EXPERIMENTAL STUDY OF GEODESIC ACOUSTIC MODE IN THE STOR-M TOKAMAK

Abstract

In this experimental study, we used a set of Langmuir probes in order to observe geodesic acoustic mode (GAM) in plasmas created in the STOR-M tokamak. At four locations of STOR-M, we measured floating potential and ion saturation current. Around 9 ms into the discharge, the end of ramp-up region and the beginning of the flat-top region, we found a dominant peak around 30 kHz in the power spectral densities of ion saturation current. The ion saturation current signals showed the sine theta dependence of the oscillation amplitude, which is one of the characteristics of GAM. Furthermore, the cross spectral analysis on the ion saturation current signals suggested the poloidal mode number of m = 1. As for the floating potential signals, while 30 kHz peak was present around 9 ms, the cross spectral analysis indicated the poloidal mode number of m = 1, which is not expected for GAM. In comparison to the theoretical calculations of GAM frequency, the frequency predicted by the fluid model matched with the experimental results. Based on the above findings, it is concluded that the plasmas in the STOR-M tokamak have a signature of GAM.