The internal magnetic structure and current drive in the SPHEX spheromak

Steady-state current drive by helicity injection has been demonstrated in t he SPHEX spheromak. In order to understand the processes by which plasma to roidal current is driven on the closed flux, it is necessary first to diagn ose the internal magnetic held and current structure. A method is described which fits force-free equilibria to magnetic held data from a linear inser table probe. It is shown that the overall field configuration and associate d global parameters can be well fitted, though the detailed shape of the in ternal current profile is sensitive to small errors in the data and the cho ice of fitting function. The current drive process is closely associated wi th 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 driv e.