V. M. Timokhin

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.

Spectroscopic diagnostics for ablation cloud of tracer-encapsulated solid pellet in LHD

In the Large Helical Device (LHD), various spectroscopic diagnostics have been applied to study the ablation process of an advanced impurity pellet, tracer-encapsulated solid pellet (TESPEL). The total light emission from the ablation cloud of TESPEL is measured by photomultipliers equipped with individual interference filters, which provide information about the TESPEL penetration depth. The spectra emitted from the TESPEL ablation cloud are measured with a 250 mm Czerny-Turner spectrometer equipped with an intensified charge coupled device detector, which is operated in the fast kinetic mode. This diagnostic allows us to evaluate the temporal evolution of the electron density in the TESPEL ablation cloud. In order to gain information about the spatial distribution of the cloud parameters, a nine image optical system that can simultaneously acquire nine images of the TESPEL ablation cloud has recently been developed. Several images of the TESPEL ablation cloud in different spectral domains will give us the spatial distribution of the TESPEL cloud density and temperature.

Study of carbon pellet ablation in ECR-heated W7-AS plasmas

The ablation of carbon pellets injected into ECR-heated plasmas of Wendelstein 7-AS (W7-AS) has been investigated using a photographic technique with the aim of studying features of the ablation process in stellarator plasmas. The ablation rate profiles were measured assuming a proportionality of the C II radiation and the ablation rate. This assumption was confirmed experimentally and by simulations performed by a quasi-three-dimensional code. The fraction of continuum radiation, which is detected together with the C II line radiation (when using the light filters with finite spectral width), did not affect these measurements.No significant drifts of pellet clouds in the toroidal, poloidal and radial directions were observed in the C II line (723 nm) emission. From fast electron temperature measurements (Δt = 10 µs) using ECE diagnostics, no indications of a pre-cooling wave were found.A difference between the ablation rates for discharges with low and high plasma density, which lies beyond the error bars, was observed. A specific zone of enhanced ablation was detected at the plasma periphery at high density. The appearance of such ablation enhancement, points to the existence of an additional local heat flux. However, the mechanism that could explain the enhanced ablation phenomena at high plasma densities remains unclear.

Study of discharge quenching in the T-10 tokamak by injecting high-Z impurity pellets

The problem of the fast quenching of a discharge in tokamaks by injecting high-Z impurity pellets is considered. Results are presented from experiments in the T-10 tokamak, in which a substantial decrease (up to 70%) in the thermal plasma energy was observed. A one-dimensional transport code is developed to describe tokamak-discharge quenching. The code is used to simulate the experiments on quenching the T-10 discharge. It is shown that the injection of a high-Z pellet into the T-10 plasma changes the transport coefficients as compared to their steady-state values derived from the energy balance or scalings.

Dust mode of carbon pellet ablation in the W7-AS stellarator

A new type of carbon pellet ablation in the high-density ion-heated plasma of the Wendelstein 7-AS stellarator is studied. In contrast to conventional ablation in the form of atoms or clusters, this type of ablation features the emission of small (micrometer) carbon particles from the surface of the pellet. The emission angles and velocities of such microparticles are determined. Their sizes are estimated with two methods. The experimental data are compared with theoretical predictions.

Studies of the Structure of C Pellet Ablation Clouds in W7-AS

The structure of the ablation clouds surrounding carbon pellets injected into the ECR-heated Wendelstein 7-AS plasma has been studied. Snapshot and integrated photographs obtained in the spectral ranges containing the CII (720 ± 5 nm and 723 ± 1 nm) and CIII (770 ± 5 nm) spectral lines were analyzed over a wide range of the bulk plasma parameters. It is found that the cloud luminosity profile along the magnetic field is exponential with either one or two characteristic decay lengths of about a few millimeters and a few centimeters. The smaller length corresponds to the zone closer to the pellet. There is good agreement between the characteristic decay lengths deduced from snapshot and integrated photographs. The characteristic decay lengths were obtained along the entire pellet trajectory and were found to change weakly in the central region and to grow at the plasma periphery (generally, in inverse proportion to the plasma electron density). In the central region, the characteristic decay lengths are about a few millimeters and 1 cm. They depend weakly on the bulk plasma temperature and decrease with increasing bulk plasma density. These lengths agree fairly well with estimates of the ionization length of carbon ions into the C2+, C3+, and C4+ charge states, respectively, assuming that ionization is provided by the hot electrons of the bulk plasma and that the cloud expands with the ion-acoustic velocity at a temperature of ∼1 eV. The results obtained prove that the cloud structure in the vicinity of the pellet is mainly determined by the bulk plasma electrons.

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.

Current shutdown in the T-10 tokamak studied by pellet injection

Processes accompanying in the tokamak discharge shutdown were studied by injecting a large number of heavy-impurity (KCl) pellets (∼1019 atoms) into the T-10 tokamak operating in the ohmic regime. A more than twofold drop in the current at an average rate of 12 MA/s was observed, which corresponded to a considerable (exceeding 75%) decrease in the magnetic energy of the plasma column. The observed phenomena showed evidence of the production of runaway electrons during the discharge current drop.

Local enhanced evaporation of carbon pellets in a Wendelstein 7-AS stellarator

Experiments with second-harmonic electron cyclotron resonance plasma heating performed on a Wendelstein 7-AS stellarator revealed a new regime of the carbon pellet evaporation, whereby sharp peaks with the characteristic spatial width of a few millimeters appeared on the evaporation rate profile. The peaks were observed in a peripheral region of the plasma column for an electron density in the central region below and the heating power above the threshold values. The appearance of these peaks is correlated with the observation of a superthermal emission in the low-frequency part of the cyclotron radiation spectrum. The observed effect is related to the excitation of superthermal electrons by the cyclotron radiation of gyrotrons. The energy and density of the superthermal electron population are estimated at 30–50 keV and 1011–109 cm−3, respectively.

Three-dimensional structure of the cloud of a carbon pellet evaporated in plasma of Wendelstein 7-AS stellarator

Experiments performed on a Wendelstein 7-AS stellarator revealed asymmetry of the cloud of an ablating carbon pellet. The cloud is elongated in the direction of large radius toward stronger fields, that is, opposite to the direction expected due to the toroidal drift. In order to study the effect in more detail, the three-dimensional structure of the cloud was analyzed by computer tomography. The results confirmed asymmetry of the cloud. This asymmetry is related to a drift flow existing in the plasma and directed along large radius toward stronger fields. The drift velocity is numerically estimated at ∼300 m/s.