Global magnetic fluctuations in spheromak plasmas and relaxation toward a minimum-energy state

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Abstract
Globally coherent modes have been observed during formation in the S‐1 Spheromak plasma [Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, Austria, 1985), Vol. 2, p. 535] by analysis of magnetic field fluctuations measured from outside the plasma. The modes are of low n number (2≤n≤5), where n is defined by the functional dependence einφ of the fluctuation on toroidal angle φ. 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, q0, 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%.