Radiative collapse and fluctuations in tokamak plasmas at up to 18 times the Greenwald density limit in the Madison Symmetric Torus

Experiments in the Madison Symmetric Torus (MST) have demonstrated sustained, disruption-free tokamak plasmas with densities far above the Greenwald limit, n_G [Hurst et al., Phys. Rev. Lett. (2024)]. This capability is aided by MST's close-fitting conductive shell with large 0.8 s resistive wall time as well as high-power feedback-controlled power supplies that can sustain a large toroidal loop voltage as the plasma resistance rises. When the electron density, n_e, approaches 2n_G, a distinct equilibrium bifurcation occurs with broadened n_e and nearly flat current density profiles, characteristics resulting from radiative collapse. This happens to occur near the Sudo limit, n_S, an empirical density limit observed in stellarators that is attributed to radiative collapse and depends on input power. We study fluctuation power spectra from an 11-chord far-infrared interferometer, edge-measured magnetics, and internal Langmuir probes across the range of Greenwald fraction that has been accessed in MST tokamak plasmas, 0.5 < n_e/n_G < 18. We focus on plasmas near n_G and n_S where changes are observed, particularly an increase in high frequency magnetic fluctuations above n_S. Experimental results are compared to several different theoretical models of the Greenwald density limit, including those involving enhanced edge turbulence or radiation-destabilized tearing modes.