Kinetic Effects in the Scrape Off Layer

Abstract

The scrape off layer (SOL) of a confined plasma (Fig. la) is in many respects in a state far from thermodynamic equilibrium: -The energy diffusing outward from the plasma core across the separatrix is condensed in the thin SOL to a strong energy flux along the magnetic field.-The recycling of cold neutrals from the limiter or divertor target gives rise to a strong temperature gradient from the hot midplane region to the relatively cold recycling zone.-For long mean free path length in the plasma relative to the extension of the recycling zone the thermalization in the recycling zone may be incomplete.-The potential gradient length in the sheath in front of the target is small as compared to the mean free path length; the velocity distributions of ions and electrons as determined by the electric field are far from being Maxwellians there.Thus the follwing topics will be discussed in this paper:-Parallel transport (electron heat flow, ion viscosity) -Recycling -Sheath structure (grazing incidence of magnetic field, secondary electrons).-Boundary conditions for fluid models Numerical illustrations for these kinetic effects are given from a Id particle-in-cell code including In studying kinetic effects in the SOL one is interested in basic features in velocity space rather than in an exact modelling of the spatial situation. The higher dimensionality due to inclusion of v-space is achieved on the expense of a simplified geometry in configuration space: The axially symmetric SOL is straightened to a prism limited by two target plates a3 shown in Fig. lb

Modification of the electron energy distribution function during lithium experiments on the National Spherical Torus Experiment

“…A variation in the gradients in the SOL adjacent to the PFC could also contribute to a change in the measured distribution. In this case, an increase in the hot-electron fraction implies the PFC is absorbing more incident plasma [16]. Both effects are the subject of ongoing studies.…”

“…In regions with steep gradients in plasma properties of density and temperature it is possible for these high-energy particles to originate in regions further away (hence the term "non-local") than the slower particles. These non-local effects were the chief concern in the works by Chodura [16] and Batishchev [17]. The degree of collisionality in the plasma is given by the ratio of a typical scale-length of the plasma with the mean-free-path of a given charged-particle population.…”

“…The collisionality is the ratio of the system scale-length with the mean-free-path of particle. In the work by Chodura [16], it was found that SOL collisionalities of about 10 would be insufficient to completely eliminate kinetic effects and that values of ν * ee 100 with respect to the temperature scale-length are necessary. This is due to the fact that the parallel heat-flux is carried in electrons of energies ≈ 10T e .…”

“…In simulations of charged particles in the SOL, non-Maxwellian EEDFs are a common feature. Independent simulations by Chodura [16] and Batishchev [17] both show that sharp spatial gradients arising from a recycling boundary were found to give rise to non-Maxwellian distributions at a divertor target plate. In addition to gradient effects, electron interactions with neutrals and ions can also lead to non-Maxwellian distributions [18].…”

ASCOT simulations of electron energy distribution at the divertor targets in an ASDEX Upgrade H-mode discharge

“…Measurements of the sheath potential typically rely on the assumption of a Maxwellian electron energy distribution and may fail to give correct results in the presence of collisionless electrons (see [8] and references therein). Furthermore, hot electrons with velocities v = 3 − 5v th contribute most to the parallel electron heat flux [4,9] and, thus, even small deviations from the Maxwell-Boltzmann distribution significantly affect the target temperature, density and heat deposition, ultimately influencing the SOL transport. Recently, these kinetic effects have been of considerable concern in edge modelling [10].…”