Stability studies and the origin of the n=1 mode in the SPHEX spheromak experiment

Oscillations with toroidal mode number n=1 are ubiquitous in helicity injected spheromaks and spherical tokamaks, and play a crucial role in current drive. It has been proposed that these arise from a current driven instability of the open flux tube. Stability calculations are presented to confirm this, and they are compared with experimental data from the Spheromak Experiment (SPHEX) [M. Rusbridge , Plasma Phys. Control. Fusion 39, 683 (1997)]. The equilibria are modelled as piece-wise constant mu profile force-free plasmas with different values for the mu in the open (mu(c)) and closed (mu(a)) flux regions. A stability map in mu(c),mu(a) space is then calculated. The SPHEX experimental data is also reduced to the same space both as a culmination of direct single point measurements of mu and as a time history of the reconstructed equilibrium from a particular shot. The results show a favorable comparison of the stability map with experiment, both in magnitude and shape. The effect of inserting a central current-carrying rod on the stability is also discussed. (C) 1999 American Institute of Physics. [S1070- 664X(99)00811-3].