Speaker
Description
The inclusion of hydrogenic Lyman line radiation absorption in drift-enabled SOLPS-ITER [1] modeling of JET-ILW L-mode hydrogen and deuterium discharges [2,3] resulted in improved predictions of the divertor target particle fluxes by 30% and higher heat fluxes at the onset of detachment. Lyman line absorption changes the plasma density distribution in the divertor volume in detachment and is expected to significantly impact the prediction and interpretation of spectroscopy signals critical for detachment control in ITER and DEMO. The divertor volume of JET-ILW is found to be highly opaque to Lyman-α and Lyman-β photons, up to ∼ 90% and ∼ 79%, respectively, in deep detachment. Photo-induced ionization occurs when Lyman line absorption populates higher excitation levels of hydrogen and deuterium atoms, thereby reducing the electron energy threshold for ionization. The proportion of photo-induced ionization to the total ionization rate of the scrape-off layer (SOL) increases with opacity, thereby reducing the effective ionization cost as the divertor plasma is increasingly detached. The lowering of the ionization cost prevents early onset of radiation condensation instability and allows stable SOLPS-ITER simulations at higher plasma edge densities. The decreasing ionization costs allow for further increase of ionization rate and therefore electron density, thus shifting the particle balance from power-limited to a highly recombinating divertor.
Synthetic Balmer-α diagnostic signals are produced from the highly recombinating divertor solution using the PESDT-Cherab [4] raytracing code to include surface reflections and Lymanβ absorption, improving agreement by a factor of 3 with the JET-ILW Balmer-α measurements compared to Lyman transparent solutions. The agreement in divertor target particle flux between SOLPS-ITER and Langmuir probe measurements in JET-ILW L-mode hydrogenic plasmas is further improved by reducing the uncertainties in two areas: the SOL radial transport parameters by using the SOLPS-ITER wide-grids version [5], and a more updated data on the hydrogenic molecular breakup rates for deuterium. PESDT-Cherab is extended to include Lyman line opacities, allowing the production of consistent synthetic Lyman line diagnostic signals useful for comparison with VUV measurements in existing and future devices.
References
[1] X. Bonnin et al., Plasma and Fusion Research 11:1403102–1403102 (2016)
[2] D. Reiter et al., Plasma Physics and Controlled Fusion 44, 8:1723–37 (2002)
[3] R. Chandra et al., Nuclear Materials and Energy 41, 101794 (2024)
[4] B. Lomanowski et al., Nuclear Materials and Energy 20, 100676 (2019)
[5] W. Dekeyser et al., Nuclear Materials and Energy 27, 100999 (2021)