Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics. Regimes with Ohmic heating and with on-and off-axis ECRH were studied. It was shown that GAMs are mainly the potential oscillations. Typically, the power spectrum of the oscillations has the form of a solitary quasimonochromatic peak with the contrast range 3-5. They are the manifestation of the torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. The frequency of GAMs changes in the region of observation and decreases towards the plasma edge. After ECRH switch-on, the frequency increases, correlating with growth in the electron temperature T e . The frequency of the GAMs depends on the local T e as f GAM ∼ c s /R ∼ T 1/2 e , which is consistent with a theoretical scaling for GAM, where c s is the sound speed within a factor of unity. The GAMs on T-10 are found to have density limit, some magnetic components and an intermittent character. They tend to be more excited near low-q magnetic surfaces.

This report summarizes the results of experimental turbulence investigations carried out at T-10 for more than 10 years. The turbulence characteristics were investigated using correlation reflectometry, multipin Langmuir probe (MLP) and heavy ion beam probe diagnostics. The reflectometry capabilities were analysed using 2D full-wave simulations and verified by direct comparison using a MLP. The ohmic and electron cyclotron resonance heated discharges show the distinct transition from the core turbulence, having complex spectral structure, to the unstructured one in the scrape-off layer. The core turbulence includes 'broad band, quasi-coherent' features, arising due to the excitation of rational surfaces with high poloidal m-numbers, with a low frequency near zero and specific oscillations at 15-30 kHz. All experimentally measured properties of low frequency and high frequency quasi-coherent oscillations are in good agreement with predictions of linear theory for the ion temperature gradient/dissipative trapped electron mode instabilities. Significant local changes in the turbulence characteristics were observed at the edge velocity shear layer and in the core near q = 1 radius after switching off the electron cyclotron resonance heating (ECRH). The local decrease in the electron heat conductivity and decrease in the turbulence level could be evidence of the formation of an electron internal transport barrier. The dynamic behaviour of the core turbulence was also investigated for the case of fast edge cooling and the beginning phase of ECRH.

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 .

Plasma periphery investigation performed in the T-10 tokamak has shown an essential increase of the perpendicular anomalous particle flux in the scrapeoff layer (SOL) with an average plasma density rise. The strengthening of the radial transport is found to occur at an average electron density above a threshold level, which depends on a plasma current I p . The value of the threshold level is about 0.3 times the Greenwald density. Langmuir probe measurements of SOL plasma parameters indicate that intermittent events can play a significant role in the cross-field transport. Intermittent behaviour of the plasma parameters is associated with formation and propagation of the plasma regions (or structures) with high density. The structures move in radial and poloidal directions. Radial movement is predominantly directed to the vacuum vessel wall in the SOL. The radial velocity of the high density plasma structures reduces from 1000 m s −1 near the last closed flux surface to 200 m s −1 at the wall of the vacuum chamber. The radial size of the structures also decreases with minor radius from 3 to 0.5 cm. The poloidal velocity is equal to 1000-1300 m s −1 and is directed towards an ion diamagnetic velocity; the poloidal size of the plasma structures is 2-3 cm. The observed plasma structures can be responsible for more than 50% of the total radial turbulent particle flux. T-10 results support the hypothesis that intermittent convection rather than diffusion can define the cross-field transport.

The velocity of plasma rotation and the potential distributions are measured in the TM-4 device in the Ohmic-heating regime. The potential is negative at the centre of the column, and its magnitude is significantly larger than the ion temperature. At the edge, the potential is positive while the rotation velocities are considerably lower than their neoclassical values.