Magnetohydrodynamic simulations of edge poloidal flows

Edge poloidal flows exceeding the poloidal sound speed lead to the formation of a pedestal structure [GUAZZOTTO, L. and BETTI, R., Phys. Rev. Lett. 107 (2011) 125002]. This result is based on the existence of “transonic” equilibria, in which the edge region of the plasma flows supersonically with respect to the poloidal sound speed (i.e., the sound speed reduced by a factor Bθ/B), while the plasma core is rotating with subsonic poloidal velocities. The ideal MHD equilibrium force balance shows that radial discontinuities must be present at equilibrium in the presence of transonic flows. The formation of the transonic discontinuity was proven with time-dependent simulations. In the present work, we prove that the transonic discontinuity can be formed with poloidal velocities no larger than a few 10s of km/s. Such relatively slow velocities are supersonic at the bottom of the pedestal where the temperature is a few tens of eVs. We also show how realistic toroidal velocity profiles can be obtained in transonic equilibria if the appropriate choice is made for the input free