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.

Direct measurements of micro-, meso-, and macroscale transport phenomena in the FT-2 tokamak are shown to be quantitatively reproduced by global full f nonlinear gyrokinetic simulation predictions. A detailed agreement with mean equilibrium E×B flows, oscillating fine-scale zonal flows, and turbulence spectra observed by a set of sophisticated microwave backscattering techniques as well as a good fit of the thermal diffusivity data are demonstrated. A clear influence of the impurity ions on the fluctuating radial electric field is observed.

The complex interaction between large-scale mean EB flows, meso-scale zonal flows and fine-scale micro-turbulence excited due to specific profiles of plasma parameters and leading to anomalous transport is an important area of experimental and theoretical research in magnetically confined plasmas. In this paper the global gyrokinetic full particle distribution particle-in-cell simulations performed for Ohmic discharge of small research FT-2 limiter tokamak are for the first time validated against experimental data obtained both with a set of standard tokamak diagnostics and by sophisticated microwave backscattering techniques characterizing the tokamak turbulent dynamics and transport phenomena at micro, macro and intermediate scales. As a result of the simulation carried out with the ELMFIRE code an overall reasonable agreement between the numerical expectations and the experimental estimations of the electron and ion effective heat conductivity profiles obtained from the primary experimental profiles is achieved. The agreement obtained at the macro-level was strengthen at the microlevel and for intermediate turbulent scales using the Doppler reflectometry (DR), radial correlation reflectometry (RCR) and Doppler Enhanced Scattering (ES) microwave diagnostics. The turbulence dynamics at the micro-scale measured by DR at FT-2 were compared to the ELMFIRE predictions using synthetic DR diagnostics. As a result, not only the experimental DR spectra frequency shift, but its width and even the shape of the spectra was well reproduced by synthetic diagnostic indicating comparable rotation and spreading of the selected turbulent density fluctuations. The turbulence radial correlation length dependence on its frequency provided by the modeling appears to be in reasonable agreement to that given by RCR. The fluctuation poloidal rotation velocity profile obtained by the DR is very close to that obtained from the synthesized DR spectrum and also close to that given by ES and to the value produced by ELMFIRE for plasma EB drift velocity. The large spectral width of the DR and ES spectra is explained by fast and strong variation of radial electric field observed in the modeling. Giant oscillations of the field at a frequency of approximately 30-50 kHz much smaller than the typical drift wave frequency, but much larger than the inverse energy confinement time, observed by DR and ES as well, are attributed to the geodesic acoustic mode (GAM). This attribution is supported by comparison of the oscillation frequency dependence on the radial position with analytical prediction for the GAM frequency accounting for the impurity contribution. The dispersion of radial electric field estimated from the ES is found to be comparable to the huge E r dispersion provided by the gyrokinetic modeling. Moreover the GAM correlation length and phase velocity determined using ES technique appears to be close to those obtained by the ELMFIRE. The interrelations of GAM and drift wave turbulence in experiment and gyrokinetic modeling are compared....

Fine scale turbulence is considered nowadays as a possible candidate for the explanation of anomalous ion and electron energy transport in magnetized fusion plasmas. The unique correlative upper hybrid resonance backscattering (UHR BS) technique is applied at the FT-2 tokamak for investigation of density fluctuations excited in this turbulence. The measurements are carried out in Ohmic discharge at several values of plasma current and density and during current ramp up experiment. The moveable focusing antennas set have been used in experiments allowing probing out of equatorial plane. The radial wave number spectra of the small-scale component of tokamak turbulence are determined from the correlation data with high spatial resolution. Two smallscale modes possessing substantially different phase velocities are observed in plasma under conditions when the threshold for the electron temperature gradient mode excitation is overcome. The possibility of plasma poloidal velocity profile determination using the UHR BS signal is demonstrated.

The results of geodesic acoustic mode (GAM) studies in the spherical torus Globus-M via Doppler reflectometry are presented. The intermittent character of the GAM evolution is similar to the limit-cycle oscillation behavior of zonal flows. The evident correlation between the GAM rotational velocity and both Dα emission and plasma density oscillations is exhibited and discussed. The obtained experimental results are compared with those from tokamaks with large aspect ratios.

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The targeted plasma parameters of the compact spherical tokamak (ST) Globus-M have basically been achieved. The reasons that prevent further extension of the operating space are discussed. The operational limits of Globus-M together with an understanding of the limiting reasons form the basis for defining the design requirements for the next-step, Globus-M2. The recent experimental and theoretical results achieved with Globus-M are discussed, the operational problems and the research programme are summarized and finally, the targeted Globus-M2 parameters are presented. The magnetic field and plasma current in Globus-M2 will be increased to 1 T and 0.5 MA, respectively. The plasma dimensions will remain unchanged. With auxiliary heating at a high average plasma density, the temperatures will be in the keV range and the collisionality parameter with ν * 1 will define the operational conditions. Noninductive current drive will be a major element of the programme. The engineering design issues of Globus-M2 tokamak are discussed and the technical tokamak parameters are confirmed by thermal load and stress analysis simulations. The experimental results obtained on Globus-M2 and the limits of its performance should clarify the feasibility of an ST-based super compact neutron source.