Fast drift Alfvén waves excited at the low-frequency band in tokamak plasmas

Abstract

It is shown that fast drift Alfvén waves can be excited in tokamak plasmas by an external antenna operating in the low-frequency band. The dispersion of these waves depends on the derivative of the q-profile or drift terms in the case of low or negative shear. The wave absorption is determined by the Alfvén continuum dissipation and has an oscillating increase with frequency. Typical global Alfvén wave resonances are found at the plasma core for different signs of poloidal/toroidal mode numbers (N∕M<0) for a standard tokamak safety factor profile with central value q0>1.

Kinetic effect on low frequency Alfvén continuum in tokamaks

“…Earlier, it was discovered that the Alfvén eigenmodes ͑like global or discrete Alfvén waves 7 ͒ could be excited below minimum of the Alfvén continuum, which explain the spectrum of some unstable low frequency modes in the tokamaks. The Alfvén continuum equation can be also represented 8 as…”

“…Here, only the first-order corrections related to the drift, Doppler shift, and toroidal terms are kept and cross-corrections are ignored. In this case, the drift and Doppler shift terms should be the same as in perpendicular components of the cylindrical tensor 8 and their evaluation is omitted. The corrections up to the second order of ͑1 / c 2 ͒ are kept in the following equations for the f 1,2 :…”

“…Recently, low frequency global Alfvén waves ͑LFGAWs͒ have been analyzed taking into account drift and Doppler effects 8 in a kinetic approximation. It is shown that LFGAW may be excited below the Alfvén continuum at the rational magnetic surfaces for the q-factor slightly higher than 1.…”

“…The cylindrical part of the tensor ͓Eq. ͑A5͔͒ is used in ideal MHD limit, 8 proved in Ref. 12 to be due to large phase velocity at the rational surfaces.…”

“…The cylindrical coordinate system ͑v Ќ , , v ʈ ͒ is set in velocity space, which is oriented with respect to the magnetic field. The equilibrium Maxwell distribution with the drift corrections 8,10 is proposed for the plasma electrons and ions as follows:…”