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The dependence was studied of the level of density fluctuations recorded at different devices on the minor and major radii of the tokamak. It is known that a high level of density fluctuations can negatively affect the operation of diagnostics, in particular, the plasma reflectometry diagnostic. The increase of density fluctuations decreases the quality of raw data by increasing the error of measuring the density profile and, when density fluctuations exceed the threshold value, it renders such measurements impossible. Based on experimental data obtained on devices with substantially different sizes, a dependence of the density fluctuation level on the major and minor tokamak radii was proposed. Since the main experiments were carried out in round limiter tokamaks in ohmic (OH) heating regimes, the extrapolation result is applicable, generally speaking, to installations of larger size with the same configuration and regimes. However, experiments with electron cyclotron heating at the T-10 tokamak also allow one to extend the obtained dependence to regimes with auxiliary heating. It was shown that the obtained dependence is applicable to limiter tokamaks Tore Supra and TFTR of larger size. The applicability of the dependence to installations with diverter configuration is discussed and the possible level of density fluctuations is extrapolated to the international reactor tokamak ITER that is being constructed in France.

In the framework of the isotope effect studies at the FT-2 tokamak, the efficiencies were compared of the lower hybrid wave heating of the ion components of the hydrogen and deuterium plasmas with high densities (close to the Greenwald limit, 〈ne〉 ≤ 1020 m–3). It was experimentally ascertained that, in accordance with the theoretical concepts, the efficient axial heating of the deuterium plasma ions occurs, as opposed to the peripheral heating of the hydrogen plasma ions. Such an isotope effect occurs due to the different localization of the plasma-RF wave interaction regions. The distinctive feature of these experiments is the fact that, in deuterium plasma, during the preliminary ohmic heating, the dependence of the energy lifetime on density τЕ(n) is linear (LOC dependence), and, with increasing density, the transition to the improved ohmic confinement mode occurs. In hydrogen plasma, on the contrary, the transition to the saturation mode is observed. In this study, the considerable decrease in τЕ was revealed, observed during the additional lower hybrid wave heating in both hydrogen and deuterium plasmas.

The possibility of local measurement of the fluctuation level of the radial plasma velocity using equatorial enhanced scattering of a narrow microwave beam in the upper hybrid resonance in internal regions of tokamak plasma is demonstrated. Restrictions of the proposed method at the plasma periphery where the density fluctuation amplitude increases and small-angle scattering of microwaves by the fluctuations along the path to the upper hybrid resonance and back becomes significant are clarified.

The dynamics of spectra formation was studied experimentally using radar stroboscopic time-of-flight measurements. Both conventional fluctuation reflectometry spectra (obtained with probing normal to the cutoff surface) and Doppler reflectometry spectra were studied. Two mechanisms of spectrum formation were distinguished even at low time resolution that did not allow capturing the spectra formed after single radiation pass to the cutoff surface and back. The first mechanism that manifests itself for the normal probing and works for the unshifted component of the Doppler reflectometry spectrum is typical for the nonlocal formation of the fluctuation reflectometry spectra in small machines. This can be associated with the radiation propagation along the lengthy trajectory between the cutoff surface and the chamber wall, as well as with the small-angle scattering that occurs during this propagation along the trajectory. The second mechanism, which manifests itself during the formation of the bulk of the Doppler reflectometry spectrum (which is dominant in power and is frequency shifted), occurs without a time delay and most likely corresponds to the classical single-pass backscattering near the cutoff surface.

The effect of the isotope content of plasma (hydrogen/deuterium) on energy confinement time was studied at the FT-2 tokamak. A strong isotope effect is discovered, which is manifested in the deuterium plasma in high-density regimes. A correlation is shown between the improvement of the energy confinement in deuterium and the drop in the turbulence level at the plasma periphery.

We propose implementing the diagnostics of long-wavelength fluctuations of the radial velocity of tokamak plasma, which is based on Doppler-enhanced microwave scattering by short-wavelength fluctuations entrained by large-scale turbulent flow in the equatorial plane of a tokamak.

