A new type of stellarator divertor: the hybrid stellarator divertor

Abstract: A new type of stellarator divertor is found. It has features of both a nonresonant divertor (A. Punjabi and A. H. Boozer, Phys. Plasmas 27, 012503 (2020)) as well as a resonant divertor. It has the outermost confining surface with sharp edges and large islands outside the outermost surface. For this reason, we have called it hybrid divertor. This divertor can be configured by adjusting the currents in external coils which produce nonresonant perturbations. We have simulated this divertor using the method developed in (A. H. Boozer and A. Punjabi, Phys. Plasmas 25, 092520 (2018)). The simulation shows that the footprints have fixed locations on the wall and are stellarator symmetric. The magnetic field lines leave and enter the outermost surface through three magnetic turnstiles. The probability exponents of the three turnstiles are 2.1, 2.25, and 4.3. The hybrid divertor confines larger plasma volume, has higher average shear, larger footprints, lower average density of strike points, lower maximum density of strike points, and longer loss-times than the nonresonant stellarator divertor. The hybrid divertor is robust against small changes in the rotational transform and large changes in the shape parameter that controls the sharp edges on the outermost confining surface.