Hyper-Resistive Modeling in the Spheromak

Current drive by coaxial helicity injection in the Sustained Spheromak Physics Experiment (SSPX) is modeled by a hyperresistive term in Ohm’s law for discharges in which magnetic fluctuations are small (1–3%). The current on the open magnetic field lines from the applied vacuum bias flux is assumed completely relaxed; interior to the spheromak, the helicity flux balances the ohmic losses. The poloidal area of the spheromak is found to depend on the strength of the hyper-resistive diffusion coefficient, allowing potentially large amplifications of the vacuum flux and discharge current. One discharge is examined in detail; the best fit to the experimental data finds that a limited fraction of the helicity injected into the flux conserver is effectively applied to current drive interior to the spheromak.