The fast linear (Born approximation) version of the X-mode Doppler reflectometry (DR) synthetic diagnostics is developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. The DR signal frequency spectra and the dependence of their frequency shift and shape on the probing antenna position are computed and shown to be similar to those measured in the high magnetic field side probing DR experiment at the FT-2 tokamak. The fluctuation poloidal velocities are determined using the DR experiment and synthetic diagnostics and shown to be within 15%. However, the computed and measured dependences of the DR signal power on the antenna position appear to be different presumably due to underestimation of the small-scale trapped electron mode turbulence component in the measurement region by the code.

Prior experimental work on the FT-2 tokamak has observed electron density fluctuations at electron Larmor radius scales using the enhanced scattering (ES) diagnostic (Gusakov et al 2006 Plasma Phys. Control. Fusion 48 A371–6, Gurchenko and Gusakov 2010 Plasma Phys. Control. Fusion 52 124035). Gyrokinetic GENE simulations of conditions at the upper hybrid resonance layer probed by the ES diagnostic show the presence of the anticipated turbulence from the electron temperature gradient (ETG) driven instability in linear and nonlinear simulations. Ion-scale turbulence is responsible for majority of the transport via trapped electron modes, while impurities act to merge the spectrum of the ion and the electron scale instabilities into a continuum. The linear spectrum at electron scales is characterized by maximal growth rate at a significant ballooning angle θ 0, and at ion scales the turbulence is broad in the ballooning angle distribution. The neoclassical shearing rate obtained from GENE breaks symmetry in nonlinear simulations of ETG turbulence, which manifests itself as an asymmetric turbulence spectrum. The electron density fluctuation spectrum obtained with GENE corresponds well to the ES measurement at electron scales, as do the fluxes obtained from the ion-scale simulations.

In experiments on lower hybrid current drive (LHCD) carried out at the FT-2 tokamak, a substantial increase in the central electron temperature T e (r = 0 cm) from 550 to 700 eV was observed. A complex simulation procedure is used to explain a fairly high LHCD efficiency and the observed additional heating, which can be attributed to a transition into the improved core confinement (ICC) mode. For numerical simulations, data obtained in experiments with deuterium plasma at 〈n e 〉 = 1.6 × 1019 m–3 were used. Simulations by the GRILL3D, FRTC, and ASTRA codes have shown that the increase in the density and central temperature is apparently caused by a significant suppression of heat transport in the electron component. The mechanism for transition into the improved confinement mode at r < 3 cm can be associated with the broadening of the plasma current channel due to the lower hybrid drive of the current carried by superthermal and runaway electrons. In this case, the magnetic shear s = (r/q)(dq/dr) in the axial region of the plasma column almost vanishes during the RF pulse. In this study, the effect of lower hybrid waves on the plasma parameters, resulting in a transition into the ICC mode, is considered. New experimental and calculated data are presented that evidence in favor of such a transition. Special attention is paid to the existence of a threshold for the transition into the ICC mode in deuterium plasma.

International audience; 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.

Radial electric field shear is crucial for turbulence suppression and transition to the H-mode, although the high shear value alone may not be sufficient for the LH-transition initiation. Temporal and spatial parameters of shear perturbation, particle source and turbulence parameters are the main factors responsible for LH-transition initiation. Different plasma discharge scenarios in two Ioffe Institute conventional tokamaks are analyzed using the model of plasma density and ion temperature evolution to clear up the role of aforementioned factors.Radial electric field shear is crucial for turbulence suppression and transition to the H-mode, although the high shear value alone may not be sufficient for the LH-transition initiation. Temporal and spatial parameters of shear perturbation, particle source and turbulence parameters are the main factors responsible for LH-transition initiation. Different plasma discharge scenarios in two Ioffe Institute conventional tokamaks are analyzed using the model of plasma density and ion temperature evolution to clear up the role of aforementioned factors.

openaire: EC/H2020/633053/EU//EUROfusion In this work, Doppler reflectometry (DR) and radial correlation DR (RCDR) nonlinear scattering effects are studied using full-wave modeling with a set of representative FT-2 tokamak turbulence as inputs. Narrowing of the RCDR correlation function and widening of the DR poloidal wavenumber spectrum are demonstrated. An effect on the dependence of the DR signal frequency shift on the probing wavenumber is found, namely, this dependence 'linearizing' in the nonlinear scattering regime. Nonlinear effects are shown to be weaker for O-mode probing than for X-mode probing, while a faster transition to nonlinear regime is demonstrated for RCDR compared to DR in both probing scenarios.

