Time-resolved laser-induced fluorescence diagnostics for electric propulsion and their application to breathing mode dynamics

Abstract

Several techniques have been developed recently for performing time-resolved laser-induced fluorescence (LIF) measurements in oscillating plasmas. One of the primary applications is characterizing plasma fluctuations in devices like Hall thrusters used for space propulsion. Optical measurements such as LIF are nonintrusive and can resolve properties like ion velocity distribution functions with high resolution in velocity and physical space. The goals of this paper are twofold. First, the various methods proposed by the community for introducing time resolution into the standard LIF measurement of electric propulsion devices are reviewed and compared in detail. Second, one of the methods, the sample-hold technique, is enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The new system is applied to study the dynamics of ionization and ion acceleration in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44-49 kHz range. A very detailed experimental picture of the common breathing mode ionization instability emerges, in close agreement with established theory and numerical simulations.

Evidence of a free-space ion acceleration layer in the plume of a quad confinement plasma source

“…The location of the acceleration front is atypical and differs from the one observed in other types of open-channel thrusters, such as Hall Effect Thusters 17 and Cusped Field Thrusters 24 . Indeed, in these latter devices, ion acceleration takes place around the exit plane: in some cases the front extends just few centimeters beyond the exit 17 , in other cases it is even located inside the channel 27 . The observation of the velocity vectors and angles on the centreline of the thruster, shown in Fig.…”

Ion plume investigation of a Hall effect thruster operating with Xe/N2 and Xe/air mixtures

“…This is the case of runs XeAir-3 and XeAir-4 where the xenon and air flow rates are 0.78 and 0.83 mg/s, respectively. In Hall Thrusters operated on xenon, Laser-Induced Fluorescence measurements synchronized with discharge current oscillations have shown that the dynamic ion flow field originates from the interplay between time-dependent ionisation and time-varying potential profile [47,59]. This framework can be extended to multi-species plasmas operated on air, however with increased complexity.…”

Numerical and Experimental Investigation of Longitudinal Oscillations in Hall Thrusters

“…Nevertheless, perturbations remain up to a certain degree and, as discussed by Jorns et al in Reference [42], a sharp transition of global plasma parameters and a downstream shift in plasma properties are observed upon probe insertion in the channel. Optical methods, such as Laser Induced Fluorescence (LIF) [44][45][46][47][48][49] and Thompson scattering [50], provide non-intrusive alternatives to fast moving ionic probes for the exploration of the steady and unsteady behavior of the near-plume plasma of Hall thrusters. In spite of being capable of providing useful information, these diagnostic systems can present signal-to-noise interpretation issues and, in most cases, require complex and expensive setups, limiting their extensive use.…”