L. N. Khimchenko

Intermittent transport in the plasma periphery of the T-10 tokamak

Plasma periphery investigation performed in the T-10 tokamak has shown an essential increase of the perpendicular anomalous particle flux in the scrape-off layer (SOL) with an average plasma density rise. The strengthening of the radial transport is found to occur at an average electron density above a threshold level, which depends on a plasma current Ip. The value of the threshold level is about 0.3 times the Greenwald density. Langmuir probe measurements of SOL plasma parameters indicate that intermittent events can play a significant role in the cross-field transport. Intermittent behaviour of the plasma parameters is associated with formation and propagation of the plasma regions (or structures) with high density. The structures move in radial and poloidal directions. Radial movement is predominantly directed to the vacuum vessel wall in the SOL. The radial velocity of the high density plasma structures reduces from 1000 m s−1 near the last closed flux surface to 200 m s−1 at the wall of the vacuum chamber. The radial size of the structures also decreases with minor radius from 3 to 0.5 cm. The poloidal velocity is equal to 1000–1300 m s−1 and is directed towards an ion diamagnetic velocity; the poloidal size of the plasma structures is 2–3 cm. The observed plasma structures can be responsible for more than 50% of the total radial turbulent particle flux. T-10 results support the hypothesis that intermittent convection rather than diffusion can define the cross-field transport.

First results of biasing experiments on the T-10 tokamak.

The first results of the electrode biasing experiments performed on the T-10 tokamak are presented. Positive voltage applied to an electrode inserted inside the tokamak limiter results in the H-mode transition characterised by a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy lifetime. Growth of core electron and ion temperatures during the electrode biasing implies the formation of the thermal barrier in addition to the particle transport barrier.

Long-range correlations in the structure of fractal films

Hierarchically granulated films formed on a material subjected to the action of a high-temperature plasma have been microscopically studied. The statistical self-similarity of the structure of the surface from nanoscale to macroscale, as well as long-range correlations in the relief that are due to the statistically inhomogeneous structure of granularity, has been revealed.

Peculiarities of electrode biasing experiments on the T-10 tokamak in regimes with Ohmic and ECR heating

Two types of electrode biasing H-mode were observed on the T-10 tokamak in regimes with electron–cyclotron resonant heating (ECRH). These types differ mainly by the dynamics of the electron temperature. Both types are characterized by a strong (130 V cm−1) radial electric field formation, a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy confinement time. However, distinguishing features of the type II H-mode are a time delay of 70–80 ms in the electron temperature growth, a slower increase of the plasma density, and a weak drop of Dα intensity. The differences between the two types of biased H-mode are likely to be connected with the conditioning of the vacuum chamber.

Internal transport barrier formation in experiments with reverse magnetic shear in T-10 tokamak

T-10 experiments with electron internal transport barrier (ITB) formation in discharges with reverse shear q(r) profile are described. Reverse magnetic shear was formed in the central region characterized by r/aL ≤ 0.3. It is shown that electron thermal conductivity decreases essentially in comparison with the value typical for the L-mode. It is found that degradation of the ITB correlates with development of MHD activity in the internal part of the plasma column.

High density experiments with gas puffing and ECRH in T-10

High density experiments were carried out in T-10 with gas puffing and electron cyclotron resonance heating (with absorbed power value up to 1.4 MW) with oblique and perpendicular power launch. Densities exceeding the Greenwald limit (nGw) by up to a factor of 1.8 were achieved in a regime with a high value of the edge safety factor at the current flat-top, q(a)8.2. The decrease of q(a) to a value of 3 led to the reduction of the ratio (e) lim /nGw to 1. Confinement degradation with density increase was not significant up to the density limit. However, the typical T-10 linear increase of energy confinement time with density saturates at e≥0.6nGw. This saturation is the result of the development of an additional transport in the electron heat channel. However, the saturated τE values exceeded the ITER L-mode scaling predictions by up to a factor of 1.2 and were close to the value predicted by the ITER H-mode scaling. Effect of the strong gas puffing on the plasma confinement and experiments with neon seeding are also discussed in this paper.

Threshold effects in the interaction of a plasma with injected pellets in the T-10 tokamak

Sharp changes (peaks and dips) of the radiation signal from carbon pellets injected into the plasma in the tokamak are related to the level of introduced disturbances. The threshold size (near 0.3 mm) is determined beginning with which a pellet in the ohmic plasma of the T-10 tokamak initiates tearing processes in a region with q < 2 and Kadomtsev reconnection in the central region with q = 1. A model describing ablation under the conditions of pellet-induced reconnections is proposed. This model satisfactorily describes the observed shape of the ablation rate curve.

Investigation of the H-mode during ECRH in the T-10 tokamak

An improved confinement regime with an external transport barrier (H-mode) is obtained during electron-cyclotron resonance heating of a plasma in the T-10 tokamak. A characteristic feature of this regime is a spontaneous density growth accompanied by a drop in the intensity of Dα line and an increase in βp by a factor of ∼1.6. The threshold power for the L-H transition is close to that predicted by the ITER scaling. The best characteristics of the H-mode are achieved with decreasing qL to 2.2. It is shown that the external transport barrier arises for electrons, whereas the heat transport barrier insignificantly contributes to improved confinement.

INFLUENCE OF PERIPHERAL RADIAL ELECTRIC FIELD ON L-H TRANSITION IN T-10 TOKAMAK *)

In this paper the peripheral plasma behaviour during and after L-H transition on the tokamak T-10 is considered. Abrupt changes of Scrape-Off Layer (SOL) plasma parameters (ion saturation current, floating potential, electron temperature) are observed in regimes with transition to the improved plasma confinement. Such changes begin (5–150) ms prior to the L-H transition. The time delay of a transition depends on auxiliary plasma heating powerPaux and plasma currentIp (at constant toroidal magnetic field inductionBt).