A detailed study of the general transport properties of the FT-2 tokamak shows significant isotopic difference in ohmically heated quasi-stationary plasma in high density regimes 〈 n e 〉 ≈ (6–9) × 10 19 m −3 . For deuterium, the signatures of a transition to the H-mode are found at a plasma density exceeding a certain value, while hydrogen plasma remains in L-mode in all comparable discharge scenarios. The origin of this isotopic discrepancy is studied both with energy transport analysis and direct turbulence and plasma velocity measurements performed with Langmuir probes and microwave diagnostics. A special, more complicated dynamic experimental series with current ramp up, performed in these high density regimes, demonstrated the stability and robustness of an improved energy confinement in deuterium, whereas a comparable confinement improvement in hydrogen plasma was observed.

In this paper a one-dimensional approach to the lower hybrid current drive (LHCD) modelling in the presence of inductive electric field is applied to calculate LHCD for deuterium plasmas at the FT-2 tokamak. The simulation results are compared to experimental data.

Numerous theoretical and experimental studies have proved the important role of radial electric field inhomogeneity, or shear, in LH-transition initiation, and established heating power threshold for transition, although some experiments provide the observation of LH-transition dependency on particle source. It is necessary to apply a concerted approach to describe LH-transition initiation possibility and dynamics, considering E-r shear, particle source and turbulence properties as the main factors responsible for LH-transition initiation.

Recent research at three small tokamaks with different parameters located at the Ioffe Institute—the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M—are reviewed. This overview covers energy confinement (Globus-M and FT-2), L–H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

The isotope effect in a tokamak confinement resulting, in contradiction to the theory expectations, in the anomalous transport decrease in numerous experiments with growth of the hydrogen isotope number remains a long-standing puzzle for the period of 40 years [1]. The novel approach to explanation of this effect, which is favorable for fusion applications, is based on accounting for the multi-scale turbulence nonlinear interactions. Within this approach the isotope effect in particle confinement, but not energy, was demonstrated recently in FT-2 tokamak in hydrogen (H) and deuterium (D) ohmic discharges with modest electron density ~ (1.5-2.5)×10 m. The higher particle confinement in D-discharges was correlated in these experiments to a higher excitation level of the GAM in agreement with results of specially performed global full-f gyrokinetic modeling by ELMFIRE code [2, 3]. In this paper we present the results of further development of energy confinement studies [4] in FT-2 tokamak at high densities. Special series of Ohmic discharges are performed in H and D plasmas within the chord averaged density range ~ (5–9)×10m. The energy confinement time calculations based on measured kinetic profiles demonstrate essential difference in τE behavior for different gases. Hydrogen plasma follows the LOC to SOC transition that happens at the densities above ~ 6×10 m. At the same time deuterium plasma behavior at the highest densities shows further increase of τE with growing density typical of LOC scenario. In vicinity of tokamak operational density limits lim~ 9×10 m the energy confinement time in D is twice as high as in H. Confinement improvement in Ddischarge is accompanied by the flattening of the electron density profile in the central region and its steepening at the edge, followed by essential decrease of radiation losses. The turbulence evolution with growing plasma density in these regimes is studied both with reflectometry diagnostics and by the gyrokinetic 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 Gamma(H)/Gamma(D) = 1.3 at the edge and Gamma(H)/Gamma D= 1.4 at r/a = 0.6. Local 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.

A new wave of interest has been raised in last few years to the isotope effect in a tokamak confinement which remains a long-standing puzzle for the period of 40 years, within which the energy anomalous transport decrease was observed in numerous experiments with growth of the hydrogen isotope number in contradiction to the theory expectations. The novel approach to explanation of this effect, which is favourable for fusion applications, is based on accounting for the multi-scale turbulence nonlinear interactions, in particular, for the coupling between small-scale turbulence and zonal flows. In this paper the isotope effect in particle confinement is investigated experimentally in FT2 tokamak (R = 55 cm, a = 7.9 cm) with a circular limiter and computationally by the global gyrokinetic (GK) code ELMFIRE [1, 2]. Experiments were performed in two pairs of similar hydrogen (H) and deuterium (D) ohmic discharges with high and low central density shown in fig. 1(a, b). GK simulations were performed at time moments pointed by black arrows, indicated as NH-ND and GH-GD cases, respectively. The density profiles for these cases are shown in fig. 1(c).