The effect of variations in the key parameter of short-wavelength turbulence—the ion-acoustic Larmor radius ρs, which determines the position of the maximum of the drift instability growth rate over poloidal wavenumbers—was studied experimentally at the FT-2 tokamak. For this purpose, helium was injected to hydrogen plasma, which resulted in a change in the electron temperature at the plasma edge. The universality of the exponential shape of the turbulence spectra over radial wavenumbers q and a substantial excess of the characteristic turbulence scale L over the ion-acoustic Larmor radius was confirmed with the help of correlative diagnostics of enhanced scattering. This excess at the discharge periphery reaches a value of 3–5 at a low electron temperature, apparently, due to an increase in the dissipation of drift waves upon their cascade transfer toward short scale-lengths.

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 at the edge and at . 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.

The cross-correlation function of high field side radial correlation X-mode Doppler reflectometry (DR) measured in the FT-2 tokamak experiment is shown to be a factor of three narrower than that computed using the fast linear (Born approximation) version of the X-mode DR synthetic diagnostics developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 ohmic discharge. This difference is observed in spite of the fact that the computed DR signal frequency spectra are shown to be similar to those measured. A modest phase modulation of the probing and backscattering waves by the long-scale turbulent density fluctuations is shown, both experimentally and in computation, to be responsible for the observed effect.

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value n LH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures T e ). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(N z ), were calculated. Simulations using the FRTC code with allowance for the P(N z ) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.

The dynamics of electron heat transport at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak was studied experimentally. Evolution of the profiles of the electron temperature and density was thoroughly investigated under conditions of fast variation in the plasma parameters. The energy balance in the electron channel is calculated with the help of the ASTRA code by using the measured plasma parameters. Correlation is revealed between the dynamics of electron heat transport and the behavior of small-scale drift turbulence measured using the enhanced scattering correlation diagnostics. The suppression of heat transfer and turbulence agrees well with the increase in the shear of poloidal plasma rotation calculated from experimental data in the neoclassical approximation.

The nonlinear nature of the rf absorption in a helicon-produced plasma was recently evidenced by the observation that the helicon wave damping as well as the level of short-scale electrostatic fluctuations excited in the helicon plasma increases with rf power. Correlation methods using electrostatic probes as well as microwave back-scattering at the upper-hybrid resonance allow identifying the fluctuations as ion-sound and Trivelpiece– Gould waves satisfying the frequency and wavenumber matching conditions for the parametric decay instability of the helicon pump wave. Furthermore, the growth rates and thresholds deduced from their temporal growth are in good agreement with theoretical predictions for the parametric decay instability that takes into account realistic damping rates for the decay waves as well as a non-vanishing parallel wavenumber of the helicon pump. The close relationship between the rf absorption and the excitation of the fluctuations was investigated in more detail by performing time- and space-resolved measurements of the helicon wave field and the electrostatic fluctuations.

A fine scale drift wave mode possessing unusually high frequency 2?3?MHz and radial wave number is observed using the correlative upper hybrid resonance backscattering technique at the FT-2 tokamak under conditions when the ETG mode should be unstable. The radial wave number spectrum of turbulence is measured and shown to be maximal at values 120?170?cm?1 corresponding to the largest ETG instability growth rate.

25th Fusion Energy Conference (FEC), Govt Russian Federat, St Petersburg, RUSSIA, OCT 13-18, 2014; International audience; In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, B-t = 0.4 T, I-p = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, B-t = 3T, I-p = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, B-t = 1.0T, I-p = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L-H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field B-t(1.29) and plasma current I-p(1.34) justifying the B-t and I-p increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of similar to 1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.

