Reduction in runaway avalanche growth rate during beam scrape-off due to drift orbit effects

The avalanche source of runaway electrons (REs) occurs when existing REs have large-angle collisions with thermal electrons, promoting them to REs. This exponentially grows the RE population, posing a critical danger to fusion reactors. Previous KORC work [1] demonstrated that drift-orbit effects, where REs travel radially across magnetic surfaces as energy increases, have a significant impact on new RE generation. High-energy REs have large drift orbits; near a beam's edge, a large fraction of the orbit may be in an open field line region. Newly generated, low-energy REs lack large drift orbits and can be rapidly deconfined if born in open field line regions. This work investigates the reduction in avalanche growth rate as a RE beam scrapes against the first wall. Guiding center KORC simulations will utilize analytic magnetic/electric fields and plasma/impurity profiles. RE beam scrape-off on vertical and horizontal first walls will be examined, mimicking center-post compression and vertically displaced scenarios. Initial RE energies and magnetic field configuration will be varied to change drift orbit size, and marginal and large avalanche growth will be explored. Results can inform reduced RE dynamics models to better estimate generation in future devices.



[1] M.T. Beidler et al., Nucl. Fusion 64, 076038 (2024)