The features of the global GAM in OH and ECRH plasmas in the T-10 tokamak

Abstract

Zonal flows and their high-frequency counterpart, the geodesic acoustic modes (GAMs) are considered as a possible mechanism of the plasma turbulence self-regulation. In the T-10 tokamak GAMs have been studied by the heavy ion beam probing and multipin Langmuir probes. The wide range of the regimes with Ohmic, on-axis and off-axis electron cyclotron resonance heating (ECRH) were studied (B t = 1.5-2.4 T, I p = 140-300 kA, ne = (0.6-6.0) × 10 19 m −3 , P EC < 1.2 MW). It was shown that GAM has radially homogeneous structure and poloidal m = 0 for potential perturbations. The local theory predicts that f GAM ∼ √ T /m i /R, that means the frequency increases with the decrease of the minor radius. In contrast, the radial distribution of experimental frequency of the plasma potential and density oscillations, associated to GAM, is almost uniform over the whole plasma radius, suggesting the features of the nonlocal (global) eigenmodes. The GAM amplitude in the plasma potential also tends to be uniform along the radius. GAMs are more pronounced during ECRH, when the typical frequencies are seen in the narrow band from 22 to 27 kHz for the main peak and 25-30 kHz for the higher frequency satellite. GAM characteristics and the range of GAM existence are presented as functions of T e , density, magnetic field and P EC .

ECRH effect on the electric potential in toroidal plasmas (Overview of recent T-10 tokamak and TJ-II stellarator results)

“…The results show a consistency of the experimental data with the theoretical prediction for GAM, excited at r=21cm (U=0.7), over the whole operational limit of the machine, in which Te varied by a factor of 4. The NBI-excited Alfven Eigenmodes (AEs) are affected strongly by ECRH: the steady frequency AEs transform to the chirping modes with frequencies up to 400 kHz [8]. A further increase of the PECRH from 0.3MW (one gyrotron on) to 0.6MW (two gyrotrons on) causes a mitigation of the AE intensity, as presented in figure 4.…”

Investigations of the Peripheral Plasma by Langmuir Probes on the T-10 Tokamak

“…Частота ГАМ-колебаний согласуется с из-мерениями, проводимыми с помощью HIBP на периферии плазменного шнура и в его центре [11]. В ряде случаев колебания ГАМ-типа, из-меренные ленгмюровскими зондами на пери-ферии плазменного шнура и с помощью пучка …”

Drift and geodesic effects on the ion sound eigenmode in tokamak plasmas

“…Typically, BAE oscillations are much easier detected by magnetic probes due to large magnetic field component, contrary to geodesic acoustic modes (BAAEs) with [9], which involves acoustic sidebands where , that was detected due to strong NB drive despite of their Landau dissipation for . Fluctuations in the GAM frequency range have been observed in tokamaks with strong NB and/or ICR heating [2,9,[12][13][14]17], as well in tokamaks with ohmic and ECR heating [18][19][20][21][22]. Strong evidence exist that experimentally observed frequencies follow the electron temperature [19] even in presence of plasma turbulence [23] suggesting the important role of geodesic and ion sound effects.…”

“…Many of these observations are generically referred as geodesic oscillations. The experimental values of geodesic oscillations in the inner plasma regions bulk have the frequency [18][19][20][21][22][23] below the theoretically predicted GAMs continuum.…”