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 field E r 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 the E × B poloidal velocity. This conclusion is also confirmed by the data obtained by Langmuir probes in the edge plasma.

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.

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.

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.

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.

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.

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.

The Thomson-scattering plasma diagnostic technique, which is based on a multipass-laser-probing and interresonator scheme, is used to study the dynamics of lower hybrid plasma heating in the FT-2 tokamak. Results of measurements of the plasma electron temperature and density are presented for two lower hybrid heating regimes with different plasma-energy condinement times.

Fast transient plasma processes are of great interest for high temperature plasma physics. They allow to study important physical phenomena which could be low manifested in steady state plasma. The experimental study of fast transient processes requires appropriate diagnostic techniques. Recently new approach to Thomson scattering based on laser multipass intracavity probing has been developed [1, 2]. It provides the measurements of evolution of electron temperature with high accuracy and repetition rates. The work reports its application to the study of lower hybrid heating of plasma in the FT-2 tokamak and analysis of first results.

The potentialities and design features of a combined diagnostic technique for studying accelerated electrons are considered, The results obtained with this technique in the FT-2 tokamak are presented. The diagnostic method is based ore simultaneous measurements of microwave synchrotron emission in the f(ce)-2f(ce) frequency range and collective emission in the f(p)-2f(p) frequency range, as well as the intensity and energy spectrum of hard X-ray emission, By using these diagnostics, a beam of accelerated electrons with energy up to 1 MeV was detected in the initial stage of the FT-2 discharge. This beam exists during the Ohmic heating stage. For N-e0 < 4 x 10(13) cm(-3), due to fairly large nonuniformity of N-e(r) and ripples of B-T, this beam continuously drives fan instability. In denser plasma, N-e0 > 4 x 10(13) cm(-3), when runaway electrons are freely accelerated up to an energy of several MeV, emission of intense microwave radiation in the frequency range f(p)-2f(p) is observed. It seems that this emission, whose power depends weakly on N-e, is not a result of conversion of plasma modes and may be the thermal-electron bremsstrahlung emission that is amplified up to saturation level by a beam of relativistic electrons moving in the rippled magnetic field B-T. In experiments On the lower hybrid current drive and ion heating, the regime of high-frequency pumping of the plasma is realized in the presence of a beam of accelerated electrons. A partial absorption of the pumping wave by accelerated electrons increases the efficiency of the current drive and leads to excitation of intense plasma oscillations. For N-e0 > 4 x 10(13) cm(-3), when the absorption region is localized near the limiter, the efficiency of lower hybrid heating can decrease.

Low-frequency (<2 MHz) plasma turbulence in the FT-2 tokamak is studied experimentally with the help of a technique based on the study of enhanced scattering and X- and O-reflectometry in the ohmic heating regime during current drive, in the transitional regime from current drive to lower hybrid heating, and during lower hybrid heating. The experimental results are analyzed and compared with the data from CO2-laser light scattering obtained in the same regimes. A characteristic size of the region in which small-scale turbulence is efficiently driven is determined.

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.

In 1994 a new vacuum vessel was installed in TUMAN-3M. During the first experimental run, the device was operated in the ohmic regime. The purpose of the experiments was to check the possibility of achieving ohmic H mode, found previously in TUMAN-3, and to study the conditions of the LH transition and parametric dependences of the energy confinement time in both OH and ohmic H mode. The plasma parameters were R-0 - 0.53 m, a(i) - 0.22 m, I-p - 80-150 kA, B-t - 0.5-0.9 T, n(e) - (0.8-1.8) x 10(19) m(-3). T-e0 - 0.4-0.8 keV and T-i0 - 0.1-0.2 keV. A clear tau(E) dependence on plasma current and no dependence on density are found in OH discharges with a circular limiter configuration. Ohmic H mode was obtained after boronization. Discharges enter H mode operational space from the low density margin. The threshold density slightly increases with plasma current and toroidal field. Input ohmic power substantially exceeds the threshold power derived from the ITER database. The transition time increases with plasma current from 0.5-1 ms at I-p = 80 kA up to 5-7 ms at I-p = 150 kA. The reduction of tau(E) in ohmic H mode under poor vacuum conditions was observed, indicating the direct influence of plasma purity on confinement. In FT-2 tokamak experiments (R-0 = 0.55 m, a(l) = 0.08 m, I-p = 20-40 kA, B-t = 2.2 T, n(e) = (1.0-3.0) x 10(19) m(-3), T-e0 = 0.5 keV, T-i0 = 0.1 keV, P-RF = 150 kW, f(RF) = 920 MHz), improved plasma confinement has been found in three discharge scenarios: lower hybrid heating (LHH), OH alone and current ramp-up (CRU) combined with LHH. Improved confinement was shown by an increase in the particle confinement time, an increase in the energy confinement time (LHH scenario) and an increase in the ion thermal energy confinement (CRU + LHH scenario). Different mechanisms are discussed as possible causes of confinement improvement: electron heating by RF waves, formation of a broadened or a hollow current density profile, an increase in the plasma rotation velocity and damping of plasma microturbulence.

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.