Progress in modeling erosion and redeposition on plasma facing materials

Addressing Research and Development Gaps for Plasma-Material Interactions with Linear Plasma Devices

“…However, if the surface is tilted with respect to the magnetic field to very shallow angles, electrons being reflected from the surface or being emitted as secondary electrons will be reabsorbed by the surface to a large fraction after only one gyro-motion. Five degrees seems to be sufficient to reabsorb almost all secondary electrons [75]. Otherwise, the ion energy distribution does not depend on the angle of the magnetic field at all, whereas the gyro-motion strongly affects the impact angle of the ion [75].…”

“…Five degrees seems to be sufficient to reabsorb almost all secondary electrons [75]. Otherwise, the ion energy distribution does not depend on the angle of the magnetic field at all, whereas the gyro-motion strongly affects the impact angle of the ion [75]. For shallow angles of 5 degrees, the angle of incidence of the ions is a distribution function with its maximum at about 20 degrees and a minimum of zero at 0 degrees and larger than 40 degrees.…”

“…As a result, these devices cannot reproduce the physics of sheath formation with an oblique magnetic field as will be present in a reactor divertor. The presence of a magnetic pre-sheath, in particular, determines prompt redeposition [72,73] and affects other PMI processes [74,75], indicating the need for a facility that can produce high-power-flux thermal plasmas without the need to bias the target. At small angles between the magnetic field and the surface, the sheath and pre-sheath will become more important.…”

“…Furthermore, the ion energy of the redeposited tungsten is low, since the mean charge state the sputtered tungsten is ionized to is much lower in the case of oblique incidence sheath [73]. Secondary electron emission can lower the potential drop over the sheath [75], which would effectively reduce the ion energy of the impinging ions and change the erosion yield. However, if the surface is tilted with respect to the magnetic field to very shallow angles, electrons being reflected from the surface or being emitted as secondary electrons will be reabsorbed by the surface to a large fraction after only one gyro-motion.…”