High-speed imaging of magnetized plasmas: when electron temperature matters

TL;DR

This study demonstrates that electron temperature significantly influences high-speed imaging of magnetized plasmas, challenging the common assumption that light fluctuations directly represent plasma density variations.

Contribution

It provides a systematic comparison between high-speed camera imaging and direct plasma parameter measurements, highlighting the importance of electron temperature in interpreting plasma fluctuations.

Findings

Light intensity is not a reliable proxy for plasma density.

Electron temperature fluctuations are crucial for accurate interpretation of imaging data.

High-speed imaging fluctuations match an Arrhenius law incorporating density and temperature variations.

Abstract

High speed camera imaging is a powerful tool to probe the spatiotemporal features of unsteady processes in plasmas, usually assuming light fluctuations to be a proxy for the plasma density fluctuations. In this article, we systematically compare high speed camera imaging with simultaneous measurements of the plasma parameters -- plasma density, electron temperature, floating potential -- in a modestly magnetized Argon plasma column at low pressure (1 mTorr, magnetic fields ranging from 160 to 640~G). The light emission was filtered around $488\pm5$~nm, $750\pm5$~nm, $810\pm5$~nm. We show that the light intensity cannot be interpreted as a proxy for the plasma density and that the electron temperature cannot be ignored when interpreting high speed imaging, both for the time-averaged profiles and for the fluctuations. The features of plasma parameter fluctuations are investigated, with a focus on ion acoustic waves (at frequency around 70 kHz) at low magnetic field and low-frequency azimuthal waves (around a few kHz) at larger magnetic fields. An excellent match is found between the high speed images fluctuations and an Arrhenius law functional form which incorporates fluctuations of the plasma density and of the electron temperature. These results explain the discrepancies between ion saturation current and narrow-band imaging measurements previously reported in the literature.