Resistive effects on helicity-wave current drive generated by Alfvén waves in tokamak plasmas

Abstract

This work is concerned with the investigation of non-ideal (resistive) MHD effects on the excitation of Alfvén waves by externally launched fast-mode waves, in simulated tokamak plasmas; both continuum range, CR ({ωAlf(r)}min <ω< {ωAlf(r)}max) and discrete range, DR, where global Alfvén eigenmodes, GAEs (ω<{ωAlf(r)}min) exist, are considered. (Here, ωAlf(r)≡ωAlf[n(r), B0(r)] is an eigenfrequency of the shear Alfvén wave.) For this, a cylindrical current carrying plasma surrounded by a helical sheet-current antenna and situated inside a perfectly conducting shell is used. Toroidicity effects are simulated by adopting for the axial equilibrium magnetic field component a suitable radial profile; shear and finite relative poloidal magnetic field are properly accounted for. A dielectric tensor appropriate to the physical conditions considered in this paper is derived and presented. When the resistive wave equation is solved and the current drive by helicity injection, IHICD, is calculated, the following illustrative results are found to hold. For CR, (i) the maximum power absorption as well the maximum helicity injection current drive increase significantly with decreasing resistivity (i.e. with increasing temperature); (ii) unlike the power absorption, which is a maximum at a frequency between the lower and the upper edge of the CR, the total current drive is a maximum at the lower edge, and decreases strongly with increasing frequency; (iii) the behaviour of the efficiency closely follows that of the current drive; (iv) the smaller the resistivity, the smaller is the radial distance from the axis (x=0) of the maximum current-drive density. For DR, (i) the maximum power absorption in the discrete GAE case increases with decreasing resistivity even more strongly than in the CR case; (ii) unlike the CR case, the total helicity-injection current has, for almost all GAEs, a symmetric frequency dependence about the line centre; its maximum value as well as the efficiency increase strongly with decreasing resistivity; (iii) unlike the continuum case, the efficiency is almost constant over the entire width of the discrete-mode range; its value increases strongly with the GAE rank.