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Description
MAST Upgrade experimental infrared thermography and Langmuir probe data have been used to study the upper to lower outer targets asymmetries, especially the ratio of the peak of the heat flux densities, for a variety of experimental discharges. SOLPS-ITER simulations with a wide range of actuator conditions and magnetic configurations (Conventional, Elongated, and Super-X; combined with Connected and Disconnected Double Null) and with all drifts and currents activated have been used to separate the relative importance on up/down asymmetries caused by the magnetic bias, drift driven fluxes, and actuator asymmetries. In fully symmetrical connected double null simulations in the Super-X divertor configuration, the activation of the drifts in SOLPS-ITER drives an upper biased asymmetry of the peak of the heat flux density to the outer targets (up to a factor of 15) that is well ordered by the difference of the upper to lower electron target temperatures [1]. The predicted upper biased asymmetries are observed experimentally, with the cause from the simulations attributed to thermoelectric currents. However, operational correlation introduced by the standard MAST-U discharge program between the inter separatrices distance and the global plasma density/temperature complicates the separate interpretation of the role of the magnetic geometry bias and the drift driven fluxes, as both are expected to constructively cause an upper bias asymmetry of the ratio of the heat flux densities. To address this issue, new experiments will be carried out in MU05 with the aim of breaking these operational correlations.
For simulations with Conventional divertor geometry the asymmetry is weaker (up to a factor of 8) and depends on the gas puff strength. The less efficiently dissipative nature of the open CD allows upper and lower divertors to be in the same collisionality regime, whereas the lower outer divertor of the closed SXD simulations is always detached. The new MU05 data will help to validate the relative importance of drift-driven fluxes and magnetic geometry bias predicted by the different sets of SOLPS-ITER simulations. Understanding these asymmetries is important to develop control strategies for spreading power evenly among divertors, as all four divertors need to simultaneously have acceptable conditions.
This work is supported by the US DOE under contract DE-AC05-00OR22725.
This work was partly funded by the EPSRC Energy Programme (Grant No. EP/W006839/1)
[1] I. Paradela Perez et al, Nucl. Fusion, 2025