Observing the evolution of self-organized helical states in the Madison Symmetric Torus

Magnetically confined plasmas in the Madison Symmetric Torus (MST) reversed-field pinch spontaneously organize into helical equilibria under conditions of at high plasma current and/or low density. This occurs when the inner-most- resonant tearing mode grows to large enough amplitude ($\sim 7\%$ of the equilibrium field strength) that the associated island envelopes the magnetic axis. A suite of x-ray diagnostics, including a new solid-state multi-purpose multi-energy soft- x-ray detector, a hard- x-ray detector, and a two-color diode-based tomographic array are used to study evolution of this quasi-single helicity (QSH) state. Evolution of temperature and impurity density profiles are inferred using an integrated data analysis procedure. Residual tearing mode activity is observed to resume during the quasi-stationary period and is correlated with intermittency in the thermal confinement. These observations are compared with the predictions of a predator-prey model by Terry, \emph{et al.} which proposes that strong magnetic or flow shear suppresses energy transfer between tearing modes, significantly extending the lifetime of the QSH state. Work supported by US Department of Energy.