High-$\beta $, improved confinement RFP plasmas at high density

In MST discharges with improved confinement, pellet injection has quadrupled the density, with n$_{e}$ reaching 4x10$^{19}$m$^{-3}$. The energy confinement time of these high density plasmas is comparable to that at low density, and $\beta _{tot}$ now reaches 26{\%}. This beta exceeds the Mercier limit for interchange stability, but as yet no indication of interchange is detected experimentally. Beta is also now high enough for global $m$ = 1 tearing modes to be linearly unstable, with the instabilities being driven by the pressure gradient rather than the current gradient. Although a $\beta $ limit has not yet been reached, both the $m$ = 1 fluctuation levels and energy transport do increase modestly at high $\beta $, suggesting the possibility that beta may eventually be limited by pressure-driven tearing. Work supported by USDOE.