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Horizontal displacement of plasma along the major radius has been found to significantly influence the fluxes of 2.45 MeV DD neutrons and high-energy charge-exchange atoms from neutral beam injection (NBI) heated plasma of the TUMAN-3M tokamak. An inward shift by ΔR = 1 cm causes 1.2-fold increase in the neutron flux and 1.9-fold increase in the charge-exchange atom flux. The observed increase in the neutron flux is attributed to joint action of several factors-in particular, improved high-energy ion capture and confinement and, probably, decreased impurity inflow from the walls, which leads to an increase in the density of target ions. A considerable increase in the flux of charge-exchange neutrals in inward-shifted plasma is due to the increased number of captured high-energy ions and, to some extent, the increased density of the neutral target. As a result of the increase in the content of high-energy ions, the central ion temperature Ti(0) increased from 250 to 350 eV. The dependence of the neutron rate on major radius R0 should be taken into account when designing compact tokamak-based neutron sources.

We consider the possibility of attaining a maximum contribution of energy to plasma due to variation in the impact parameter of a beam of high-energy neutral atoms and in the slope angle of the injection line to the equatorial plane at different parameters of the discharge. The influence of energy of particles on their confinement is also considered. It is shown that particles can be both passing and trapped depending on energy, which is connected with nonconservation of the adiabatic invariant W⊥/B.

We have experimentally studied the influence of toroidal magnetic field (BT) and plasma current (Ip) on the capture and confinement of energetic ions (EIs) formed upon ionization of a neutral beam injected in a tokamak. Based on the results of measurements of the flux of 2.45-MeV fusion neutrons, it is concluded that the amount of EIs significantly grows with increasing BT from 0.7 to 1.0 T and Ip from 140 to 180 kA. In addition to the classical Coulomb slowing down, a supplementary channel of EI losses is found that accounts for a 15% decrease in their confinement time.

The influence of toroidal magnetic field on the energy confinement time τE in the ohmic H-mode has been studied in the TUMAN-3M tokamak with low magnetic field. The experiments were performed at a toroidal magnetic field of BT = 0.68−1.0 T, which is about twice as large as that (0.25–0.5 T) studied in analogous experiments on the NSTX and MAST spherical tokamaks. The results are indicative of a strong dependence of the energy confinement time on toroidal magnetic field: τE ∝ BT0.75–0.8. This scaling is much stronger than that projected for the ITER (τE_IPB98 ∝ BT0.15), while being somewhat weaker than the scalings observed on the NSTX and MAST devices. The stronger (as compared to the ITER scaling) dependence of τE on BT observed in these experiments should be taken into account in designing thermonuclear facilities with small aspect ratios and toroidal magnetic fields—in particular, fusion neutron sources.

The behavior of a geodesic acoustic mode (GAM) in the TUMAN-3M tokamak has been experimentally studied using the heavy-ion beam probing technique. Oscillations of the electric potential under the action of a GAM localized at the plasma periphery have been detected. The GAM was observed in the regime of low confinement (L-mode) with low plasma density (∼0.8 × 1019 m−3) and disappeared upon the transition to a high confinement regime (H-mode). The possible role of GAM as a precursor of the improved confinement (LH-transition) is discussed.

Radial electric field is known to be an important factor affecting transport and confinement in toroidal fusion plasmas. Langmuire probe measurements of peripheral radial electric field evolution in the presence of a rotating MHD island were performed on the TUMAN-3M tokamak in order to clear up the possible connection between the radial electric field and the island rotation, both in L and H-modes. The measurements showed that Er became positive, if the island was large enough, in spite of the constant direction of the island's rotation. Comparing similar ohmic H-mode discharges with or without a rotating MHD island, it was found that in the presence of the large island Er was always more positive. Possible explanations of this observation are discussed.

The experiments described in the paper are aimed at investigating the possible influence of the low frequency magnetohydrodynamic (MHD) activity burst on the Ohmic H-mode in the TUMAN-3M tokamak. During the MHD burst a transient deterioration of improved confinement was observed. The study has been focused on the measurements of plasma fluctuation poloidal velocity performed by microwave Doppler reflectometry. The plasma fluctuation rotation observed before the MHD burst in the vicinity of the edge transport barrier was in the direction of plasma drift in the negative radial electric field. During the MHD activity the measured poloidal velocity was drastically decreased and even changed its sign. Radial profiles of the poloidal velocity measured in a set of reproducible tokamak shots exhibited the plasma fluctuation rotation in the ion diamagnetic drift direction at the location of the peripheral transport barrier. The possible reasons for this phenomenon are discussed.

