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Improved operation of the SSPX spheromak

We present significant advances in the performance and understanding of the sustained spheromak physics experiment (SSPX). High performance discharges are obtained by adjusting the injected current and bias magnetic flux to produce a flat to slightly peaked radial current profile, consistent with reducing the drive for tearing and other MHD modes. Wall conditioning and density control are also essential for obtaining high performance discharges. With these optimizations, plasmas with peak electron temperatures of ∼350 eV, edge magnetic fluctuation levels less than 1% and core thermal diffusivities (χe) <10 m2 s−1 are produced. Most SSPX temperature data are bounded by βe ∼ 5% but differentiating between a stability limit and simple energy transport, until now, has not been possible. Achieving these high performance discharges enables us to consider the possibly of auxiliary heating of a spheromak for the first time.