Scoping studies of plasma detachment in long-leg divertor geometries

A new class of magnetic divertor geometries with long poloidal legs have been proposed as a solution to handle the extreme power exhaust projected for future tokamak reactors. At present, it is not clear how the existing understanding of the underlying physics of the detached divertor regime will translate to these new geometries. To assess the physics of plasma detachment in these newly-proposed configurations, scoping studies of tokamak plasmas in a long-leg divertor geometry are performed using 2D plasma edge transport codes. The model includes a standard leg divertor at the inner target and a narrow, tightly-baffled long poloidal leg divertor at the outer target. A scan of plasma parameters is generated by using a so-called “closed gas box” particle model and increasing total particle count with fixed input power to assess the physics of the transition to detachment in the long-leg regime. Simulations of pure deuterium plasmas show that the long-legged outer divertor plasma demonstrate onset of detachment in an isolated flux tube at similar criterion to that of standard divertors [S. I. Krasheninnikov, A. Kukushkin, A. Pshenov, Phys. Plasmas 23, 055602 (2016)]. Strong ionization at the outer divertor target and an increase in flux to the side walls of the long divertor leg are also observed.