Relaxation of spheromak plasmas toward a minimum-energy state and global magnetic fluctuations.

Globally coherent modes which are observed during formation in the S-1 Spheromak plasma occur during flux conversion and plasma relaxation toward a minimum-energy state, suggesting that these modes provide a means for relaxation. A significant finding is the temporal progression of these modes through a sequence n = 5, 4, 3, 2, m = 1, as q rises through rational fractions m/n, where n and m are defined by the functional dependence e' "~+ ~ of the fluctuations on toroidal angle @ and poloidal angle 8 Co.mparison with theory of the observed modes and the sequence of occurrence suggests that these modes are due to resistive MHD instabilities. A spheromak' is a toroidal magnetic-confinement configuration for plasmas. The toroidal field in the plasma is sustained entirely by poloidal plasma currents, eliminating the need to link the plasma topo-logically. An important observation in spheromak experiments is the tendency for plasmas to relax toward the force-free, minimum-energy Taylor state. In recent S-1 Spheromak experiments, magnetic flux conversion between the poloidal and toroidal fluxes of the plasma was observed during and after formation. This Letter presents the experimental identification and origin of magnetic fluctuations which occur during flux conversion and which, thus, may play a significant role in the relaxation process. Flux conversion has become a topic of strong interest in the general plasma physics community since it has applications also to reversed-field pinches (RFP's) and tokamaks. This phenomenon is important not only for relaxation of these plasmas to a stable minimum-energy state, but also for sustainment. Plasma formation in the S-1 Spheromak device is based on an inductive transfer of magnetic flux from a toroidal "flux core" to the plasma6 (Fig. 1). Currents from the toroidal-and poloidal-field capacitor banks induce poloidal and toroidal plasma currents simultaneously so that the magnetic configuration can be guided toward the Taylor state. Properly detailed programming of the formation process was found not to be essential since plasmas were observed to adjust themselves during formation to a final equilibrium near the Taylor state. The ratio of the toroidal plasma current to the toroidal magnetic flux in the plasma,