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Description
The power exhaust problem in the future large-scale fusion reactors necessitates operational regimes that can avoid extreme heat fluxes onto the plasma-facing components. One promising regime is the X-point radiator (XPR), which relies on a highly radiative, cold and dense plasma volume forming above the X-point, and which can be accessed via impurity seeding. Experimentally, the height of the XPR can be controlled by tuning the seeding rate and heating power [M. Bernert et al., NF 2021].
This contribution presents axisymmetric (2D) simulations of the XPR regime in ASDEX Upgrade (AUG) using the nonlinear MHD code JOREK extended with a kinetic particle framework for the main species neutrals and nitrogen impurities. From the time-dependent simulations, the progression from attached divertors to complete detachment with the XPR formation is shown, highlighting the effects of the neutrals and impurities separately.
After the XPR is well-formed at the height of 6.8 cm, the fueling and seeding rates are adjusted so that the XPR remains stationary. From the stationary case, the seeding rate is then changed to see how the XPR location reacts. By increasing the seeding rate, the XPR moves further inside the main plasma, and a MARFE-like (unstable XPR) scenario is eventually achieved. In the core of the MARFE, the electron temperature is reduced to well below 1 eV. On the other hand, by reducing the seeding rate, the XPR moves vertically downwards and is gradually lost. As the XPR retreats, the ionization front shifts from above the impurity radiation peak to below it, confirming the importance of neutral density for the XPR development [U. Stroth et al., NF 2022].
These simulations show JOREK’s capability of simulating time-varying XPR, which will provide a baseline for the transition to 3D simulations, so the MHD activities and their interaction with the XPR can be studied.