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 β. Some critical values βc are found. Different current profiles are obtained in different β ranges. Three typical current profiles are presented. The key features of the first case with β< 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 β from 7.1 to 9.65. There exist reversals of both jφ and Bφ in the central part of the plasma when β becomes higher than βc ≈ 9.65. For a selected geometry the values of βc depend weakly on other parameters. The different β 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 λ = μ0jφ/Bφ is non-uniform in the plasma and a larger deviation from force free states appears when β becomes higher.
Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing