On the generation of runaway electrons and their impact to plasma facing components

Runaway electrons generated by inductive or non-inductive plasma currents in a tokamak have severe interactions with plasma facing materials of a first wall, influencing the first wall structure due to activation and damage. In this paper, modelling of runaway electron generation near the wall in a tokamak is carried out. This includes the evaluation of acceleration along magnetic surfaces for relativistic electrons with energies larger than the runaway threshold. The penetration of runaway electrons of energy ranges from a few MeV to several tens of MeV leads to gamma ray photon production by bremsstrahlung. One of the specific features of their impact on first wall technology is that these runaway electrons give rise to activation due to the giant resonance of the (γ, n) nulcear reaction and, as a consequence, cause a requirement of remote maintenance. Additionally, these runaways induce energy deposition at brazing areas between low Z material and metal, or at a metal itself, leading to melting, cracking and grain growth. The methods to estimate these effects of runaway electrons on plasma facing components are introduced and examples of estimation are given.