Electron temperature a key element for interpreting high-speed plasma imaging

High-speed imaging provides a nonintrusive technique for probing astrophysical, fusion, and laboratory plasmas. The method supplies instantaneous access to the full spatial features of plasma with a high time resolution, routinely reaching hundreds of thousands of frames per second with current technology.

Light emission in a plasma depends on the plasma density, neutral density, and electron temperature. However, numerous experimental investigations demonstrated the light fluctuations seen in high-speed images are highly correlated to the plasma density fluctuations, so the former is often used as a simple proxy for the latter.

Vincent et al. demonstrated the importance of incorporating electron temperature for the interpretation of plasma light fluctuations.

The team measured the plasma density and electron temperature with a probe in a linear plasma column while taking high-speed images. They specifically investigated ion acoustic waves, which occur at high frequencies and high wavenumbers, and low-frequency azimuthal waves at low wavenumbers.

The two types of waves demonstrate different spatial and temporal behavior, but they both showed high correlation between the light fluctuation intensity and the electron temperature. The correlation was higher than that between the light and plasma density fluctuations.

By combining the electron temperature and plasma density fluctuation parameters into a simple model based on the Arrhenius equation, the researchers found the highest correlation with the light fluctuations.

“Going even further, this means if you have both high-speed video and the density fluctuations, then you can work out the electron fluctuations,” said author Nicolas Plihon.

Source: “High-speed imaging of magnetized plasmas: When electron temperature matters,” by Simon Vincent, Vincent Dolique, and Nicolas Plihon, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0083130 .