Plasma rotation in a quasisymmetric stellarator

The equilibrium plasma rotation in a general toroidal magnetic field is nearly always subsonic and is determined by the requirement that the collisional particle transport should be ambipolar in lowest order in the small-ion-gyroradius expansion. Gyrokinetic turbulence can only appreciably modify the rotation on radial length scales of order the ion gyroradius. Only in quasisymmetric fields, where collisional particle transport is intrinsically ambipolar, can the plasma rotate freely and then only in the quasisymmetry direction. Sonic rotation velocities are allowed in this case. However, the quasisymmetry gets violated when the rotation speed exceeds the diamagnetic speed appreciably, leading to reappearance of the non-ambipolar 1/ν-transport regime. Fortunately, this transport scales with the fourth power of the rotation Mach number and is expected to be modest in most plasmas of interest.