A regime with electron heat confinement improvement was recently found in the initial phase of discharges in the TUMAN-3M tokamak. An internal transport barrier (ITB) formation in this regime was confirmed by Thomson scattering measurements and by transport modelling. Two possible reasons for the ITB formation are discussed in the paper: by reduction of turbulent transport in the presence of low magnetic shear or by plasma sheared rotation. It is demonstrated that low magnetic shear formation is possible in the current ramp-up phase of the Ohmic discharge. The low magnetic shear does not seem to be the only reason for the transport reduction. Results of Doppler reflectometry measurements of poloidal rotation of density fluctuations are presented. It is found that core confinement improvement correlates with the appearance of sheared rotation of the density fluctuations and with a burst of the MHD activity. The ITB formation in the regime seems to be a result of a combined action of reduced magnetic shear and plasma sheared rotation.

Generation of radial current Ir during the ionization of fast deuterons in their neutral beam injected into a tokamak is considered. The current Ir and force F determined by the [Ir × B] product vary in a complicated manner along the tokamak radius. For this reason, the torque that arises together with force F also nonuniformly varies along the radius, which can lead to a shear in the rotation velocity. A sufficiently large magnitude of this shear can result in reduction of both micro- and macroinstabilities in tokamak plasma [1, 2]. The influence of injected beam characteristics and other experimental parameters on the spatial distribution of Ir in the TUMAN-3M tokamak is considered.

Radial electric field and associated sheared plasma rotation is known to play crucial role in the turbulent transport suppression and improved confinement development. Toroidal and poloidal rotation velocity evolutions at the plasma edge were studied on the TUMAN-3M tokamak using four-tip movable Mach probe. Measurements were performed in two modes of plasma confinement: H-mode transition (featured by peripheral transport barrier formation), and transient internal transport barrier (ITB) formation; both in ohmic heating scenario. The measurements were performed in the vicinity of a last closed flux surface, both in scrape-off layer and core plasmas. In ohmic H-mode, virtually no perturbation of toroidal rotation velocity was observed, whereas poloidal rotation velocity profile underwent gradual evolution after the transition. Surprisingly, poloidal rotation changed not in tact with radial electric field, but followed density gradient built up at the edge transport barrier region. In transient ITB scenario, a noticeable perturbation of toroidal rotation velocity was observed just after the ITB decay, indicating, probably, the outflux of the toroidal momentum due to confinement degradation. The Mach probe data obtained in both the scenarios are compared to available data on plasma density and temperature profiles, radial electric field and rotation velocity of impurity ions.

Results of an experimental study of geodesic acoustic modes (GAM) in the TUMAN-3M tokamak are reported. With Doppler backscattering (DBS) the basic properties of the GAM such as frequency, conditions for the GAM existence and the GAM radial location have been identified. The two-frequency Doppler reflectometer system was employed to reveal an interplay between low frequency sheared poloidal rotation, ambient turbulence level and the GAM intensity. Bicoherence analysis of the DBS data evidences the presence of a nonlinear interaction between the GAM and plasma turbulence.

The results of the experimental study of confinement mode bifurcation performed on the TUMAN-3M tokamak are reported. As a trigger of confinement mode switching, plasma current ramp-up/-down or magnetic compression/decompression is used. It is found that the possibility and direction of confinement mode switching are correlated not with plasma current profile perturbation (peaking or broadening) but with the sign of toroidal electric field perturbation. A model connecting confinement bifurcation and toroidal electric field perturbation through the perturbation of the radial electric field is used to describe the phenomena observed in all eight scenarios investigated. This model ascribes the radial electric field generation to the non-compensated Ware drift of banana electrons at the TUMAN-3M peripheral plasma, where . Measurements of turbulence level and poloidal plasma rotation using microwave reflectometry are used to validate the model.

This paper reports observations of the LH transition at very low density in the experiments on counter-current neutral beam injection (NBI) in the TUMAN-3M tokamak. The transition has been found at a target average density of as low as 0.5 × 1019 m−3, which is lower by a factor of 2.5 than the LH transition density boundary in ohmic and co-current NBI heated plasmas. Relatively low input power in counter-NBI experiments is noticed: P input = 130–270 kW. In contrast, in the case of co-NBI the LH transition is difficult at low density. No transition is possible at the above density with co-NBI P input up to 500 kW. A model that predicts the generation of a negative radial electric field E r , which is thought to help LH transition during counter-NBI, is suggested. The model conjectures the development of the E r and toroidal rotation V ϕ in the presence of large ion orbit losses in the counter-NBI scheme. Measurements of the plasma potential by the heavy ion beam probe technique and edge radial electric field by electrostatic probes indicating negative E r emergence in the counter-NBI scenario are presented. Doppler spectroscopy of B3+ impurity ions has shown an increase in the V ϕ of 16 ± 6 km s−1 after counter-NBI switch on. The measured V ϕ agrees with the above model estimations.

Radial electric field evolution has been studied on the TUAMN-3M tokamak in different modes of operation: ohmic and NBI heating, L- and H-modes, with and without strong MHD activity. Peripheral rad ...