The energy confinement time dependence on the tokamak ohmic discharge plasma parameters is usually described by empirical scaling formulas benchmarked against the data base obtained at numerous machines. The two most popular dependencies are given by neoAlcator τ nA(ms) = 0,007 aRneq and Goldstone τ Gs(ms) = 0,01 aRneqA scalings. They roughly agree predicting the linear dependence of the energy confinement time on the plasma density, however the Goldstone scaling in addition includes the proportionality of the energy confinement time to the square root of the hydrogen isotope atomic number, the socalled confinement isotope effect. The linear dependence on the plasma density corresponding to the linear Ohmic confinement (LOC) in many experiments saturates at high plasma density leading to the saturated ohmic confinement (SOC) regime. The saturation is attributed to the ITG mode turbulence contribution to the anomalous transport, whereas the LOC is explained by the dominance of the TE mode in the drift-wave turbulence [1]. In the present paper we determine the electron energy confinement time scaling at the small research FT-2 tokamak (a=0.08m, R=0.55m, 19kA < Ipl <35 kA, 1.2T < Bt < 2.5T, q~5) using the ASTRA code transport modeling of the experimental data base parameter profiles obtained using laser Thomson scattering, microwave interferometry, NPA and bolometric diagnostics. In parallel we investigate the anomalous transport in several typical discharges from the data base using local gyrokinetic code Gene and global full-f code ELMFIRE. It is shown that the Goldstone scaling better fits to the set of data obtained in different gases (hydrogen, deuterium and helium), however the experimental points do not show a clear isotope effect for the electron energy confinement time, unlike the scaling predictions. It should be mentioned that the isotope effect consistent with the Goldstone scaling (proportionality to the square root of atomic mass) is demonstrated for the energy transport in the ion channel. No transition to SOC with growing plasma density is observed in experiment, which is explained by the gyrokinetic modeling by the dominance of the TE mode.

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Over the past decade new regimes of tokamak operation have been identified, whereby electrostatic and magnetic turbulence responsible for anomalous transport, can be externally suppressed, leading to improved confinement. Although turbulence measurements have been performed on many confinement devices, the insight gained from these experiments is relatively limited. To make further progress in the understanding of plasma turbulence in relation to improved confinement and transport barriers, an extensive experimental and theoretical research programme should be undertaken. The present INTAS project investigates the correlations between on the one hand the occurrence of transport barriers and improved confinement in the tokamaks TEXTOR & T-10 and Tore Supra as well as on the smaller-scale tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution. This is done in a strongly coordinated way and exploiting the complementarity of TEXTOR and T-10 and the backup potential of the other tokamaks, which together have all the relevant experimental tools and theoretical expertise. Advanced theoretical models and numerical simulations are used to check the experimental results.

An extensive (INTAS) research programme started in 2002 to investigate the correlations between on the one hand the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution and of various active means to externally control plasma transport. This has been done in a strongly coordinated way and exploiting the complementarity of TEXTOR and T-10 and the backup potential of the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise.

The diagnostic system of TEXTOR comprises about 50 individual diagnostic devices. Since the start of the Trilateral Euregio Cluster collaboration, part of the emphasis in the experimental program has shifted toward the study of physics processes in the plasma core. To aid these studies several new and advanced core diagnostics have been implemented, whereas a number of other core diagnostics have been upgraded to higher resolution, more channels, and better accuracy. In this paper a brief overview is given of the present set of plasma core diagnostics at TEXTOR.

Inhomogeneous plasma rotation, according to the present day understanding, can play a substantial role in energy confinement in toroidal plasmas, suppressing drift micro turbulence and thus reducing anomalous heat and particle fluxes. In this paper the temporal variation of the poloidal plasma rotation and small-scale turbulence are studied under the incidence of RF heating power in Lower Hybrid (LH) frequency range. The experiment is performed at research FT-2 tokamak (R0 = 55 cm, a = 8 cm, BT ≈ (1.7 ÷ 2.2) T, Ip ≈ (19 ÷ 37) kA, ne(0) ≈ (0.5 ÷ 5)⋅10 13 cm -3 , Te(0) ≈ 500 eV), where the RF power up to PRF ≈ 120 kW at frequency fRF = 918 MHz is launched into the plasma by a two-waveguide grill. The plasma poloidal rotation profile is measured using recently found enhanced Doppler frequency shift effect of the highly localized microwave back scattering (BS) in the Upper Hybrid Resonance (UHR) [1], as well as by Doppler reflectometry. The UHR BS or enhanced scattering (ES) [2] utilizes for local diagnostics of small-scale plasma fluctuations the growth of wave vector and electric field of the probing extraordinary wave in the UHR, where condition ) ( ) ( 2 2 2 r f R f f pe ce i + = is fulfilled for the probing