Variations in fluctuation activity below and above the Greenwald limit in Madison Symmetric Torus tokamak plasmas

The Madison Symmetric Torus (MST) is capable of creating and sustaining tokamak plasmas of a high normalized density, up to 18 times the Greenwald limit (n_G). When the electron density exceeds 2n_G, the current density profile is observed to flatten. This is likely due in part to radiative collapse, which causes global cooling. Another potential reason for this loss of energy and profile flattening is turbulence driven by resistive interchange instability. We used an 11-chord interferometer, internal Langmuir probes, and edge magnetic coils to study a set of plasmas over a range of densities from below 1 n_G to above 2 n_G. We compared fourier and wavelet transforms to study fluctuations with frequency in the range 1 kHz to 50 kHz to identify changes in fluctuation activity over time. The wavelet analysis reveals a burst of density fluctuations during startup for plasmas with densities above 1n_G which does not appear below the limit. And, following current-profile flattening events, all frequencies of the magnetic field data showed an increase in amplitude. These studies aim to determine some of the underlying mechanisms of the Greenwald limit and understand the behavior of plasmas when the limit is surpassed.