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 [1]. Gyrokinetic turbulence can only appreciably modify the rotation on radial length scales of order the ion gyroradius [1,2]. Only in quasisymmetric fields, where collisional particle transport is intrinsically ambipolar, can the plasma rotate freely and then only in the quasisymmetry direction [1]. In particular, sonic rotation velocities are allowed in this case [3]. However, the quasisymmetry is broken when the rotation speed exceeds the diamagnetic speed appreciably, leading to reappearance of the non-intrinsically-ambipolar $1/\nu$-transport regime. Herein, we explicitly evaluate the electron radial particle flux caused by such a quasisymmetry breaking. We find that this flux scales with the fourth power of the rotation Mach number and is therefore expected to be modest in most plasmas of interest.\\[4pt] [1] P. Helander and A. N. Simakov, Phys. Rev. Lett. {\bf 101} 145003 (2008).\\[0pt] [2] P. Helander and A. N. Simakov, Contrib. Plasma Phys. {\bf 50}, 1 (2010).\\[0pt] [3] P. Helander, Phys. Plasmas {\bf 14} 104501 (2007).