JOREK simulations of the X-point radiator formation and its movement in ASDEX Upgrade

TL;DR

This study investigates the formation and dynamics of the X-point radiator (XPR) in ASDEX Upgrade using axisymmetric 2D JOREK simulations that couple a reduced MHD fluid model with full-f kinetic neutrals and nitrogen impurities. The results show that a quasi-stationary XPR can be maintained at a characteristic height (~6.8 cm) with a radiative fraction around 51%, and that varying the impurity seeding can move the XPR upward or downward; extreme seeding leads to a MARFE-like state, while turning off seeding causes the XPR to retreat and be lost. Including impurity-Background collisions localizes the radiating mantle and poloidal impurity distribution, bringing the model closer to experimental observations. The work provides a time-dependent baseline for transitioning to 3D simulations to study XPR–MHD interactions, ELM suppression, and impurity transport in more realistic conditions and under active pumping and molecular processes.

Abstract

Future large-scale magnetic confinement fusion reactors require 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 adjusting the seeding rate and heating power. This contribution presents axisymmetric (2D) simulations of the XPR regime in ASDEX Upgrade using the nonlinear MHD code JOREK extended with a kinetic particle framework for the main species neutrals and nitrogen impurities. With the time-dependent simulations, the progression from attached divertors to a complete detachment with the XPR formation is shown, highlighting the effects of the neutrals and impurities separately. Amidst this progression, the formation and the loss of the high-field-side high-density are observed. After the XPR is well-formed at the height of 6.8 cm, the fuelling 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 and decreasing the seeding rate, the XPR responds by moving upwards and downwards, respectively. 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.

Figures (11)

• Figure 1: (a) the flux-aligned grid with the grid-to-wall extension to the ASDEX Upgrade first wall. The magnetic equilibrium corresponds to AUG discharge #38773 at 3.65 seconds. (b) an enlarged view of the region of interest for the presented XPR simulations, with the blue quadrilaterals marking where the kinetic particles enter the simulation.
• Figure 2: The deuterium fuelling and nitrogen seeding rates through the three simulation phases before reaching the reference time point, with the time window of maintaining a quasi-stationary XPR marked in gray.
• Figure 3: Cross sections of the (a--d) electron density, (e--h) electron temperature with contours at 20, 2 and 1 eV, (i--l) deuterium neutral density, (m--p) impurity density, and (q--t) impurity radiation density at the time points of the four main events (HFSHD formation, partial detachment, complete detachment and XPR formation).
• Figure 4: 1D profiles of the (a) total heat flux and (b) total particle flux onto the simulation boundary at the same time points as figure \ref{['formation-2D']}, with the vertical lines marking the inner and outer strike points. Both the heat and particle fluxes are from the background plasma.
• Figure 5: (a) cross section of the impurity radiation density and (b) the vertical profile above the X-point in the stationary case, with the horizontal lines marking the height of the radiation peak and the upper bound of the radiating mantle.