S.V. Krylov

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption. (Some figures in this article are in colour only in the electronic version)

Conceptual analysis of a tokamak reactor with lithium dust jet

The steady-state operation of tokamak reactors requires radiating a substantial part of the fusion energy dissipated in plasma to make more uniform the heat loads onto the first wall and to reduce the erosion of the divertor plates. One of the approaches to realize this goal uses injection of lithium dust jet into the scrape-off layer (SOL). A quantitative conceptual analysis of the reactor parameters with lithium dust jet injection is presented here. The effects of the lithium on the core and SOL plasma are considered. The first results of developing the lithium jet injection technology and its application to the T-10 tokamak are also presented.

Reduced core transport in T-10 and TEXTOR discharges at rational surfaces with low magnetic shear

It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τE), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ~ 1 × τE.

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and qL<4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/aL≈0.6, where aL is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr∼0 with q≈1 in the barrier region.

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.

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.

Improvement of plasma energy confinement in tokamak under radiative cooling of the edge plasma

Improvement of plasma energy confinement in the T-10 tokamak by injection of impurity gases was studied experimentally. Injection of Ne and He in the ohmic and ECR heating regimes allows one to separate the dependences of energy confinement on the plasma density and on the edge plasma cooling rate. It is shown that the well-known dependence of the energy confinement time on the plasma density is, in fact, the dependence on the radiative loss power. This phenomenon can be explained by plasma self-organization. The experiments are described by a thermodynamic model for self-organized plasma in which the transport coefficient depends on the difference between the actual and self-consistent pressure profiles. The reduction in the heat flux at the plasma edge due to radiative cooling leads to a decrease in the transport coefficient in this region and, accordingly, improves energy confinement. Results of approximate model calculations for experiments with Ne injection are presented.

EFFECT OF NOBLE GAS INJECTION ON DISCHARGE DISRUPTION IN T-10 TOKAMAK

Cрывы разряда в крупных токамаках и будущих реакторах являются серьёзной проблемой, ограничивающей срок их службы. Представлены результаты исследований влияния напуска благородных газов на динамику развития срыва разряда в токамаке Т-10. Срывы разряда вызывались разными способами: наращиванием плотности плазмы до предельной и/или инжекцией дейтериевой или примесной (углеродной) макрочастицы. Инжекция благородных газов (гелия, аргона или ксенона) проводилась как в стационарной стадии для инициации срыва разряда, так и во время спада тока. Обнаружено, что спад тока при срыве разряда в Т-10 происходит в две фазы (медленную и быструю), отличающиеся существенно разным характерным временем. Инжекция дейтериевых макрочастиц при параметрах имеющегося инжектора не влияет на динамику спада тока, а массивный напуск благородных газов при выcоком давлении приводит к переходу спада тока из медленной фазы в быструю и инициации вторичных МГД-возмущений во время не слишком быстрого спада тока, а также к подавлению пучка ускоренных электронов.

Time Evolution of the Ion Temperature in the T-10 Tokamak during Simultaneous Pellet Injection and Electron-Cyclotron Resonance Heating

Results are presented from studies of the time evolution of the ion temperature in the T-10 tokamak in the course of injection of several (up to five) deuterium pellets into a deuterium plasma the electron component of which is heated at the second harmonic of the electron gyrofrequency. It is shown that, at an electron cyclotron heating power of 900 kW, the injection of five pellets is accompanied by an increase in the ion temperature to 1200 keV, which is a record value for T-10. It is noted that energy exchange between the electron and ion components in these experiments is a purely classical, Coulomb process.