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Stellarators are among the leading concepts for future fusion reactors, with a key advantage over tokamaks being their practical immunity to large-scale disruptions. The work [1] shows that a rapid shutdown of stellarator coil currents - leading to fast dissipation of poloidal magnetic flux - can nonetheless generate a strong toroidal electric field and drive a runaway-electron avalanche, even without any interruption of the net toroidal plasma current. Simultaneously, the E×B drift displaces these energetic electrons outward. Consequently, the runaway population drifts to the wall and can deposit energy while carrying an appreciable toroidal current. Thanks to the much longer quench timescales the problem is far less serious than in a tokamak.
For parameters representative of Wendelstein 7‑X, it is found that during routine plasma operation the avalanche multiplication remains weak, implying a negligible runaway hazard. The more vulnerable phase is coil ramp-down between discharges, when neutral density is low and collisional friction is reduced; the outcome then depends critically on whether even a small seed population is present. A straightforward preventive measure is to maintain sufficiently high neutral-gas pressure between discharges.
In reactor-scale stellarators, more dangerous runaway generation may occur due to stronger induced fields. 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. Some form of dedicated intervention is therefore likely to be necessary, although there is much more time to mitigate such events than in tokamak disruptions.
[1] Pavel Aleynikov, Per Helander, and H\aa kan M. Smith, Phys. Rev. Applied, Accepted 2 January, 2026 https://doi.org/10.1103/v4s5-p4fr