Portable Thomson scattering system for temporally resolved plasma measurements under low density conditions.

Abstract

We present the development of a portable Thomson scattering diagnostic system allowing simultaneous spatially and temporally resolved plasma property measurements for low density plasmas. The setup uses a compact pulsed Nd:YAG laser (532nm) as the light source with suppression by two volume Bragg grating notch filters and dispersion with a single-stage spectrometer before measurement with an intensified camera. A key issue is the detailed light collection and how it impacts the sensitivity and elastic light suppression, for which we have investigated two optical configurations, one based on a 7 × 1 linear fiber bundle and the other based on a slit spatial-filter. We find that the configuration with the slit spatial-filter provides a higher sensitivity by a factor of ∼2 along with more uniform spatial response. We have developed a custom pulsed-plasma setup with a modulation at 20kHz, representative of the Hall thruster breathing mode oscillation, to show the possibility of temporally resolved measurements for electric propulsion applications. We have successfully recorded the variations in electron number density and temperature with sub-mm spatial resolution and capturing ten temporal points over the 50 µs modulation period. The detection limit of electron density (with the spatial-filter configuration) is ∼1.6 × 1017m-3, which is ∼1/10 of the plasma density in the acceleration channel of Hall thrusters.