Effects of Two-Fluid Flows on Free-Boundary Tokamak Equilibria

As is widely known, in ideal MHD theory particles reside on magnetic flux surfaces, with particles leaving these surfaces only on longer (drift-order) timescales. However, deviations from magnetic surfaces can occur even within the MHD ordering. For the past 25 years it has been known that for non-static, multi-fluid, ideal MHD equilibria, massive particles will instead reside on so called “drift surfaces” (or “flow surfaces”) which deviate from the magnetic surfaces. For example, with experimentally relevant toroidal flow speeds, these deviations have been calculated to be up to 5cm for NSTX-like equilibria. This issue is particularly noteworthy in free-boundary calculations, as particles’ behavior near the LCFS (or any separatrix), will differ between species. This may 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).