Author: Alessandro Geraldini
Requested Type: Pre-Selected Invited
Submitted: 2018-02-27 16:41:57
Co-authors: F. I. Parra, F. Militello
University of Oxford and Culham Centre for Fusion
1 Keble Road
Oxford OX13NP, 0000000
A distance of a few ion gyroradii from a typical divertor or limiter target, the ion gyro-orbits are travelling along the magnetic field, which makes a small angle α<<1 to the wall. Thus, the ion orbits are moving towards the wall at a timescale that is longer, by a factor of 1/α, than their gyration period. This region is known as the magnetic presheath . The difference in electrostatic potential between the bulk plasma and the wall implies that there is an electric field which usually repels the fast-moving electrons from the wall and accelerates the slower ions towards it. The length scale of the electric field variation is the distance from the wall, which is approximately an ion gyroradius. Hence, the electric field distorts the ion orbits, which are therefore not circular when they are a few gyroradii from the wall .
We exploit the separation of timescales due to α<<1 to solve for the ion distribution function and density in a collisionless magnetic presheath for an arbitrary potential profile. Assuming Boltzmann-distributed electrons, the quasineutrality equation (valid because the Debye length is much smaller than the ion gyroradius) is solved numerically using an iteration scheme. Contrary to previous fluid models, the effect of ion temperature is included in a rigorous way via the distribution function. We obtain the electrostatic potential, ion density, and ion flow profiles, and show that these approach the results of Chodura’s fluid model in the cold ion limit. The distribution function entering the Debye sheath is found to be narrower for decreasing α, with an increasing number of ions travelling near-tangentially to the wall. Sputtering studies require the distribution function since the sputtering yield of an ion depends on its speed and direction of impact with the wall.
 R Chodura, Physics of Fluids, 25, 1628 (1982)
 A Geraldini, F I Parra, and F Militello, Plasma Physics and Controlled Fusion, 59, 025015 (2017)