Tokamak current drive via AC magnetic helicity injection

With AC helicity injection, low frequency modulations of toroidal and/or poloidal magnetic fields produce an effective steady DC toroidal electromotive force which can sustain a DC toroidal current. Both toroidal-poloidal and poloidal-only modulation schemes are discussed. For the toroidal-poloidal scheme, analysis (in both one-dimensional and axisymmetric toroidal geometry) of the effect of the motion of the plasma-vacuum boundary shows that this motion may largely cancel the DC electromotive force. Thus, if toroidal-poloidal helicity injection is to work, there must be (i) no moving boundary or (ii) a less symmetric geometry. A low power toroidal-poloidal experiment (Caltech Encore tokamak) shows a change ΔI in plasma current that is dependent on toroidal-poloidal phasing, but, because of an offset, ΔI is always negative. The poloidal-only scheme is based upon the experimentally observed anomalous skin penetration during the startup phase of tokamaks. Analytic and numerical results indicate feasibility, and scaling of steady state current and power dissipated with frequency and plasma resistivity is presented. This scheme is potentially more efficient in terms of current generated per unit power dissipated than other current drive schemes. It is shown that the resistive MHD activity responsible for anomalous current penetration (and therefore required for the success of this current drive scheme) must be fully three-dimensional. A comparison of the effectiveness of this scheme with that of F-θ pumping in reversed field pinches is presented. (author)