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arXiv · 2509.15721

Runaway electrons during a coil quench in stellarators

Pavel Aleynikov·Per Helander·Håkan M Smith
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Abstract

It is shown that avalanches of runaway electrons can arise in stellarators, even if there is no net toroidal current in the plasma or the magnetic-field coils, if the current in the latter varies rapidly enough, e.g. due to a superconductor quench. In present-day devices such as W7-X, significant runaway generation is theoretically possible only at very low gas or plasma density in the vacuum vessel, e.g. between discharges, if a seed population of free electrons is present. In reactor-scale stellarators, more dangerous runaway generation may occur both between discharges and during low-density plasma operation. Since a radiation-induced seed population is necessarily present in an activated device, an accidental coil ramp-down could convert substantial magnetic energy into wall-damaging runaway currents. There is however much more time to mitigate such events than in tokamaks disruptions.

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Pavel Aleynikov, Per Helander, Håkan M Smith. 2025-09-19. Runaway electrons during a coil quench in stellarators. https://doi.org/10.1103/v4s5-p4fr

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