Global magnetic fluctuations in S-1 Spheromak plasmas and relaxation toward a minimum-energy state

Globally coherent modes have been observed during formation in the S-1 Spheromak plasma by analysis of magnetic field fluctuations measured from outside the plasma. The modes are of low n number (2 less than or equal to n less than or equal to 5), where n is defined by the functional dependence e/sup in phi/ of the fluctuation on toroidal angle phi. These modes are shown to be related to flux conversion and plasma relaxation toward a minimum-energy state during the spheromak formation. The modes are active while the q profile is rapidly changing, with q on-axis, q/sub 0/, rising to 0.7. A significant finding is the temporal progression through the n = 5, 4, 3, 2; m = 1 mode sequence as q rises through rational fractions m/n. During formation, peak amplitudes of the n = 2, 3, 4 modes relative to the unperturbed field have been observed as high as 20%, while more typical amplitudes are below 5%