Analysis of plasma current profile driven by coaxial helicity injection in a low aspect ratio tokamak

The plasma current profile driven by coaxial direct current helicity injection in a low aspect ratio toroidal configuration is investigated by applying the principle of minimum energy dissipation rate. It is shown that current profile modes are mainly determined by the Lagrange multiplier beta. Some critical values beta (c), are found. Different current profiles are obtained in different beta ranges. Three typical current profiles are presented. The key features of the first case with beta < 7.1 agree well with current experiments. Larger driven plasma current and the typical current profiles of normal tokamaks can be obtained in the region of beta from 7.1 to 9.65. There exist reversals of both j phi and B phi in the central part of the plasma when beta becomes higher than 13, 9.65. For a selected geometry the values of beta (c), depend weakly on other parameters. The different beta values and corresponding current profiles can be achieved by adjusting certain parameters. There exist critical values of plasma temperature, bias voltage and vacuum toroidal magnetic field that induce the transformation of the current profile mode. The profile of lambda = mu oj phi /B phi is non-uniform in the plasma and a larger deviation from force free states appears when beta becomes higher.