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Abstract The experiments at the FT-2 tokamak are described that were focused on clearing up the role of the parametric decay instabilities in decreasing the generation efficiency of the non-inductive current excited by the electromagnetic waves in the lower hybrid frequency range. The most discussed instability of such kind is the decay of the pump wave into the daughter high-frequency waves and the low-frequency ion–acoustic quasi-modes. The studies performed have shown that, under conditions of the FT-2 experiment, the ion–acoustic instability has no decisive effect on the decrease in the efficiency of the lower hybrid current drive.

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 systematization of experimental data related to development of the parametric decay instability in tokamak plasma during RF power input within the lower hybrid frequency range was performed, which allows its characteristics to be compared with theoretical representations. The spectra of pumping waves and radiation from the plasma are obtained, which illustrate the appearance of satellites that are shifted from a pump frequency by a value that is a multiple of the ion cyclotron frequencies, toward the “red” and “blue” sides. It is noted that with a rise in density, a considerable broadening of the line of a pumping wave occurs. Values of the RF power and plasma density are observed, which are threshold values for the onset of the development of parametric decay instability. Using the diagnostic complex, the analysis of the low-frequency region of the plasma oscillation spectrum was carried out. The conditions of the development of instability were analyzed within the existing theoretical representations.

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.

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.

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.

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.

Diagnostics of a plasma edge is an extremely important field of tokamak physics because this region determines the plasma-surface interaction. In the paper the plasma density, electron temperature and radial electric field at the plasma edge of the FT-2 tokamak are analyzed using the results of probe measurements. The measurements were performed in the experiments conducted with a plasma pinch being in a symmetrical vertical position and were compared with the results derived by the ELMFIRE code simulations to correct the model calculations. The experimental observations are shown to be in a reasonable agreement with the gyro-kinetic model predictions.

A gamma-ray spectrometer based on LaBr 3 (Ce) scintillator has been used for measurements of hard X-ray emission generated by runaway electrons in the FT-2 tokamak plasmas. Using of the fast LaBr 3(Ce) has allowed extending count rate range of the spectrometer by a factor of 10. A developed digital processing algorithm of the detector signal recorded with a digitizer sampling rate of 250 MHz has provided a pulse height analysis at count rates up to 107 s−1 . A spectrum deconvolution code DeGaSum has been applied for inferring the energy distribution of runaway electrons escaping from the plasma and interacting with materials of the FT-2 limiter in the vacuum chamber. The developed digital signal processing technique for LaBr 3 (Ce) spectrometer has allowed studying the evolution of runaways energy distribution in the FT-2 plasma discharges with time resolution of 1–5 ms.

Study of behavior of accelerated electrons (AE) in the ohmically heated (OH) plasma and low-hybrid current drive (LHCD) in tokamak has the practical interest especially for estimations of AE radiation energy losses and solving of the non-inductive CD problem for the ITER. Studies based on measurements of non-thermal microwave radiation (MR) intensity and hard X-ray (HXR) spectra are conducted in the FT-2 tokamak having large local magnetic ripples under LHCD in the omically heated (OH) plasma with the fan instability excited. The paper presents experimental data first obtained in such conditions witnessing about some AE behavior features. Nature of MR abnormal intensity and short giant flashes together with the fast additional electron heating in the plasma core are discussed. The first high additional fast electron heating was registered together with the synchrotron radiation (SR) intensity increase. It was accompanied by short MR spikes observed in the narrow frequency range (53 ÷ 78) GHz. They arise owing to the AE transverse energy and pitch angle increase under each crossing the cyclotron auto-resonance region and SR maser gain. It is proposed the heating mechanism owing to absorption of SR and Bernstein waves arising at linear transformation the extraordinary component of intensive MR spikes in the black plasma layers. It was found that the non-thermal MR of abnormal intensity arising during OH in the frequency range (10 ÷ 40) GHz is due to the fan instability development and the substantial local magnetic ripples. MR is accompanied by short giant flashes having a narrow frequency spectrum. Together with the SR growth the less intensive MR flashes appear in the range (57 ÷ 75) GHz. In our case it becomes possible the maser amplification of both SR and collective radiation. Appearance of the giant flashes may be initiated under transition of the maser - amplifier into the self - excitation regime, when low-frequency quasi- coherent MR flashes are generated.

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.

