X-point radiation: From discovery to potential application in a future reactor

Power exhaust is a crucial issue for future fusion reactors. Divertor detachment and the required power dissipation fractions of about 95% are foreseen to be achieved by impurity seeding. In a tokamak, at high seeding levels the radiation often concentrates in a small region inside the confined plasma near the X-point. In early observations the so-called X-point radiator (XPR) often led to back-transitions to L-mode or disruptions. In metal tokamaks or with higher available heating power, these regimes can be stabilized and are now established on AUG, JET, TCV, KSTAR and WEST. The XPR is a cold, dense plasma inside the confined region in the vicinity of the X-point, that breaks the paradigm of poloidal symmetry of density and temperature on closed flux surfaces. On AUG, the poloidal extent of the XPR is a few centimeters and it is observed up to 15 cm above the X-point. The long connection length in this region and the access of neutral particles from the divertor region facilitate the creation of the XPR, as predicted by an analytical model. Numerical simulations with SOLPS-ITER match the observations at AUG and TCV and allow predictions towards a power plant, where a lower impurity concentration is required to trigger an XPR. Since the XPR greatly reduces power and particle fluxes to the targets, simpler and more efficient divertor concepts, such as the compact radiative divertor, can be envisaged for future devices. A scenario with an XPR, however, comes at the cost of an increased impurity concentration and a potential reduction in confinement, which has to be further quantified. The XPR location can be well detected by various diagnostics, enabling responsive real-time control, even through large transients like an LH transition. The active control helped to access a new regime of ELM suppression at AUG, which is now also observed at TCV and JET. The observation of the XPR on multiple tokamaks, the demonstration of its active control, and the emergence of theoretical models that scale favourably towards fusion reactors have opened up a new phase of advanced power exhaust research.