A study of the influence of plasma-molecule interactions on particle balance during detachment

TL;DR

This study provides experimental evidence that plasma-molecule interactions significantly influence particle and power balance during divertor detachment in a tokamak, affecting diagnostic interpretations and understanding of detachment physics.

Contribution

It offers new experimental insights into the role of plasma-molecule interactions, especially molecular activated recombination and ionisation, in divertor detachment processes.

Findings

Molecular activated recombination is a major ion sink, exceeding electron-ion recombination by about five times.

Plasma-molecule interactions significantly enhance Balmer line emissions near the target during detachment.

These processes may complicate the use of Balmer series lines for divertor diagnostics.

Abstract

In this work we provide experimental insights into the impact of plasma-molecule interactions on the target ion flux decrease during divertor detachment achieved through a core density ramp in the TCV tokamak. Our improved analysis of the hydrogen Balmer series shows that plasma-molecule processes are strongly contributing to the Balmer series intensities and substantially alter the divertor detachment particle balance. We find that Molecular Activated Recombination (MAR) ion sinks from $H_2^+$ and/or $H^-$ are a factor $\sim$ 5 larger than Electron-Ion Recombination (EIR) and are a significant contributor to the observed reduction in the outer divertor ion target flux. Molecular Activated Ionisation (MAI) may also be significant during detachment. Plasma-molecule interactions enhance the Balmer line series emission strongly near the target as detachment proceeds. This indicates enhancements of the Lyman series, potentially affecting power balance in the divertor. As those enhancements vary spatially in the divertor and are different for different transitions, they are expected to result in a separation of the $Ly\beta$ and $Ly\alpha$ emission regions. This may have implications for the treatment and diagnosis of divertor opacity. The demonstrated enhancement of the Balmer series through plasma-molecule processes potentially poses a challenge to using the Balmer series for understanding and diagnosing detachment based only on atom-plasma processes.