Magnetic flux conversion and relaxation toward a minimum-energy state in spheromak plasmas

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Abstract
S‐1 spheromak [Plasma Physics and Controlled Nuclear Fusion, 1984 (IAEA, Vienna, Austria, 1985), Vol. 2, p. 535] currents and magnetic fluxes have been measured with Rogowski coils and flux loops external to the plasma. Toroidal plasma currents up to 350 kA and spheromak configuration lifetimes over 1.0 msec have been achieved at moderate power levels. The plasma formation in the S‐1 spheromak device is based on an inductive transfer of poloidal and toroidal magnetic flux from a toroidal ‘‘flux core’’ to the plasma. Formation is programmed to guide the configuration into a force‐free, minimum‐energy Taylor state. Properly detailed programming of the formation process is found not to be essential since plasmas adjust themselves during formation to a final equilibrium near the Taylor state. After formation, if the plasma evolves away from the stable state, then distinct relaxation oscillation events occur that restore the configuration to that stable state. The relaxation process involves reconnection of magnetic field lines, and conversion of poloidal to toroidal magnetic flux (and vice versa) has been observed and documented. The scaling of toroidal plasma current and toroidal magnetic flux in the plasma with externally applied currents is consistent with the establishment of a Taylor state after formation. In addition, the global magnetic helicity in the plasma is proportional to that injected from the flux core, independent of how that helicity is generated.

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