RESISTIVE EFFECTS ON HELICITY-WAVE CURRENT DRIVE GENERATED BY ALFVEN WAVES IN TOKAMAK PLASMAS

This work is concerned with the investigation of non-ideal (resistive) MHD effects on the excitation of Alfven waves by externally launched fast-mode waves, in simulated tokamak plasmas; both continuum range, C R ({omega(Alf)(r)}(min) < omega < {omega(Alf)(r)}(max) and discrete ra nge, DR, where global Alfven eigenmodes, GAEs (omega < {omega(Alf)(r)} (min)) exist, are considered. (Here, omega(Alf)(r) = omega(Alf)[n(r), B-0(r)] is an eigenfrequency of the shear Alfven wave.) For this, a cy lindrical current carrying plasma surrounded by a helical sheet-curren t antenna and situated inside a, perfectly conducting shell is used. T oroidicity effects are simulated by adopting for the axial equilibrium magnetic field component a suitable radial profile; sheer and finite relative poloidal magnetic field are properly accounted for. A dielect ric tensor appropriate to the physical conditions considered in this p aper is derived and presented. When the resistive wave equation is sol ved and the current drive by helicity injection, I-CD(HI), is calculat ed, the following illustrative results are found to hold. For CR, (i) the maximum power absorption as well the maximum helicity injection cu rrent drive increase significantly with decreasing resistivity (i.e. w ith increasing temperature); (ii) unlike tile n power absorption, whic h is a maximum at a frequency between tile lower and the upper edge of the CR, the total current drive is a maximum at the lower edge, and d ecreases strongly with increasing frequency; (iii) the behaviour of th e efficiency closely follows that of the current drive; (iv) the small er the resistivity, the smaller is the radial distance from tile axis (x = 0) of the maximum current-drive density For DR, (i) the maximum p ower absorption in the discrete GAE case increases with decreasing res istivity even more strongly than in the CR case; (ii) unlike the CR ca se, the total helicity-injection current has, for almost all GAEs; a s ymmetric frequency dependence about the line centre; its maximum value as well as the efficiency increase strongly with decreasing resistivi ty; (iii) unlike the continuum case, tile efficiency is almost constan t over the entire width of the discrete-mode range; its value increase s strongly with the GAE rank.