Helicity injection in the FACT spheromak experiment

A spheromak configuration consisting of bias flux surrounding a core region of closed flux surfaces has been successfully sustained in the Flux Amplification Compact Torus (FACT) device by DC/Coaxial helicity injection. In this experiment, the energy transfer efficiency is estimated to be about 30%. The relaxed configuration possesses a low q profile (1/3<q<1/2) whose shape implies that the current density is concentrated in the core and which is maintained by the process of MHD relaxation. The current conversion and rapid inward diffusion of the injected current is found to be significantly related to the n=1 helical deformation of the open field lines along the geometric axis. In this paper, we present some design parameters for the planned Helicity Injected Spherical Torus (HIST) which will permit a corresponding investigation to the above to be made for a tokamak. 5 refs., 4 figs., 1 tab