L. A. Esipov

Investigation of small-scale tokamak plasma turbulence by correlative UHR backscattering diagnostics

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.

Investigation of ETG mode-scale component of tokamak plasma turbulence by correlative enhanced scattering diagnostics

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.

Observation of the ETG mode component of tokamak plasma turbulence by the UHR backscattering diagnostics

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.

Observation of turbulence exponential wave number spectra at ion sub-Larmor scales in FT-2 tokamak

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.

Investigation of Small-scale Plasma Fluctuations by Radar Backscattering in the Upper Hybrid Resonance

The radar enhanced scattering (RES) diagnostic is shown to be a feasible tool for studying small-scale plasma fluctuations and waves. This method is based on the effect that an RF wave launched into the plasma is considerably slowed down near the hybrid resonance so that the backscattered signal is received with a large time delay. Fairly good spatial and wavenumber resolution of this diagnostic are demonstrated in various laboratory plasmas.

Mechanism of the transport barriers formation at lower hybrid heating in the FT-2 tokamak experiments

The possibility of controlling the transport processes in the tokamak plasma in the lower hybrid heating (LHH) experiment has been demonstrated. We illustrate experimentally the observed transport barrier formation initialized by the LHH for different plasma experiment scenarios. First, it was found during LHH. The next method to trigger improved confinement is a combination of fast current ramp-up with LHH. The mechanisms of internal barrier formation have been put forward to explain the observed regime of improved core confinement. The increased shear of the radial electric field stimulates the internal barrier formation.

Impact of isotopic effect on density limit and LHCD efficiency in the FT-2 experiments

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.

Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

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 various active means to externally control plasma transport. It also requires one to characterize fluctuations of various important plasma parameters inside and outside transport barriers (TBs) and pedestal regions with high spatial and temporal resolution using advanced diagnostics, and to elucidate the role of turbulence driving and damping mechanisms, including the role of the plasma edge properties. Furthermore, one needs to determine the cross-field transport from the measurements and compare this with available theoretical models. This has been done in a strongly coordinated way, 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. Physical mechanisms of several TBs have been studied: electron internal transport barriers in T-10 and TEXTOR, ergodization-induced TB in TEXTOR, TB in ohmic discharges in TUMAN-3M, periodic bias-induced TBs in CASTOR. Geodesic acoustic modes (GAM) have been investigated in T-10, TEXTOR and TUMAN-3M. Core turbulence has been characterized in T-10, and small-scale turbulence has been revealed in FT-2.

Poloidal inhomogeneity of turbulence in the FT-2 tokamak by radial correlation Doppler reflectometry and gyrokinetic modelling

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.

Dynamics of the transport barrier formation on the FT-2 tokamak caused by lower hybrid heating

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.