-expanded jet featuring sharp radial velocity gradients and microscale flow structures, which are probed through the spectroscopic signatures in two different signal beams. Velocity component extracted from the acquired spectra reveals flow-dependent signatures. The method presented here is inherently scalable through optical design modifications, offering a powerful, flexible diagnostic tool for detailed thermodynamic characterization of neutral gas and plasma flows in complex environments.