Equilibrium Dynamics of Tokamak Plasmas above the Greenwald Density Limit in the Madison Symmetric Torus

Recent experiments in the Madison Symmetric Torus (MST) have demonstrated the capability of sustaining tokamak plasmas with densities up to ten times the Greenwald limit, n_G. [Hurst, et al., accepted to Phys. Rev. Lett.]. Here, we present observations of shifts in plasma equilibrium that occur with increased density, particularly focusing on two phenomena: rapid density oscillations near and just above n_G, and a sudden broadening of electron density and current density profiles near 2 n_G. The capability of sustaining plasmas with n > n_G in MST is due in part to a high-voltage feedback power supply system driving the plasma current and may also be due to a close-fitting conductive shell with resistive wall time 0.8 s. Plasma profiles are obtained from toroidal equilibrium reconstructions using the MSTFit code, which is constrained by magnetic sensors on the plasma-facing wall, an 11-chord far-infrared interferometer and, in some cases, inserted magnetic sensors. With plasma current I_p = 40-55 kA and toroidal field B_T = 0.13 T, dependence of plasma behavior on edge safety factor in the range q(a) = 2.2-3, nearing the regime of most other tokamak experiments, is explored. Efforts to utilize additional diagnostics to further characterize high-density plasmas in MST are presented, including impurity ion spectroscopy for ion temperature and insertable probe measurements to characterize edge turbulence.