Characterization of geodesic acoustic modes in the ISTTOK edge plasma

Abstract

ISTTOK is equipped with two probe systems that allow the simultaneous measurement of the three-dimensional characteristics of the edge fluctuations with high temporal resolution. Electrostatic fluctuations consistent with the geodesic acoustic mode (GAM) are observed in the edge plasma. The radial, poloidal and toroidal structure of the fluctuations are investigated and good agreement with the GAM theoretical predictions is found. Furthermore, experimental evidence is presented suggesting that the GAM is modulating long-range correlations and the ambient turbulent fluctuations.

Experimental investigation of geodesic acoustic modes on JET using Doppler backscattering

“…As illustrated, GAMs are generally most intense in the edge density gradient region near the pedestal top with a radial extension of about 3 cm, coinciding with the location of the radial electric field (E r ) well. The GAM amplitude is reduced before the separatrix and is undetectable in the scrape-off layer as typically observed in different devices [13][14][15][16][22][23][24]. However, the existence of GAMs in the core plasma cannot be excluded as this region is not accessible to the DBS diagnostic at B T = 3 T except at low densities.…”

“…The intermittent character of GAMs has been reported in different devices [13,14,23,24]. Figure 7 shows the temporal evolution of the perpendicular velocity derived from the DBS master signal bandpass filtered around GAM frequency (f GAM ± 3 kHz).…”

“…as typically observed in different devices [13][14][15][16][22][23][24]. However, the existence of GAMs in the core plasma cannot be excluded as this region is not accessible to the DBS diagnostic at B T = 3 T except at low densities.…”

Plasma Turbulence in the Scrape-off Layer of the ISTTOK Tokamak

“…The goal of the present paper is the study of turbulence properties in a poloidally limited geometry, such as the one of ISTTOK [16,17], a large aspect ratio tokamak (R/a ∼ 5. 4, where R and a are the major and minor radius respectively) with a circular cross section.…”

Geodesic acoustic modes in toroidally rotating tokamaks with an arbitrary β

“…The GAM is basically a kind of electrostatic perturbation with the toroidal and poloidal symmetrical structures (m ¼ n ¼ 0). It has been experimentally observed in many tokamak plasmas [6][7][8][9][10][11] and extensively investigated by theoretical analyses [12][13][14][15][16] as well as numerical simulation. 17,18 As pointed out by Qiu et al, 19 most of the previous theoretical investigations neglect the radial structures of GAMs while the radial inhomogeneity leads to the continuous spectrum of the GAM.…”