Performance assessment of tightly-baffled long-leg divertor geometries in the ARC reactor concept*

Advanced divertor configurations have been proposed as potential solutions to the divertor heat-load problem, including double-nulls, long-legs and magnetic field flaring with secondary X-points. Modelling of tightly-baffled, long-leg divertor geometries in the divertor test tokamak concept ADX has shown the potential to access passively stable, fully detached regimes over a broad range of parameters. To explore how these advanced divertors may perform in a reactor setting, we have performed numerical simulations in UEDGE of these configurations in the context of the ARC reactor (projected power exhaust of 105 MW). Initial studies employing a Super-X Divertor and 0.5% fixed-fraction neon impurity radiation have shown that a passively stable detached regime exists for power exhaust in the range of 80 to 108 MW. Employing an X-point target geometry, without any impurity seeding, detachment extends up to 90 MW exhaust power, and possibly further, when separations between the flux surfaces of the magnetic X-points are small. Simulations are extended to further study the X-point target divertor in ARC, and to explore solution sensitivity to model parameters.