Parametric dependence and control of 3D helical structures in MST

MST plasmas with a central 3D helical structure are now routinely achievable. As in RFX-mod, the stellarator-like structure arises in otherwise toroidally axisymmetric plasmas when the innermost-resonant m = 1 tearing mode grows to large amplitude and dominates the core-resonant-mode spectrum. The tearing mode spectrum in MST varies strongly with toroidal plasma current. Peaked spectra with a helical structure occur most reliably at high Ip (0.6 MA), while at low Ip (0.2 MA), the spectra are quite flat. This trend is like that in RFX-mod and may reflect a dependence on the Lundquist number. Preliminary MST data also show that manipulation of the inductive electric field has a significant impact, driving the dominant mode to large amplitude more robustly, and reducing the other modes. At high Ip, Te(0) $>$ 1 keV with a possible several-fold increase in the energy confinement time. Supported by USDOE.