Resistive g-modes in a reversed-field pinch plasma

The first direct experimental evidence of high frequency, high toroidal mode number magnetic fluctuations due to unstable resistive interchange modes (g-modes) resonant in the edge region of a RFX-mod reversed-field pinch device is presented. Experimental characterization of time and space periodicities of the modes is provided by means of highly resolved in-vessel edge and insertable magnetic diagnostics. Although the saturated energy spectrum of the measured modes is expected to be highly nonlinear, it is found that the spectral mode properties are in good agreement with the predictions of a simple linear resistive magnetohydrodynamic stability analysis. Also a simple quasi-linear saturation model is proposed to explain the observed mode amplitudes.