Results of observation of anomalously intense microwave radiation (MR) in the electron plasma frequency range of 10–20 GHz that is generated continuously during ohmic heating of moderately dense plasma in the FT-2 tokamak are presented. Its appearance is associated with development of the “fan” plasma instability, and the presence of a large number of the local ripples of the toroidal magnetic field. It was found that MR is accompanied by the short gigantic and less intense flashes in the range of magnetic broadening of the first harmonic of the electron gyrofrequency (57–75 GHz). As is known, the latter occurs upon maser amplification of the synchrotron radiation of accelerated electrons interacting with the harmonics of local magnetic ripples in an autoresonance cyclotron mode. In our conditions, due to the nonlinear transformation of plasma waves into electromagnetic ones under the buildup of the fan instability, collective radiation (CR) is generated. Therefore, an abnormally high MR-intensity is probably due to CR maser amplification, whereas gigantic flashes occur during the self-excitation of maser amplifier under suitable conditions.

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.

In experiments with lower hybrid current drive (LHCD) on the FT-2 tokamak, lower hybrid (LH) waves have been successfully used for the first time to ensure effective additional heating of plasma electrons from 450 to 600 eV (I Pl = 32 kA, Δt RF = 14 ms, P RF = 100 kW, F = 920 MHz). Several factors influencing the efficiency of plasma heating have been discovered. In particular, significant growth of radiation losses in the LHCD regime has been found, which is probably related to an increase in the intensity of synchrotron radiation from accelerated electrons. The increase in this intensity in the 53–156 GHz frequency range was accompanied by short spikes of microwave radiation, which were observed only in a narrower frequency range (53–78 GHz) and apparently resulted from interaction of a runaway electron beam with significant local mirrors of toroidal magnetic field. A model of the additional heating of plasma electrons due to absorption of the microwave radiation generated by a beam of accelerated electrons is proposed.

The experiments carried out at the FT-2 tokamak in which additional pulsed puffing of helium into the hydrogen plasma was used for diagnostic purposes are considered. To estimate the necessary content of helium ions in the experiments on studying short-scale plasma oscillations, the ionization-recombination balance was simulated numerically under the assumption of a toroidally homogeneous influx of the working gas onto the boundary of the plasma column. In these simulations, the effective density of the neutral gas incident on the plasma boundary was determined by the iteration method, which made it possible to provide agreement between the obtained solution and the experimental discharge conditions. In particular, the correspondence of the determined admixture content to both the plasma quasineutrality condition and the value of the effective charge Zeff, as well as agreement between the calculated and measured plasma density profiles, was ensured. The simulations were performed under the assumption of anomalous diffusion coefficients for all plasma components. The temporal variations of the ionization-recombination balance were checked by comparing them with the measured spectra of radiation in the HeI, HeII, and Hα lines. In the current drive experiments, variations in ne(r) at the discharge periphery were examined by the method based on the proportionality of the intensity ratio of the helium spectral lines, HeI(668 nm)/HeI(728 nm), to the plasma density. In these calculations, the factors relating the intensity ratio of these lines to the plasma density were taken from the literature on spectral diagnostics.

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.

Studies of the super-thermal and runaway electron behavior in ohmic and lower hybrid current drive FT-2 tokamak plasmas have been carried out using information obtained from measurements of hard x-ray spectra and non-thermal microwave radiation intensity at the frequency of 10 GHz and in the range of (53 ÷ 78) GHz. A gamma-ray spectrometer based on a scintillation detector with a LaBr3(Ce) crystal was used, which provides measurements at counting rates up to 107 s−1. Reconstruction of the energy distribution of RE interacting with the poloidal limiter of the tokamak chamber was made with application of the DeGaSum code. Super-thermal electrons accelerated up to 2 MeV by the LH waves at the high-frequency pumping of the plasma with low density ~ 2 × 1013 cm−3 and then up to 7 MeV by vortex electric field have been found. Experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the FT-2 plasmas is presented in the article and compared with the numerical calculation of the maximum energy gained by runaway electrons for given plasma parameters. In addition, possible mechanisms for limiting the maximum energy gained by the runaway electrons are also calculated and described for a FT-2 plasma discharge.

Probe measurements in a peripheral plasma region of the FT-2 tokamak have been performed in experiments with lower hybrid heating and during the subsequent transition to an improved confinement regime. The average characteristics of high-density plasma structures and the velocity of their propagation in the radial and poloidal directions are determined by the conditional averaging technique and compared to the results obtained using some other methods of data processing. It is established that a quasi-coherent mode can exist on the inner side of the torus.

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.

