A.S. Kukushkin

Self-sustained oscillations in the divertor plasma

A simple analytical model for a high recycling edge plasma, which relies on the presence of a small parameter – the ratio of the particle flows crossing the magnetic field to those impinging on the target – is proposed. The concept of a one-dimensional steady state (OSS) is introduced as a zero approximation in the small parameter. The mean number density of the particles – ions plus neutrals – in the magnetic flux tube is chosen as the most representative and convenient parameter of the problem. The OSS is shown to be ambiguous in a certain range for sufficiently high values of the energy flow entering the scrape-off layer from the bulk plasma. This ambiguity is confirmed by 1-D calculations. An equation describing quasi-steady variations in OSS is derived, and a mechanism of exciting self-sustained oscillations is developed. These oscillations are related to peculiarities of the neutral gas transport in the edge plasma and to the ambiguity of the OSS. The results of a 2-D simulation of the edge plasma oscillations are found to be in a good agreement with this mechanism, which might be responsible for the edge localized oscillations observed in H-mode divertor experiments.

Helium transport in the edge plasma of a tokamak reactor

A simple linear diffusive/convective model is proposed for helium transport in the edge plasma of a tokamak reactor. Kinetic effects related to weak collisionality of the plasma are assessed. Results of two-dimensional numerical modelling of helium transport in the edge plasma are presented. Issues of ITER divertor optimization with respect to helium exhaust are considered, with a realistic geometry of the divertor plates taken into account.

Modelling of plasma performance and transient density behaviour in the H-mode access for ITER gas fuelled scenarios

ITER operations require effective fuelling of the core plasma for conditions in which neutral dynamics through the scrape-off layer is expected to significantly affect the efficiency of gas penetration. On the basis of the previous analysis for stationary conditions, pellets are foreseen to provide core fuelling of high-Q DT scenarios. In this paper we present a modelling study of the gas fuelling efficiency in ITER providing an estimate of the maximum plasma density achievable with gas fuelling only in various DT reference scenarios. Dynamical integrated core-edge plasma simulations for various phases of ITER DT H-mode discharges have been carried out with the JINTRAC suite of codes. Simulations of the L-mode phase show that divertor detachment sets the maximum density achievable at the separatrix by deuterium-tritium gas fuelling. The maximum volume-average density is achieved for 15 MA/5.3 T and it is close to the requirement for stationary application of neutral beam injection heating at full power (16.5 MW per injector) and ion energy (1 MeV) compatible with acceptable shine-through loads on the first wall. The achievable density in gas fuelled H-modes is typically a factor of 2–3 larger than in L-modes. The fusion performance of gas fuelled H-modes at 15 MA is typically found to be moderately high (Q ~ 6–8). Sensitivity of the above predictions to modelling assumptions and validation of the physics models are discussed.

Neutral recirculation-the key to control of divertor operation

Interaction of the plasma with neutral gas in the divertor affects virtually all aspects of divertor functionality (power loading of the targets, pumping and fuelling, sustaining the operational conditions of the core plasma). In the course of ITER design development, this interaction has been the subject of intense modelling analysis, supported by experiments on various tokamaks. Neutral gas puffing is found to be the most effective means of divertor control. The results of those studies are summarized and assessed in the paper.

A possibility of wall-protecting cold-plasma layer formation in a tokamak reactor

It is shown that, on the assumption of neoclassical transport processes, a stationary regime of thermonuclear reactions in a thermonuclear reactor is possible, in which case, in the region close to the wall, a protective layer of dense, cold plasma capable of protecting the wall from sputtering is formed.