Optical emission spectroscopy of helium helps deduce plasma electron density and temperature

Plasma spectroscopic line emissions provide a wealth of information for analyzing fusion devices. Helium line emissions have found use in gleaning such information for plasma diagnostics. One group looks to deduce plasma density and temperatures from the helium line emissions.

Kajita et al. report findings from using helium line emissions to measure electron density and temperature in a linear plasma device called Magnum-PSI. Using optical emission spectroscopy, the group was able to observe the line emissions and deduce the electron density and temperature using a collisional radiative model.

“This is the first time that comparative investigation was done for this measurement method under the high-density conditions,” said author Shin Kajita. “It was also found that at least five line emissions would be necessary to deduce plasma parameters.”

Within a wavelength range of 388 to 728 nm, the group observed nine line emissions. Without accounting for the effect of radiation trapping, electron density was overestimated and temperature contradicted results calculated through Thompson scattering. These discrepancies were largely mitigated by using the optical escape factor to account for radiation trapping, leading the authors to conclude that radiation trapping is also essential to understand the high-density regime.

“Because the neutral atomic density and temperature are unknown parameters in the high-density regime, those could influence the helium line intensity and trapping of UV light as well,” Kajita said.

Kajita hopes their will work will inspire others to explore helium emissions and looks to further investigate the discrepancies between the group’s approach and the Thomson scattering method.

Source: “Emission spectroscopy of He lines in high-density plasmas in Magnum-PSI,” by Shin Kajita, Gijs Akkermans, Keisuke Fujii, Hennie van der Meiden, and M. C. M. van de Sanden, AIP Advances (2019). The article can be accessed at https://doi.org/10.1063/1.5143481 .