Pellet-fueled, high-$\beta $, improved confinement RFP plasmas

Pellet fueling of improved confinement MST plasmas has resulted in a four-fold increase in the density, reaching 4x10$^{19}$ m$^{-3}$, and a total beta of up to 26{\%}.~ At high beta, the Mercier limit for interchange stability is exceeded, but there is as yet no experimental evidence of interchange modes.~ The linear stability threshold for pressure-driven tearing modes is also exceeded, and one does observe increased tearing mode amplitudes.~ At high density, the energy confinement time is still improved, but the degree of improvement is reduced.~ This may be due to the tearing modes, suggesting that pressure-driven tearing could ultimately limit the achievable beta in the RFP.~ MST's pellet injector has now been upgraded to accommodate pellets with roughly twice the particle inventory used to achieve the above results.~ Tests with these larger pellets are underway.~ Work supported by U.S.D.O.E.