Two Fluid Free-Boundary Tokamak Equilibria with Flows

For the past 25 years, it has been known that charged particles in a plasma will only reside on magnetic flux surfaces in cases of static equilibria. In the general case where mass flows are present, different particle species will instead reside on so called “drift surfaces” (or “flow surfaces”), which deviate from the magnetic surfaces. In particular, for experimentally relevant toroidal flow speeds, these deviations can be on the order of 1cm for a NSTX-like spherical tokamak. In free-boundary calculations, this issue is noteworthy as the behavior of particles near a last closed-flux surface (LCFS) will differ between species. This could affect how or where particles leave the plasma, eventually affecting the location of strike lines or the distribution of particles at divertor targets. In this work, we have adapted the FORTRAN code FLOW2 [1] to, for the first time, calculate two-fluid, free-boundary equilibria with both toroidal and poloidal flows. We investigate the consequences of two-fluid effects on free-boundary equilibria, focusing on the differences between the last closed-magnetic and closed-flow surfaces, and where surfaces in the scrape-off layer intersect the divertor.

[1] L. Guazzotto and R. Betti, Physics of Plasmas 22(9), 092503 (2015).