Results of an experimental study of geodesic acoustic modes (GAM) in the TUMAN-3M tokamak are reported. With Doppler backscattering (DBS) the basic properties of the GAM such as frequency, conditions for the GAM existence and the GAM radial location have been identified. The two-frequency Doppler reflectometer system was employed to reveal an interplay between low frequency sheared poloidal rotation, ambient turbulence level and the GAM intensity. Bicoherence analysis of the DBS data evidences the presence of a nonlinear interaction between the GAM and plasma turbulence.

Radial electric field is known to be an important factor affecting transport and confinement in toroidal fusion plasmas. Langmuire probe measurements of peripheral radial electric field evolution in the presence of a rotating MHD island were performed on the TUMAN-3M tokamak in order to clear up the possible connection between the radial electric field and the island rotation, both in L and H-modes. The measurements showed that E r became positive, if the island was large enough, in spite of the constant direction of the island's rotation. Comparing similar ohmic H-mode discharges with or without a rotating MHD island, it was found that in the presence of the large island E r was always more positive. Possible explanations of this observation are discussed.

The concept of geodesic acoustic mode (GAM) was introduced in 1968 by Winsor et al. [1] in order to explain strong low frequency oscillations observed in the Model C stellarator. Since then, similar oscilla tions have been observed in many toroidal installa tions. In recent years, GAMs and other zonal flows (GAMs being a particular kind of these) attracted much attention and have been extensively studied both experimentally and theoretically [2,3] in respect of their influence on the plasma confinement. While not participating directly in the radial transport of energy and particles, GAMs can nevertheless influence the level of turbulence, the intensity of turbulent transport in plasma, and eventually the efficiency of plasma and energy confinement. Interrelation of the background turbulence, GAMs, and confinement regime was demonstrated by experiments on many toroidal devices and, in particular, in recent experiments on the AUG tokamak [4].This Letter presents the results of experiments on the TUMAN 3M tokamak [5] in a regime with the ohmic LH transition for the following parameters of plasma: minor radius, 22 cm; major radius, 52 cm; tor oidal magnetic field, 0.7 T; plasma current, 130 kA; safety factor, 2.5-3.3; average plasma density: from (0.8-0.9) × 10 19 m -3 in L mode to (3-4) × 10 19 m -3 in H mode. Figure 1 shows the temporal variation of some parameters in the H mode. Oscillations of the plasma potential at the GAM frequency were observed at the initial stage of discharge using the heavy ion beam probing (HIBP) techniques [6,7]. In these experi ments, the HIBP measurements were performed using 70 keV K + probing ions. The point of secondary ion ization of the injected ions was situated near the plasma column center and shifted relative to the initial position as a result of the temporal variation of the tor oidal magnetic field: r HIBP ~ 2.7 cm at t = 35 ms;Abstract-The behavior of a geodesic acoustic mode (GAM) in the TUMAN 3M tokamak has been exper imentally studied using the heavy ion beam probing technique. Oscillations of the electric potential under the action of a GAM localized at the plasma periphery have been detected. The GAM was observed in the regime of low confinement (L mode) with low plasma density (~0.8 × 10 19 m -3 ) and disappeared upon the transition to a high confinement regime (H mode). The possible role of GAM as a precursor of the improved confine ment (LH transition) is discussed.

In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, B t = 0.4 T, I p = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100 kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, B t = 3 T, I p = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, B t = 1.0 T, I p = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L-H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field B 1.29 t and plasma current I 1.34 p justifying the B t and I p increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of ∼1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.