SPECTRAL DEPENDENCE, EFFICIENCY AND LOCALIZATION OF NONINDUCTIVE CURRENT DRIVE VIA HELICITY INJECTION BY GLOBAL ALFVEN-WAVES IN TOKAMAK PLASMAS

A systematic study of non-inductive current drive via helicity injecti on by global Alfven eigenmode (GAE) waves is carried out. For illustra tion, the first radial mode of the discrete resonant GAE spectrum is c onsidered. The following aspects are given special attention: spectral analysis, radial dependence and efficiency - all of these as function s of the characteristics of the waves launched by an external, concent ric antenna (i.e. wave frequency and poloidal and toroidal wavenumbers ). The tokamak plasma is simulated by a current-carrying cylindrical p lasma column surrounded by a helical sheet current and situated inside a perfectly conducting shell, with incorporation of equilibrium (simu lated) toroidal field, magnetic shear and a relatively large poloidal magnetic field component. Within the framework of low-beta MHD model e quations and for typical tokamak physical parameters, the following ba sic results are obtained: (1) in the range of poloidal wavenumbers -3 less than or equal to m less than or equal to 3 and toroidal wavenumbe rs -20 less than or equal to n less than or equal to 20, resonant GAE peaks below the Alfven continuum are found; (2) the power absorption ( P), current drive (I) and corresponding frequency of the GAE modes dep end strongly on the sets of (m, n) values considered; (3) the 'net' cu rrent drive is positive (i.e. flows in the direction of the equilibriu m current j(0z)) for m = -1, -2, -3 and -20 less than or equal to n le ss than or equal to -1 as well as for m = +1, +2, +3 and n > 10; (4) i n the cases m = -1, -2, -3, the efficiency of current drive, I/P, incr eases with \m\ and 1/\n\; (5) the radial localization of the current d rive in each of the cases considered is determined and tabulated.