On plasma rotation induced by traveling fast Alfvén waves

Abstract

Absorption of fast Alfvén waves by the minority fundamental ion–cyclotron resonance, coupled with finite banana width physics, generates torque distributions, and ultimately rotational shear layers in the bulk plasma, even when the toroidal wave number kφ=n/R of the fast wave vanishes (n=0) and cyclotron absorption introduces no angular momentum nor canonical angular momentum [F. W. Perkins, R. B. White, P. T. Bonoli, and V. S. Chan, Phys. Plasmas 8, 2181 (2001)]. The present work extends these results to traveling waves with nonzero n where heating directly introduces angular momentum. Since tokamak fast-wave antennas have approximately one wavelength per toroidal field coil, the toroidal mode number n lies in the range n=10–30, independent of machine size. A zero-dimensional analysis shows that the rotation rate arising from direct torque is comparable to that of the rotational shear layer and has the same scaling. Nondimensional rotation profiles for n=(−10, 10) show modest changes from the n=0 case in the expected direction. For a balanced antenna spectrum, the nondimensional rotational profile (averaged over n=−10, 10) lies quite close to the n=0 profile.