Turbulence radial correlation length measurements by making use of Doppler reflectometry are analysed in this paper assuming the linear (in density perturbation amplitude) regime of the probing wave scattering. Based on the Born approximation applicable in this case, we get a general integral representation of the scattering signal amplitude, which is then analysed in detail both analytically and numerically. Asymptotic analytical evaluation of the scattering signal amplitude gives a criterion on the incidence angle, exceeding which leads to suppression of the poor localized forward scattering contribution to the Doppler reflectometry signal. In the latter case, fast decay of correlation in the two frequency channels appears to be similar to that expected from the turbulence correlation function. The analytical estimations and predictions are confirmed by numerical evaluation of the scattering signal amplitude. The analysis performed proves the feasibility of the radial correlation Doppler reflectometry technique for measurements of the correlation length of a modest level turbulence at oblique launching of the probing wave beam.

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.

A computational model is developed that allows one to estimate in a real 3-D geometry the electrodynamic characteristics of complicated waveguide antennas ('grills') generating plasma waves in the lower hybrid frequency band. The antenna coupling efficiency and the shape of the wavenumber spectrum are found as a solution of a self-consistent problem taking into account a complete set of waveguide eigenmodes and finite waveguide dimensions in both directions and an arbitrary orientation to the plasma magnetic field. Electric and magnetic fields inside the waveguides are equated to the outside fields represented as a Fourier sum over wavenumbers Ny, Nz in the plane parallel to the plasma surface. The fields at the plasma edge are determined by the 2*2 plasma surface impedance matrix found as a numerical solution of the wave equation in the cold plasma approximation using the finite element (Galerkin) method. The solution is found on a 1-D mesh, i=1, 2, ..., N, in the form E(xi,Ny,Nz) exp(i(Nyy+Nzz- omega t)) so that the fourth order differential equation with appropriate boundary conditions is reduced to a set of 4N+2 algebraic equations. The developed model is applied to modelling a quite new antenna design for generating the fast H waves, which can be used for plasma heating and current drive in tokamaks of ITER scale. The antenna parameters are optimized to obtain the best coupling efficiency. The structure of waves excited in the plasma at various angles between the antenna and the plasma magnetic field is considered. The nature of the waves with Nz<1 excited by the grill is discussed

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.

The first experiments on noninductive current drive (CD) using lower hybrid waves in a spherical tokamak are described. Waves at 2.45 GHz were launched by a 10 waveguide grill with 120° phase shift between neighbouring waveguides. The experimental results for a novel poloidal slowing-down scheme are described. The CD efficiency is found to be somewhat larger than that predicted theoretically whilst at the same time being somewhat less than that for standard tokamak lower hybrid CD. Geodesic acoustic modes (GAM) have been discovered in Globus-M. GAMs are localized 2-3 cm inside the separatrix. The GAM frequency agrees with theory. The mode structures of plasma density and magnetic field oscillation at the GAM frequency have been studied. Fast particle confinement during neutral beam injection has been investigated and numerically simulated. Alfvén instabilities excited by fast particles were detected by a toroidal Mirnov probe array. Their excitation conditions are discussed and the dynamics of fast ion losses induced by Alfvén eigenmodes is presented. Preliminary experiments on the isotopic effect influence on global confinement in the ohmic Nuclear Fusion

The problem of lower hybrid (LH) wave excitation and current drive (CD) in tokamaks with a small aspect ratio (spherical tokamaks) is discussed. It is proposed to solve this problem by exciting the waves slowed down in the poloidal rather than the toroidal direction. As a result, due to the strong poloidal inhomogeneity of the magnetic field in spherical tokamaks, even the waves with comparatively weak slowing down (N ≈ 3-5) excited by a waveguide antenna in the equatorial plane can penetrate into the dense plasma and be absorbed via the Landau mechanism. This approach was applied for modeling the LHCD experiments in the low aspect ratio tokamak Globus-M (R = 0.36 m, a 0 = 0.24 m, B 0 = 0.4 T, I p = 0.25 MA, vertical elongation k = 1.6, operating frequency 2.45 GHz). The modeling was carried out using four independent codes: (i) the self-consistent antenna coupling code GRILL3D, (ii) the ray-tracing code incorporating a specially developed ray-tracing technique including some corrections necessary in strongly inhomogeneous plasma, (iii) the 2D full-wave code WAVETOP2D and (iv) the Fokker-Plank code combined with the ray-tracing code allowing simulation of the driven current density profile. The results of simulations were cross-checked and appeared to be in a good agreement. It was demonstrated that the proposed scenario provides a possibility for the efficient LHCD in a spherical tokamak.

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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.