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 small-scale 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 new diagnostic technique for investigation of ETG mode-scale tokamak turbulence–correlative enhanced scattering is developed at the FT-2 tokamak. Fine scale drift wave modes possessing unusually high frequency are observed using this technique in the ohmic discharge under conditions when the ETG mode should be unstable.

In experiments on the FT-1 tokamak, the effect is observed of the cross-polarization scattering of microwave radiation by low-frequency plasma microturbulence in the upper hybrid resonance region. The radar diagnostics based on this effect was used to measure the frequencies and wavenumbers of the fluctuations that cause cross-polarization scattering.

This paper deals with the new spectral and microturbulence experimental data and their analysis, which show, that the radial electric field Er generated at the LH heating (LHH) in the FT-2 is high enough to form the transport barriers. The ETB is formed when LHH is switched off. The radial fluctuation-induced EB drift flux densities near LCFS in SOL are measured at two different poloidal angles. For this purpose two Langmuir probes located at low and high field sides of the torus are used. Registration of the poloidal and radial components of the electric field and density fluctuations at the same time during one discharge permits to measure the poloidal asymmetry of the transport reduction mechanism of the radial and poloidal particle fluxes in the SOL. The absolute E(~) fluctuation levels show dependence on the sign of Er shear. The modification of the microscale turbulence by the poloidal Er x B rotation shear EB at the L - H transition near LCFS is also studied by X-mode fluctuation Reflectometry. The new data were obtained by spatial spectroscopic technique., Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)

The wavenumber-resolved radar backscattering diagnostics in the upper hybrid resonance (UHR) region was used to study low-frequency short-scale turbulence in the FT-1 tokamak. The scattered spectra were measured for different delay times of the scattered signals. It is shown that the width of the spectrum of enhanced scattering by spontaneous fluctuations is proportional to the delay time. Possible mechanisms for the formation of the scattered spectra are proposed and discussed. The results of simulations and additional experiments were used to determine the dominant mechanisms governing the formation of the scattered spectra in the FT-1 tokamak. These mechanisms are related to the effect of multiple small-angle scattering of both the probing wave and the waves backscattered in the UHR region by long-scale density fluctuations and to the Doppler effect caused by the entrainment of short-scale fluctuations by the long-scale turbulent flow.

The propagation of lower hybrid (LH) waves in a tokamak plasma in the presence of an LH resonance surface is studied experimentally with the use of a specially elaborated technique based on the backscattering of the probing microwave radiation in the upper hybrid resonance region. The technique provides resolution in the wave vectors of the scattering density fluctuations. The conditions are determined under which the LH wave propagates in accordance with the predictions of linear theory and is converted into the short-wave-length ion Bernstein mode. The parameter range is found in which the predictions of linear theory fail to hold and the nonlinear effects come into play during LH wave conversion. The radial wavelengths of the LH and ion Bernstein waves are determined.

Experiments at the FT-2 tokamak had demonstrated effective plasma LH heating, which was accounted for by both direct absorption of RF power and plasma transport suppression. The improved core confinement accompanied by Internal Transport Barrier (ITB) formation was observed. The RF pulse switch off is followed by triggering of LH transition and the External Transport Barrier (ETB) formation near the last closed flux surface. The present paper is devoted to a much more detailed study of the radial electric fieldEr behaviour in the region of ITB and ETB and its influence on the tokamak microturbulence in these regions. The new experimental data were obtained by spatial spectroscopic technique using additional pulse helium puffing in hydrogen plasma. Simultaneously microscale plasma oscillations in the frequency band (0.01–2) MHz are observed with local enhanced microwave scattering diagnostics and by x-mode fluctuation reflectometry. Experiments demonstrate that the improved confinement is associated with the modification of microturbulence by the shear of theE×B poloidal velocity. This conclusion is also confirmed by the data obtained by Langmuir probes in the edge plasma.

