Modeling of deuterium and carbon radiation transport in MAST-U tokamak advanced divertors

Abstract

Modest effects of deuterium and carbon radiation opacity in the Super- X and snowflake divertor plasmas are predicted for MAST Upgrade tokamak with core plasma input power 2.5-5 MW and plasma current 1 MA. The radiation transport modeling is based on the SOLPS-EIRENE and UEDGE code divertor plasma predictions. Two radiation transport models are used: one is based on a full radiation transport equation implemented in the radiation transport and collisional- radiative code CRETIN (without feedback on the background plasma), and another is an internal self-consistent UEDGE model with ionization, recombination, and heating rates corrected for Lyα line trapping based on the escape probability model implemented in CRETIN. In MAST-U, the Super-X and snowflake divertor plasmas are predicted to reach detached regimes at lower upstream densities than the standard divertor, and the conclusion still holds with radiation transport effects included. At neutral densities ≤ 1020 m−3, modest Lyα deuterium line trapping with optical depths 10-15 is predicted in the Super-X divertor. Divertor plasmas are optically thin to other Lyman and Balmer lines, as well as to strong C III and C IV lines that are responsible for most of divertor radiated power. Insignificant changes (within a few percent) to divertor deuterium ionization and recombination rates are found. Radiation fluxes on outer divertor target are modified within a factor of 2-3 when the radiation transport is accounted for, and a similar variation is found due to the line shape models that define the absorption and emission line profiles in the radiation transport modeling. The predicted Lyman and Balmer spectral intensities are significantly modified due to radiation trapping. A measurement of divertor radiation transport effects is discussed using the Lyβ /Baα line ratio. In the snowflake divertor configuration, divertor plasmas are found to be optically thin to Lyman series lines within a large range of parameter variations that include upstream density, divertor transport coefficients, and magnetic configurations. Modest radiation transport effects are only found in a few cases with strongest divertor transport and magnetic configurations closest to the ideal snowflake configuration, however, the plasma background models that were used are yet to be validated with an experiment.

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

“…If the plasma is not optically thin, it could greatly impact the analysis results, as explained in [21,22], and could alter both the reaction rates as well as the emission coefficients [24,67]. Although photon opacity may occur in MAST-U conditions [3,68], it is expected to occur at much higher powers (P sep = 2.5-5 MW compared to 0.5 MW (this work)) and electron densities (n e > 10 20 m −3 compared to n e ∼ 10 19 m −3 in this work) [68]. However, this will require further monitoring in the future.…”

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

“…If the plasma is not optically thin, it could greatly impact the analysis results, as explained in [14,15], and could alter both the reaction rates as well as the emission coefficients [43,44]. Although photon opacity may occur in MAST-U conditions [3,45], it is expected to occur at much higher powers and electron densities [45]. However, this will require further monitoring in the future.…”

Investigations of atomic and molecular processes of NBI-heated discharges in the MAST Upgrade Super-X divertor with implications for reactors

“…It is assumed that the plasma is optically thin in all presented atomic emission analyses (see further discussion in section 4.3.3). For the MAST-U cases studied, the amount of photon opacity is expected to be negligible, as the divertor densities and power levels obtained are significantly lower than required for conditions in which photon opacity strongly impacts hydrogenic emission, according to CRETIN modelling [37]. Ion source and sink profiles, obtained through spectroscopic analysis [11,26], are shown in figure 2 as detachment progresses during a line-averaged core density scan.…”