NONINDUCTIVE CURRENT DRIVE VIA HELICITY INJECTION BY ALFVEN WAVES IN LOW-ASPECT-RATIO TOKAMAKS

A theoretical investigation of radio-frequency (RF) current drive via helicity injection in low aspect ratio tokamaks is carried out. A curr ent-carrying cylindrical plasma surrounded by a helical sheet-current antenna and situated inside a perfectly conducting shell is considered . Toroidal features of low-aspect-ratio tokamaks are simulated by inco rporating the following effects: (i) arbitrarily small aspect ratio, R (0)/a = 1/epsilon; (ii) strongly sheared equilibrium magnetic field; a nd (iii) relatively large poloidal component of the equilibrium magnet ic field. This study concentrates on the Alfven continuum, i.e. the ca se in which the wave frequency satisfies the condition {omega(Alf)(r)} (min) less than or equal to omega less than or equal to {omega(Alf)(r) }(max), where omega(Alf)(r) = omega(Alf) [n(r), B-0(r)] is an eigenfre quency of the shear Alfven wave (SAW). Thus, using low-beta magnetohyd rodynamics, the wave equation with correct boundary (matching) conditi ons is solved, the RF field components are found, and subsequently cur rent drive, power deposition and efficiency are computed. The results of our investigation clearly demonstrate the possibility of generation of RF-driven currents via helicity injection by Alfven waves in low-a spect-ratio tokamaks, in the SAW mode. A special algorithm is develope d that enables one to select the antenna parameters providing optimal current drive efficiency.