Understanding Detachment Processes in DIII-D via Bayesian Analysis of Balmer Emissions

Experiments at DIII-D were conducted to improve understanding of power and particle balance during plasma detachment in well detached L-mode discharges with an open divertor. Electron-ion excitation (EIE) dominates over molecular-assisted recombination (MAR) and electron-ion recombination (EIR) at both inner and outer targets. Contributions of atomic and molecular processes were systematically analyzed using a Bayesian probabilistic approach, treating measured Balmer emissions as likelihood parameters. Since Balmer line ratios are strong indicators of detachment (Te < 1.5 eV), they serve as diagnostic tools to probe detachment physics macroscopically.

To investigate 1D emission profiles, poloidal strike-point sweeping was performed, and repeated for different Greenwald fractions , f_GW ~ 35%, 40%, and 45%. This study uses extensive spectroscopic measurements of and , and also high-n Balmer emissions using a high resolution Multi-Chordal Spectrometer (MDS) and filterscope, with views covering the complete lower divertor. Modeling employs a Bayesian-based probabilistic approach [1-2]. EIE, MAR, and EIR predictions are based on atomic and molecular photon emissivity coefficients (PECs) for set constrained parameter ranges. These findings are critical for predictive divertor modeling and heat flux management in future high-density devices.

References:

[1] Verhaegh K, et al. Nucl. Fusion 63 016014

[3] Verhaegh K H A Spectroscopic investigations of detachment on TCV 2019