GLOBAL ANALYSIS OF RADIOFREQUENCY CURRENT DRIVE

A theoretical analysis of radio-frequency current drive in a tokamak a t frequencies much less than the electron cyclotron frequency is prese nted. The global model discussed includes kinetic hot plasma effects a nd collisions. It is found that all kinds of wave-plasma interactions (resonant wave-electron interaction, resonant wave-ion interaction and collisions) can contribute to the current drive. The analysis present ed discusses the following important current drive components: the mom entum-transfer current and polarization current, the helicity current, the resonant and non-resonant currents, the currents caused by kineti c and collisional effects. The two most important new current drive co mponents are introduced in this paper, the resonant helicity current a nd the collisionless part of the non-resonant current. Helicity curren t drive is proportional to the parallel helicity flux. The helicity co nversion coefficient, expressing the conversion of the parallel helici ty flux into the parallel induced current, is defined by the wave-plas ma interaction. The collisionless part of the non-resonant current is proportional to wave damping on ions. However, this is an electron cur rent, and wave-ion interaction just creates necessary polarization of the wave to drive a current. Circularly polarized waves drive a curren t differently from linearly polarized waves, and wave polarization is an important factor for current drive efficiency. Expressions for all discussed current drive components are derived, and comparison with th e results on helicity current drive obtained by other researchers is g iven.