S. Kuhn

Effect of E × B Drift on the Plasma Flow at the Magnetic Presheath Entrance

The boundary condition for ion fluid velocity at the magnetic presheath entrance is derived in the presence of the E x B drift. This condition takes into account gradients of the drift velocity and differ from the classical Bohm-Chodura condition.

Kinetic (PIC) simulations of the magnetized plasma–wall transition

We give a brief review of different particle-in-cell (PIC) simulation methods used for studying the magnetized plasma–wall transition (PWT), and present new results on the steady state multi-ion-component PWT.

Including the effect of secondary-electron emission at the divertor targets in code modelling

We present a self-consistent method to include in edge plasma simulations the mutual interactions of the secondary electrons emitted by a solid divertor target and the near-target plasma properties. Based on a semi-analytical model, our procedure accounts for the re-absorption of the secondary electrons hitting the wall during their first gyro-period. Theoretical considerations and numerical simulations performed with the B2Solps-5.0 code show that, for parameters in the operational range of present-day tokamaks, the effective electron-induced secondary-electron emission coefficient can be quite small. However, it increases sharply where the electron temperature has a maximum, i.e. near the separatrix. We compare the results of the self-consistent determination of secondary-electron emission with the commonly adopted constant-value assumption. The exact profile of the secondary-electron emission turns out not to have a dominant influence, at least for cases of low temperature at the plates, which have the biggest experimental relevance. The main limitations of our model, in particular the exclusion of roughness effects, are also briefly discussed.

Kinetic and fluid study of classical transport phenomena in scrape-off layer plasmas

Fluid modeling of the Tokamak edge region is a well-developed tool, employed both to understand the processes governing the edge plasma and to support the design of present and future experiments. However, this approach suffers from internal inconsistencies, due to the relevant effects induced by kinetic phenomena. The kinetic method is, in principle, general, but cannot be applied to regions of macroscopic size, due to its computational requirements. In this paper we apply both the fluid and kinetic approaches to similar plasma edge model problems, in order to point out their relative strengths and weaknesses.

Computational Modelling of a Purely Neoclassical Scrape-Off Layer

We study the Scrape-off Layer of ASDEX Upgrade in a scenario where the neoclassical transport phenomena are dominant with respect to the anomalous contributions. To represent such a neoclassical scenario, we implemented the expressions for the energy and momentum radial transport, toroidicity effects being introduced automatically by the tokamak geometry. Correspondingly, we reduced the anomalous transport coefficients, and accounted partially for drift effects. An extremely large difference between the two divertor plates was found, especially regarding the electron energy flux and temperature profiles.