The statistical properties of fluctuations of the plasma density and radial drift particle flux in the peripheral region of the FT-2 tokamak are analyzed using data from probe measurements. It is found that the probability distribution functions of the quantities under study vary over the radius and poloidal angle and change significantly after a transition to an improved confinement mode during auxiliary lower hybrid heating. Using experimental data and existing theoretical models, an analytic expression for the probability distribution function of the plasma density fluctuations is derived in a strongly nonlinear approximation. The expression is shown to agree well with experimental observations.

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.

Results are presented from experimental studies of the time behavior of the transport processes in the edge plasma of the FT-2 tokamak during auxiliary lower hybrid heating when an internal transport barrier and then an external transport barrier form in the plasma. An analysis of the data on turbulent particle transport in the edge plasma shows that the radial electric field generated inside the plasma column during auxiliary heating plays an important role in both the formation of a transport barrier and the suppression of anomalous transport at the plasma periphery in the postheating phase of the discharge. The mechanism for the formation of a negative radial electric field E r near the last closed flux surface after the end of the lower hybrid heating pulse is considered. Fluctuation spectra of the particle density and poloidal electric field are presented that characterize the process of suppression of microturbulence at the plasma periphery. The experimental data were obtained with the use of movable multielectrode Langmuir probes.

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)

Results are presented from numerical simulations that show that, in a plasma with well-developed turbulence, the radial electric field can be positive in the region where the gradients of the plasma parameters are steep. In a plasma in which the turbulence is suppressed (as is the case with auxiliary lower hybrid heating), the radial electric field is found to exhibit a nearly neoclassical behavior during the formation of a transport barrier and transition to the H-mode.

Experimental data on the processes in edge plasma that accompany the transition to an improved confinement regime during lower hybrid heating in the FT-2 tokamak are presented. The poloidal and radial distributions of the plasma parameters and drift particle fluxes were measured with the use of mobile mulitielectrode Langmuir probes and were found to be substantially nonuniform in the poloidal direction. The evolution of the plasma parameters in the course of heating and during an L-H transition is investigated. It is shown that, in FT-2 experiments, the drift of plasma particles in a slowly varying (quasi-steady) electric field and the fluctuation-induced particle fluxes make comparable contributions to the radial particle transport, whereas the contribution of fluctuations to poloidal plasma fluxes is negligibly small. The effective coefficient of radial diffusion is determined. The measurement results show that the L-H transition is accompanied by a substantial decrease in this coefficient.

Studies of processes in the scrape-off layer (SOL) of the tokamak show a direct influence of periphery effects on confinement parameters of the plasma core. This paper illustrates experimentally observed transport barrier formation initialized by the low hybrid (LH) heating of the hydrogen plasma. The experimental data near last close flax surface (LCFS) and in SOL were obtained by means of an enhanced movable multielectrode Langmuir probe and spatially resolved spectroscopic technique retooled with additional helium puffing. The edge diagnostics show a strong change of a radial electric field and plasma parameters near LCFS. The alteration at the plasma edge is generated by high LH ion heating. It was found that the L-H transition is accompanied by a noticeable reconstruction of the poloidal and radial plasma parameters profiles in the SOL and in the limiter shadow region.

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.

Results of comparative experimental studies of the efficiency of lower hybrid current drive (LHCD) and lower hybrid heating (LHH) in the FT-2 tokamak in hydrogen and deuterium plasmas are presented. In the new comparative experimental runs in deuterium/hydrogen plasmas suppression of the LHCD and beginning of the interaction of LH waves with ions is controlled by the plasma density rise. Role of parametric instabilities in CD switch-off is considered. In order to analyze the experimentally observed effect of LHCD the GRILL3D and FRTC codes has been used.

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field E r and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field E r is proposed to explain the formation of a transport barrier.

This contribution presents recent results obtained with a pyroelectric bolometer installed on the Globus-M spherical tokamak. By results of processing signals, the information about a time lag between signals of different channels was received, using a correlation analysis. This part of work is aimed to estimate a velocity of macroscopic movements of radiation sources. Radiation losses during recent experiments with NBI and ICRH operation are presented and considered as well.

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.

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.

We report on the first measurement of the absolute values of radiative losses in the Globus-M spherical tokamak. Measured by a wide-aperture pyroelectric bolometer, the radiative losses amount to about 30% of the total power deposited in the discharge, which is indicative of a sufficiently high purity of plasma in the experimental setup. Radiative losses on such a level have to be taken into account in determining the energy confinement time of the tokamak plasma. A new method is proposed for calibrating the bolometer using the data of other diagnostic techniques for the current ramp up stage, which provides for a coincidence up to within 15% with the calibration data obtained on a test bench.

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.

