Based on experimental observations using the TUMAN-3M and FT-2 tokamaks, and the results of gyrokinetic modeling of the interplay between turbulence and the geodesic acoustic mode (GAM) in these installations, a simple model is proposed for the analysis of the conditions required for L-H transition triggering by a burst of radial electric field oscillations in a tokamak. In the framework of this model, one-dimensional density evolution is considered to be governed by an anomalous diffusion coefficient dependent on radial electric field shear. The radial electric field is taken as the sum of the oscillating term and the quasi-stationary one determined by density and ion temperature gradients through a neoclassical formula. If the oscillating field parameters (amplitude, frequency, etc) are properly adjusted, a transport barrier forms at the plasma periphery and sustains after the oscillations are switched off, manifesting a transition into the high confinement mode with a strong inhomogeneous radial electric field and suppressed transport at the plasma edge. The electric field oscillation parameters required for L-H transition triggering are compared with the GAM parameters observed at the TUMAN-3M (in the discharges with ohmic L-H transition) and FT-2 tokamaks (where no clear L-H transition was observed). It is concluded based on this comparison that the GAM may act as a trigger for the L-H transition, provided that certain conditions for GAM oscillation and tokamak discharge are met.

It is demonstrated that measurements of radial electric currents excited from biased electrodes in TUMAN 3 and other tokamaks provide a crucial test in validating different models for L-H transitions. The results are assessed from the viewpoint of a previously developed theory, which is briefly described. There is evidence from the voltage-current characteristics in TUMAN 3 which corroborates features of this theory. A spontaneously occurring Ohmic H-mode is switched off when a substantial positive biasing voltage is applied to the electrode

Ion cyclotron emission (ICE) observation in neutral beam injection (NBI)-heated plasma in the TUMAN-3M tokamak is reported. Experiments were performed in deuterium or hydrogen target plasmas, with the neutral heating beam consisting of 60% deuterium and 40% hydrogen atoms accelerated up to 16 keV. High-frequency internal magnetic probes were used as a diagnostic tool for ICE detection. In deuterium plasmas, emission with ~13 MHz frequency was observed, with 1–2 ms delay after the NBI pulse front; this frequency corresponds approximately to fundamental ion cyclotron (IC) resonance for hydrogen near the magnetic axis. In hydrogen plasmas, ICE with frequency ~7 MHz was observed. In both cases, the observed frequency scales as IC resonance of minority ions. In deuterium plasmas, the hydrogen minority ICE spectral line was found to consist of several narrow (width ~ 50 kHz) spectral components, typically three or more, with different spacing (of the order of 50–200 kHz), and temporal dynamics in synchronicity with sawtooth oscillations. The ICE with the frequency corresponding to IC resonance for majority ions was also observed in several hydrogen and deuterium discharges. Theoretical models developed for the explanation of NBI ICE on other tokamaks are not easily applicable for the phenomena observed on the TUMAN-3M. In pure ohmically heated deuterium and hydrogen plasmas, i.e. in the absence of fast ions, a weak ICE was also observed, with frequency scaling as IC resonance condition in close proximity to the probe location.

Several types of edge plasma perturbations in the TUMAN-3 tokamak [Proceedings of the 13th International Atomic Energy Agency Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 509] have been demonstrated to trigger the Ohmic H-mode transition. It is shown that three different methods, (1) radial electric field of either sign imposed by an electrode biased up to 500 V, (2) perturbation of the edge plasma density by strong gas puffing, and (3) LiD pellet injection, bring about the Ohmic H mode. In biasing experiments, the degree of improvement of particle and energy confinement depends on the polarity of the electric field and is higher for negative biasing. The evaporation of a LiD pellet (V∼150 m/sec, size ∼0.3 mm, density perturbation∼50%) in the peripheral region of the plasma column can also lead to the H-mode transition. Experimental results are shown to be in reasonable agreement with the theory of radial electric fields in tokamaks.

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.