On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Numerical solution of momentum balance equations for plasmas with two ion species

In plasmas bounded by material surfaces the Bohm criterion has to be satisfied at the entrance of the Debye sheath near the surface. With a single ion species this constraint prescribes a boundary condition for the momentum balance equation governing the ion mass velocity. If, however, several ion species are present a generalization of the Bohm criterion does not provide enough number of boundary conditions. Additional "intermediate" conditions follow from the requirement that spatial derivatives of the ion velocities are finite everywhere within the plasma volume. The amount of such independent conditions is sufficient to determine, in an iterative way, also the position in the plasma where they have to be imposed. A numerical approach to find unique regular solutions of fluid motion equations, satisfying the generalized Bohm criterion at the plasma boundary, is elaborated and realized for the case of two ion species. (C) 2011 Elsevier Inc. All rights reserved

Modification of local plasma parameters by impurity injection

Plasma behavior in the vicinity of strong sources of impurities is considered. A model, based on a fluid description of electrons, main and impurity ions and taking into account the plasma quasi-neutrality, Coulomb collisions of background and impurity charged particles, radiation losses, sinks of particles to bounding material surfaces, is elaborated. Particle, momentum and energy balances are deduced by integrating transport equations within the clouds of neutral and singly charged impurities, both inside and beyond the scrape-off layer of the puffing limiter. These provide algebraic non-linear equations for the characteristic densities and temperatures of the plasma components in the cloud and for the cloud dimension itself. Computations are done for the conditions of impurity seeding experiments in the tokamak TEXTOR. The model allows us to simulate two-dimensional images of radiation losses which can be directly compared with experimental observations

Optimization of isotope composition of fusion plasmas

The radial profiles of deuteron and triton densities in a fusion tokamak reactor are computed by taking into account anomalous transport due to ion temperature gradient and trapped electron drift instabilities in the plasma core and neoclassical transport in plateau-banana regimes, residual anomalous diffusion and convection triggered by edge localized modes in the edge transport barrier. The charged particle sources due to influx of neutral particles through the separatrix and ablation of frozen pellets in the plasma interior are optimized by searching for conditions providing the minimal outflow of unburned tritons to divertor target plates and their exhaust by pumping systems. It is demonstrated that such conditions correspond to fuelling of deuterium plasma component with gas puffing and of tritium with pellets and a ratio of deuteron/triton densities close to 2/1 at the separatrix. Effects of the absolute level and the ion mass dependence of neoclassical and anomalous transport contributions, of the position where neutrals enter the confined volume through the separatrix on the radial profiles of the ion densities are investigated