The upper hybrid resonance (UHR) scattering technique possessing such merits as one-dimensional probing geometry, enhancement of cross section, and fine localization of scattering region is modified in the new diagnostics under development to achieve wave number resolution. The fluctuation wave number is estimated in the new technique from the scattering signal time delay measurements. The feasibility of the scheme is checked in the proof of principal experiment in a tokamak. The time delay of the UHR scattering signal exceeding 10 ns is observed. The small scale low frequency density fluctuations are investigated in the UHR RADAR backscattering experiment. The UHR cross-polarization scattering signal related to small scale magnetic fluctuations is observed. The lower hybrid (LH) wave propagation and both linear and nonlinear wave conversion are investigated. The small wavelength (λ⩽0.02 cm) high number ion Bernstein harmonics, resulting from the linear wave conversion of the LH wave are observed in a tok...

A new experimental technique is presented for the study of zonal flows in tokamak plasmas—upper hybrid resonance Doppler backscattering. The feasibility of the diagnostics is demonstrated allowing a complete characterization of the temporal and spatial behaviour of geodesic acoustic modes. The experimental results are obtained at the FT-2 tokamak.

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 poloidal dependence of the drift-wave turbulence characteristics is investigated at the FT-2 tokamak by radial correlation Doppler reflectometry (RCDR) technique and using the full distribution function global gyrokinetic modelling by ELMFIRE code. The poloidal variation of the turbulence radial correlation length from 0.2–0.55 cm is demonstrated both by measurement and computation. The turbulence correlation length rapidly decreases from the top of the poloidal cross-section to the high field side and then steadily grows in the poloidal direction. A well-pronounced excess of the turbulence radial correlation length in deuterium over its value in hydrogen discharges is demonstrated.

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 $E\ifmmode\times\else\texttimes\fi{}B$ 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.

Implementation of the correlative enhanced scattering technique in the FT-2 tokamak has resulted in measurements of both frequency and wave number spectra of a small-scale microturbulence. It is found that during the dynamic current ramp-up discharge the turbulence possesses a wide wave number spectrum which could be described by universal exponential dependence in the range of 3–4 orders of amplitude characterized by two parameters—the turbulence level and scale length. Both parameters are found to decrease substantially when the shear of the poloidal plasma rotation estimated from Doppler frequency shift of the enhanced scattering signal increases at plasma periphery. Simultaneously transition to an improved confinement resulting in the suppression of anomalous electron transport and decrease in the electron energy confinement time is observed in the experiment.

The paper pays the main attention to the LHCD efficiency in experiments on the FT‐2 tokamak. LH wave spectra P(Nz) excited by the grill computed by GRILL3D code are presented. Physical processes responsible for termination of the CD with density rise are discussed for lower and higher plasma currents that are for relatively cold and relatively hot plasmas. A role of parametric processes and additional impurity influx for CD switching‐off in the plasma are considered for those conditions.

Current drive by lower hybrid waves (LHCD) is the most effective method to sustain the plasma current, but it is feasible only at the plasma density not exceeding some density limit nDL. In the present work the main attention is paid to the investigation of this effect on the FT-2 (R = 0.55 m, a = 0.08 m, BT ≤ 3 T, Ipl = 19–40 kA, f0 = 920 MHz) tokamak. The dependence of LHCD efficiency on isotopic plasma content (hydrogen/deuterium) is studied. Characteristic features of such an experiment are a strong influence of the isotope plasma composition on the LH resonance density nLH. For hydrogen plasma nLH ≈ 3.5 × 1019 m−3, while for deuterium plasma nLH ≈ 2 × 1020 m−3. The suppression of the LHCD and beginning of the interaction of LH waves with ions are determined by the hydrogen/deuterium plasma density rise. In the hot hydrogen plasma (Te(r = 0 cm) ≈ 700 eV) the density limit nDL of LHCD is approximately equal to the resonance value nLH ≈ nLC, where nLC is the point of linear conversion. In the hot deuterium plasma one could expect an increase of nDL because of a much higher value of nLH ≥ nLC ≈ 1020 m−3. However it appeared that the observed density limit for LHCD generation nDL ≈ (3.5–4) × 1019 m−3 is not determined by nLH. The role of parametric instabilities in CD switch-off is considered in both cases. The cooling of the plasma column and density rise could lead to a reduction of the threshold for the parametric decay of f0 and result in early suppression of LHCD. In both cases the LHCD was inversely proportional to the density, which corresponds to the theoretical predictions. In order to analyse the experimentally observed LHCD efficiency the GRILL3D and FRTC codes have been used.

