Equilibrium reconstructions of tokamak and RFP plasmas above the Greenwald density limit in MST

Recent experiments in the Madison Symmetric Torus (MST) have demonstrated the capability of sustaining tokamak and reversed-field pinch (RFP) plasmas with density above the Greenwald limit, n_G. Here, we present toroidal equilibrium reconstructions of these plasmas using the MSTFit code, constrained by line-integrated electron density measurements from an 11-chord far-infrared interferometer and, in some cases, insertable probe measurements of the magnetic field. The capability of sustaining plasmas with n > n_G is likely due in part to a high-voltage feedback power supply system driving the plasma current and a close-fitting conductive shell with resistive wall time 0.8 s. In the tokamak configuration with toroidal field B_T = 0.13 T, variations in density and toroidal current profiles are studied for central density ranging up to 10 n_G, with a focus on sudden broadening of the profiles near 2 n_G, which persists to higher densities. In the RFP configuration with plasma current I_p ≤ 150 kA, the maximum density is not precisely known but is thought to be around 2 n_G. Efforts to resolve this uncertainty are discussed, as well as plans for further diagnosis of high-density plasmas in MST.