Processes in the boundary (near-wall) plasma of the FT-2 tokamak upon transition into an improved plasma confinement regime under the action of lower hybrid heating were studied by a refined plasma diagnostic method using multielectrode Langmuir probes. The experimental data show evidence of suppression of the correlation and coherency of fluctuations in the plasma density and the electric field strength as well as a reduction in the transverse particle transfer caused by the fluctuational drift flows.

A change in the heat transfer conditions and a mechanism of the internal circulation constraint formation were modeled using a self-consistent BATRAC code. It is demonstrated that conditions favoring a sharp drop in the heat transfer may arise in experiments on the FT-2 tokamak as a result of the ion temperature rise from 100 to 300 eV at the center of the plasma column caused by the effective lower hybrid heating power absorption by the ions. A key point in this phenomenon is related to a growth in the radial electric field strength and the poloidal rotation velocity shear.

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.

The magnitude of the radial electric field (Er), resulting from nonambipolar fluxes of neoclassical origin, is evaluated using Monte Carlo guiding-center simulations for a low-current plasma corresponding to the FT-2 tokamak {Fisichiskii Tokamak-2, Ioffe Institute, St. Petersburg [S. I. Lashkul, V. N. Budnikov, E. O. Vekshina et al., Plasma Phys. Rep. 27, 1001 (2001)]}. The Er-values are found to significantly exceed those given by the standard neoclassical theory, based on thin-orbit assumption, when the plasma current is sufficiently low and the pressure gradient is sufficiently high. Strong Er-structures are found to form in the same low plasma-current range where enhanced confinement is reported in the FT-2 tokamak. In the simulations, the physics behind the strong increase in the field values is intimately related to the poloidal Mach-number, together with the wide ion orbits.

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.

Collective scattering of CO2-laser radiation is used to study the microturbulence of the plasma in the FT-2 tokamak when the current is rapidly raised, while the magnetic-field shear is varied appreciably. The experiment exhibited suppression of the plasma fluctuations, the appearance time of which was correlated with the transition of the discharge to the improvedconfinement regime. The resulting data are evidence that the suppression of the oscillations occurred predominantly in the central zone of the tokamak, and a suppression-extending phenomenon or hysteresis is detected. The evolution of the spectral characteristics of the fluctuations during the suppression is analyzed, using model calculations of the magnetic-field shear.

The problem of optimizing the conditions for central lower-hybrid heating has stimulated experiments with improved confinement at the center of the discharge in analogy to discharges with an inverted shear of the q profile. To this end, a current pulse rising rapidly from 22 to 30 kA over 0.5 ms was used in the FT-2 tokamak during lower-hybrid heating. In these experiments a substantial increase in the lifetime of the energy in the ionic component was observed. A decrease of the fluxes of high-energy charge-exchange atoms and a suppression of the microoscillations of the plasma determined in the central regions of the discharge are observed. These and other data were used together with the computer simulation to clarify the mechanism leading to the improvement of energy and particle confinement at the center of the discharge. The influence of variations in both the q profile and the electric fields, which accompanied the rapid current rise, on the transport processes is studied.

An analysis is made of the transition to improved confinement (H-mode) observed in lower hybrid heating experiments in the FT-2 tokamak. Particular attention is paid to processes taking place near the wall, including the suppression of microfluctuations accompanying the L-H transition and the buildup of edge-localized modes (ELM activity). The conditions for transition to the H-mode are discussed only for Ohmic heating. The data are compared with the results of large tokamak experiments.

In low-current tokamaks, in the absence of radial electric fields (Er), the widths of the drift orbits are large and the direct orbit losses can extend deep into the plasma. Furthermore, in such a plasma even a modest Er can produce rotation with a poloidal Mach number (Mp) that exceeds unity. Using the Monte Carlo code ASCOT, which follows charged particle orbits in the five-dimensional phase space, the formation of an internal transport barrier (ITB) in such a tokamak is investigated. Carrying out the simulations for the geometry corresponding to the FT-2 tokamak, it is shown that if, under these conditions, a steep density gradient is created in the plasma, the plasma responds by generating a strong (much stronger than needed to compensate the diamagnetic drift) Er in the region of the strong gradient. The generation appears to be a pure neoclassical effect, but a global solution over the entire plasma cross section is required to fully identify it. As a result, an ITB-like situation with a strongly sheared E*B flow is obtained inside the plasma. In these circumstances Mp>1, and thus the orbits of the majority of ions become strongly squeezed. The neutral fluxes observed by neutral particle analysers are also simulated to find out if the neutral spectra can be utilized to estimate the Er values across the plasma cross section in the FT-2 tokamak.