E. R. Its

Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

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 Er 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 Er is proposed to explain the formation of a transport barrier.

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.

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.

Processes in SOL plasma at the transition into improved confinement mode in FT-2 tokamak

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.

A study of low-frequency microturbulence by CO2-laser collective scattering in the FT-2 tokamak

Results are presented from studies of small-scale plasma density fluctuations in the FT-2 tokamak by the method of far-forward CO2-laser collective scattering. The frequency and wavenumber spectra of fluctuations are measured using parallel k analysis at various positions of the scattering volume in the plane of the minor cross section of the torus. The data obtained are interpreted using numerical simulations. In phenomenological models, plasma fluctuations are substituted by a superposition of two-dimensional noninteracting cells with Gaussian profiles. A comparison of the calculated and experimental spectra shows that plasma fluctuations should be described based on the concept of strong microturbulence. The poloidal rotation velocity and the characteristic scale length of the scattering fluctuations, as well as the radial position of the region where they are located, are determined. The diffusion coefficient of the cells introduced in the model turns out to be close to the thermal diffusivity determined from the electron energy balance in the ohmic phase of the discharge.

Mechanism for the suppression of lower hybrid current drive in the FT-2 tokamak

The mechanism for the switching off of the lower hybrid current drive in the FT-2 tokamak is studied. It is shown that the lower hybrid wave-driven current is switched off when a parametric instability, which causes lower hybrid waves to decay into slowed waves interacting with plasma ions, develops at the plasma periphery. The onset of a parametric instability is attributed to the fact that the electron temperature falls off in the course of discharge, thereby lowering the instability threshold.

Plasma current profile diagnostics on FT-2 based on correlation reflectometry measurements of MHD modes

Abstract The purpose of this study was to demonstrate the possibility of plasma current profile estimation by the correlation reflectometry. The method is based on the determination of the MHD mode oscillations localized in the vicinity of resonant magnetic surfaces by the correlative analysis of multichannel reflectometry signals. The described diagnostic technique allows one to make estimation of the location of different magnetic surfaces, which can provide information on the plasma current profile. Besides, the diagnostics gives frequency spectra and poloidal rotation velocities of the MHD perturbations on rational magnetic surfaces.

Observation of improved confinement at the center of a FT-2 tokamak plasma with rapid current rise and lower-hybrid heating

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.

Wall effects in improved confinement modes in the FT-2 tokamak

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.