The internal magnetic structure and current drive in theSPHEX spheromak

Abstract

Steady-state current drive by helicity injection has been demonstrated in the SPHEX spheromak. In order to understand the processes by which plasma toroidal current is driven on the closed flux, it is necessary first to diagnose the internal magnetic field and current structure. A method is described which fits force-free equilibria to magnetic field data from a linear insertable probe. It is shown that the overall field configuration and associated global parameters can be well fitted, though the detailed shape of the internal current profile is sensitive to small errors in the data and the choice of fitting function. The current drive process is closely associated with fluctuations having toroidal mode number n = 1. We investigate in detail shots with Ti gettering, when the n = 1 mode is episodic, showing that the current drive mechanism is switched on and off for periods during a single shot, corresponding to the presence or absence of the mode. This provides definitive evidence that the n = 1 mode is responsible for the current drive.