Observation of geodesic acoustic mode in SINP-tokamak and its behaviour with varying edge safety factor

Abstract

The spectral analysis of floating potential fluctuations measured in the edge plasma region (0.87 < r/a < 1.0) of Saha Institute for Nuclear Physics tokamak (SINP-tokamak) using Langmuir probes reveals the existence of a highly coherent mode with a frequency in the range of 15–21 kHz. Long range correlations in poloidal and toroidal directions are observed over a wide range of plasma discharges having different values of the edge safety factor, from very low qedge (<2) to high qedge (>3). These coherent modes are simultaneously observed in density and radial electric field fluctuation spectra as well. These coherent modes are identified as geodesic acoustic modes (GAMs) having different characteristics over the entire qedge range. In discharges with qedge greater than 3, the local wave number spectra of the mode exhibit the properties of continuum GAM with the observed poloidal and toroidal mode numbers of m ∼ 0 and n ∼ 0, and the mode is radially localized. The observed frequency and its variation with the safety factor for qedge > 3 closely agree with the theoretical predictions using the measured values of temperature. In contrast, for qedge < 3.0, the GAM nature changes from continuum to the Eigenmode as the associated GAM frequency remained uniform at ∼13–17 kHz over the q edge range of 1.5 to 3 and ceased to depend on local temperature. Furthermore, the poloidal wave number of the coherent mode no longer remains zero and is observed to increase when qedge falls below 2.5. Coherent modes in magnetic fluctuations having similar frequencies to those of electrostatic fluctuations are also observed in the discharges with the q edge below 2.5. The coupling of these electrostatic and magnetic modes may be responsible for triggering the Eigenmode GAM.