A novel hydrogenic spectroscopic technique for inferring the role of plasma-molecule interaction on power and particle balance during detached conditions

TL;DR

This paper introduces a new spectroscopic method using hydrogen Balmer lines to distinguish plasma-molecule interactions from plasma-atom interactions, enabling quantification of their effects on detachment in fusion devices.

Contribution

The study develops and verifies a novel hydrogenic spectroscopic technique to quantify plasma-molecule interactions' contributions to detachment, which was previously difficult to measure directly.

Findings

Plasma-molecule interactions significantly influence hydrogen Balmer line emissions during detachment.

The technique successfully distinguishes between atomic and molecular contributions to $H\alpha$ emission.

Experimental results indicate molecular interactions can account for tens of percent of hydrogenic line emissions.

Abstract

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma-atom and plasma-molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. In this work, we investigate a new spectroscopic technique to utilise Hydrogen Balmer line measurements to 1) disentangle the Balmer line emission from the various plasma-atom and plasma-molecule interactions; and 2) quantify their contributions to ionisation, recombination and radiative power losses. During detachment, the observed $H\alpha$ emission often strongly increases, which could be an indicator for plasma-molecule interactions involving $H_2^+$ and/or $H^-$. Our analysis technique quantifies the $H\alpha$ emission due to plasma-molecule interactions and uses this to 1) quantify the Balmer line emission contribution due to $H_2^+$ and/or $H^-$; 2) subsequently estimate its resulting particle sinks/sources and radiative power losses. Its performance is verified using synthetic diagnostic techniques of both detached TCV and MAST-U SOLPS-ITER simulations. Experimental results of this technique on TCV data show a bifurcation occurs between the measured total $H\alpha$ and the atomic estimate of $H\alpha$ emission, indicative of the presence of additional $H\alpha$ due to plasma-molecule interactions with $H_2^+$ (and/or $H^-$). An example analysis shows that the hydrogenic line series, even $Ly\alpha$ as well as the medium-n Balmer lines can be significantly influenced by plasma-molecule interactions by tens of percent during which significant Molecular Activated Recombination (MAR) is expected.