Effect of Drift Driven Fluxes, Magnetic Geometry Bias, and Operational Actuators on Upper/Lower Asymmetries in MAST-U

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. 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, 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.

The SOLPS-ITER predicted upper biased asymmetries [1] are also observed experimentally, within the limitations of the current experimental database of MAST-U.

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.