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.

The isotope effect in tokamak anomalous transport of energy and particles is a longstanding puzzle for physicists. It was first reported almost 30 years ago and since that time observed in many machines. Already in the TFTR [1] it was shown that coming from hydrogen to deuterium and then to the mixture of deuterium and tritium experimentalists improved the energy transport, which goes down for the heavy isotopes. This effect is promising for fusion applications, but the reason why it happens is still unclear. The typical orbit's widths for circulation of particles in a tokamak go in the opposite direction: the Larmor radius and the banana orbit are larger for heavy isotopes. In terms of turbulent transport, the typical width of the drift-wave turbulent eddy scales like an ion Larmor radius, and therefore for heavier isotopes larger eddies are predicted.Based on these arguments one could expect growing transport with increasing isotope mass, nevertheless, in numerous experiments an opposite direction of effect was observed [2,3]. In addition, it was shown that the isotope effect is much stronger in tokamaks compared to stellarators.Recently it was proposed that the ion mass dependence of the multi-scale turbulence component with long-range correlation could be responsible for the isotope effect in tokamak anomalous transport [4]. Geodesic acoustic mode (GAM), the finite frequency zonal flow, as a global mode, possesses long correlation length in poloidal and toroidal directions and is often reported to be more intensive in deuterium compared to hydrogen discharges [4][5][6][7]. Moreover it is routinely observed in tokamaks, but only rarely reported in stellarators due to its strong Landau damping there. The latter makes the GAM a possible element in explanation of the transport isotope effect.

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Irzak, M. A.; Niskala, P.; Esipov, L. A.; Kiviniemi, T. P.; Leerink, S. Fast synthetic X-mode Doppler reflectometry diagnostics for the full-f global gyrokinetic modeling of the FT-2 tokamak S. (2018). Fast synthetic X-mode Doppler reflectometry diagnostics for the full-f global gyrokinetic modeling of the FT-2 tokamak.

The elementary process of turbulence control by Geodesic Acoustic Modes (GAMs) leading to modulation of its level at the GAM frequency is for the first time supported by experimental observations at the FT-2 tokamak. The modulation effect is confirmed by the global total-distribution-function gyrokinetic modelling of the tokamak discharge, predicting strong modulation of the electron thermal diffusivity induced by GAMs, which propagates inward and possesses the GAM temporal and spatial structure.

In the experiments on tangential fuel pellet injection in the TUMAN-3M tokamak an initiation of LH-transition or, in several scenarios—temporal (1–2 ms) confinement improvement with the following backwards transition was observed. To understand the possibility of the transitions, a model calculating the evolution of density and ion temperature profiles under the effect of source profile perturbation and plasma cooling created by pellet evaporation was developed. In the model, a diffusion coefficient depending on radial electric field shear value was used. Turbulence parameters are defined using a gyrokinetic simulation of the experiments with ELMFIRE code. Modeling results are in good agreement with experiments. Using the data obtained from the modeling, non-linear particle flux dependency on density gradient was analyzed; the existence of two stabile solutions for two confinement modes dependent on particle flux value was proved, in agreement with experiments.

Two versions of the X-mode Doppler reflectometry (DR) synthetic diagnostics are developed within the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. In the 'fast' version the DR signal is computed in the linear theory approximation using the reciprocity theorem, utilizing the probing wave field pattern provided by computation and taking into account the 2D plasma inhomogeneity effects; whereas the alternative 'slow' version DR synthetic diagnostic is based on the full-wave code IPF-FD3D describing the probing and scattered wave propagation in turbulent plasma. The DR signal frequency spectra and the dependence of their frequency shift, width and shape on the probing antenna position are computed and shown to be similar to those measured in the high-field side probing DR experiment at the FT-2 tokamak. The geodesic acoustic mode characteristics provided by the measurements and by the synthetic DR are close within a 12% accuracy. However, a substantial difference was found in the decay of the DR signal cross-correlation functions with growing frequency shift in the probing wave channels. The quick decrease in the radial correlation DR coherence observed in the experiment and full-wave synthetic diagnostic, compared to the fast synthetic DR, is attributed to the nonlinear effect of the probing wave phase modulation by the turbulence in the former two cases. The variation in the DR signal at a growing incidence angle in the experiment is also shown to be slower than predicted by both of the synthetic diagnostics, presumably due to underestimation of the probing wave phase modulation and consequent nonlinear saturation of the DR signal at lower incidence angles in modeling.

Isotope effect allows fusion devices to perform better when heavier hydrogen isotopes are used as fuel, but the reason for this improvement is not yet understood. We present the first direct evidence of the isotope effect on particle confinement in the FT-2 tokamak and investigate it via gyrokinetic simulations. Experimental measurements for comparable hydrogen and deuterium discharges show that the particle confinement time increases by 40% for the heavier isotope species. The isotope effect on particle flux is reproduced in global and local gyrokinetic simulations. Global ELMFIRE simulations demonstrate a systemic reduction in particle fluxes across the radial range, showing a ratio of fluxes H DG G = at the edge and 1.4Local GENE simulations agree qualitatively with the result. Besides the fluctuation level, smaller scales and a favorable shift in the cross-phase between the turbulent fluctuations are found to contribute to the isotope effect in the simulations.