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.

Geodesic acoustic modes in magnetic confinement devices

“…In ohmic conditions the E × B shearing rate was less than the turbulence decorrelation rate, but with biasing the sum of both the mean and GAM/ZFO shearing rate together exceeded the turbulence rate and the turbulence was reduced. A similar effect was also observed in SINP, where a roughly linear increase in the GAM-like mode amplitude with applied external electrode positive biasing was found [396].…”

“…Pairs of Doppler reflectometers have also been used on several devices; HL-2A [345,430], AUG [43], DIII-D [282], EAST [42], with both toroidal and poloidal separations showing LRC in the GAM flow perturbation with zero cross-phase shift, consistent with an m = n = 0 structure. Similar results were obtained using a range of other diagnostic techniques: HIBP on JFT-2M [318], BES on DIII-D [435], paired poloidal CR antennas on TEXTOR [337] and between CR at the machine top and Li-beam at the mid-plane [343] (here the slight phase-shifts could be attributed to radial mismatches-consistent with predicted linear phase shift as function of minor radius [277,318]), also using floating potential from LP arrays on HT-7 [285], J-TEXT [390] and SINP [395].…”

“…Behavioural discrepancies where noted in SINP where the GAM amplitude was independent of q edge = 1.5-6 in low density limiter circular plasmas [395]. However, the effect of the T e /T i ratio could be influential here.…”

“…Pairs of DR systems separated toroidally or poloidally have also provided information on the long-range correlation (LRC) properties of the zonal/GAM flow perturbation. DR has been employed on several machines including: AUG [304], DIII-D [280,282], Tore Supra [372], Globus-M [384,386], [395] 2018: effect of plasma biasing [396] STOR-M R/a = 0.46/0.12 m, circular, OH: LPA 2018: GAM-like oscillation suppressed by RMPs [397] TJ-II [417], TUMAN-3M [35,387], FT-2 [428], JET [358], EAST [42,368], HL-2A [430], FT-2 [255] and W7-AS [399].…”

“…Since varying definitions are used in the literature it is necessary to specify also the γ i used. Aside from the three JIPP T-IIU [36,327], SINP [395] and TCV [382] cases where the ions were particularly cold T e T i , a γ i = 1 was preferred. There is also a notable variation in G between ∼1 and √ 2, with lower values appearing for cases where ions were cold or where T i ≈ T e was assumed.…”