Structure of the n=1 mode responsible for relaxation and current drive during sustainment of the SPHEX spheromak

The structure of the n = 1 mode in the SPHEX spheromak, which plays a central role in relaxation during sustainment, is investigated by analysing the measured voltage fluctuations in the central plasma column. By combining these results with a suitably defined helical magnetic Bur function, the mode is found to be due to a rotating helical distortion of the open linked flux. We propose that the distortion is due to a saturated current-driven kink mode of the open flux tube. The prolongation of this 'helical column' an its return around the outside of the closed flux is found to be strongly asymmetric. Previously published measurements of the Poynting Bur and mu-profile are re-analysed in the light of these results, and implications for the mechanism of relaxation and non-inductive current drive are discussed.