Quantitative reproduction of selected micro-, meso- and macro-scale transport phenomena as measured in the FT-2 tokamak is reached by Elmfire global full-f nonlinear gyrokinetic particle-in-cell simulation predictions. A detailed agreement with mean equilibrium flows, oscillating fine-scale zonal flows and turbulence radial correlation length observed by a set of sophisticated microwave backscattering techniques, as well as a good fit of the thermal diffusivity data in the central and gradient region of discharge are demonstrated. Both the shift and the broadening of the power spectrum of synthetic and experimental Doppler reflectometry diagnostics have been found to overlap perfectly at various radial positions, indicating similar rotation and spreading of the selected density fluctuations. At the same time similar radial electric field dynamics, spatial structure and outward geodesic acoustic mode (GAM) propagation have been observed by comparisons of the probability distribution function, the dominant frequency, the coherence and the cross-phase of the simulated and experimentally measured radial electric field fluctuations, identifying the turbulent driven GAM as a key contributor to the observed strong temporal variation of the radial electric field affected by impurity ions.

Experimental study of ion sub-Larmor-scale turbulence by microwave scattering diagnostics developed at FT-2, DIII-D and NSTX tokamaks is reviewed. Peculiarities of electron temperature gradient (ETG) turbulence identification in dynamical experiments at these machines are discussed. The FT-2 experimental results concerning the role of the small-scale turbulence in the anomalous electron transport are compared with those published by DIII-D and NSTX teams. Possibilities of small-scale turbulence control by plasma rotation shear are discussed for marginal ETG turbulence in NSTX and FT-2.

openaire: EC/H2020/633053/EU//EUROfusion The interplay of flows and turbulence in Ohmic FT-2 tokamak plasmas is analysed via gyrokinetic simulations with the flux-driven ELMFIRE code. The simulation predictions agree qualitatively with analytical estimates for the scaling of the neoclassical radial electric field as a function of collisionality for ad hoc parameters. For the experimental parameters, the global full-f modeling agrees well with the analytical estimates in a neoclassical setting, while including kinetic electrons and impurities has a small impact. Allowing turbulence to develop modifies the flow profile through relaxation of profiles caused by turbulent transport, non-adiabatic response of passing electrons around rational surfaces, and turbulent flow drive. Geodesic acoustic mode (GAM) is the main zonal flow component in the simulations, and its frequency and amplitude agree with theoretical predictions and experimental measurements. In the simulations, the non-linear energy transfer from the turbulence to the flows through the Reynolds force is balanced by the collisional flow dissipation. Temporal relationship between the oscillating flow, Reynolds force, and turbulent particle flux is consistent with the fundamental physics picture of GAM modulating turbulent transport on the time scale of the mode. Experimental evidence also suggests anti-correlation of GAM amplitude and turbulent fluctuations.

A comparative investigation of the isotope effect in multi-scale anomalous transport phenomena is performed both experimentally by highly localized turbulence diagnostics in comparable hydrogen and deuterium FT-2 tokamak discharges and theoretically with the help of global gyrokinetic modeling. Substantial excess of the geodesic acoustic mode (GAM) amplitude, radial wavelength and correlation length in a wide spatial region of deuterium discharge resulting in stronger modulation of drift-wave turbulence level is demonstrated by both approaches. A larger turbulence radial correlation length is found at LFS in D-discharge in experiment and a stronger modulation of gyrokinetic particles and energy fluxes is shown there by the gyrokinetic code. The gyrokinetic modeling demonstrated comparable levels of drift wave density and electric field fluctuations in hydrogen and deuterium discharges. Nevertheless, the mean value of the ion energy and particle anomalous flux provided by modeling shows the systematic isotope effect at all radii.