Non-inductive current drive via helicity injection by Alfven waves in low-aspect-ratio tokamaks

摘要

A theoretical investigation of radio-frequency (RF) current drive via helicity injection in low aspect ratio tokamaks is carried out. A current-carrying cylindrical plasma surrounded by a helical sheet-current'antenna and situated inside a per?fectly conducting shell is.considered. Toroidal features of low-aspect-ratio tokamaks are simulated by incorporating the following effects: (i) arbitrarily small aspect ratio, R0/a — l/e; (ii) strongly sheared equilibrium magnetic field; and (iii) relatively large poloidal component of the equilibrium magnetic field. This study eon1 centrates on the Alfven continuum, i.e. the case in which the wave frequency satisfies the condition {wAlf(r)}min≤w≤{wAlf(r)}max. where wAif(r) = wAir [n(r), B0(r)] is an eigenfrequeney of the shear Alfven wave (SAW). Thus, using low-β magneto-hydrodynamics, the wave equation with correct boundary (matching) conditions is solved, the RF field components arc found, and subsequently current 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-aspect-ratio tokamak, in the SAWmode. A special algorithm is developed thai enables one to select the antenna parameters providing optimal current drive efficiency.