Double helix (j-tilde x b-tilde) current drive in the RF tokamak device rythmac

Travelling wave fields in m=1, n=4 double helix structures surrounding a toroidal plasma vessel are used to drive toroidal and poloidal plasma currents by means of the non-linear Hall effect technique (j-tilde x b-tilde) in high power experiments of short duration at 330 kHz. The evolution of the magnetic configuration has been studied using magnetic probe data to calculate and plot the poloidal flux function. Contours of equal driven toroidal current extracted from the same probe data reveal negative toroidal current at a small radius and at early times. This observation is consistent with the model presented. The rapidly rising quasi-steady driven toroidal current distributed over the minor section creates a changing magnetic flux and thus a back electromotive force (emf) observable as the loop voltage. This emf drives negative current along the path of minimum impedance. The configuration approaches a steady state where this back emf and negative current die away without any evidence of disruptions. A series of experiments with 40 ms of current drive is planned to study the equilibrium and stability of the configuration on this longer time-scale, beyond the start-up phase studied here. Extension of the (j-tilde x b-tilde) RF current drive technique to an m=+1 and -1 helical mesh antenna which drives purely toroidal current with no poloidal current is presented. The data for this case are contrasted with those for the m=+1 double helix antenna and the m=-1 double helix antenna applied separately, and a theoretical discussion of the drive by this helical mesh antenna is given. (author)