Taming Runaway Electrons from Fusion Plasma
Written by Tim Lash, Focus Fusion Society Contributor. Edited by Ignas Galvelis, Supervising Director.
A team of European scientists has published findings revealing a better understanding of plasma dynamics allowing tokamak designs to take another step forward. In tokamak reactors, a catastrophic effect can occur known as runaway electrons. In these scenarios, free electrons in the plasma can form coherent currents of as much as one million amperes. These electron currents have the potential to breach the plasma containment fields and cause serious damage to the reactor.
In a recent Letter to Physical Review, the team describe a method for countering such rogue currents thereby protecting the reactor from damage. Their method entails injecting neon (atomic number 10) or argon (atomic number 18) ions into the reactor. The heavy ions interact with runaway electrons significantly dissipating their energy. This research explored the Coulomb forces exerted by the ions on these electron beams.
Prior to these results, most calculations used the net ionic charge of the partially ionized argon and neon to model particle interactions. Since these ions will be weakly ionized in the plasma, it was assumed electrons would only experience Coulomb forces from the net ionic charge. This could be as little as +1 eV. However, this research demonstrates that the relativistic electrons in the runaway currents can penetrate the ion electron cloud and experience the full force of the total nuclear charge. The fast electrons could see the full nuclear charge of +10 eV (for neon) or +18 eV (for argon). Electrons interacting with the nucleus in this manner experience increased scattering and drag. These nuclear interactions decelerate the beams much more than previously expected when accounting only for the net charge carried by the ion.
Using only the net ionic charge to account for behavior of runway electrons underestimates the dampening power of these injected ions in two ways. First, elastic collision momentum transfer is underestimated since the full nuclear charge can be available to influence the electron. Secondly, inelastic electron collisions that excite the heavy ions occur at higher than expected rates due to these significant nuclear incursions. This has the effect of increasing the electron density of the plasma from the perspective of the fast electrons. Each effect exposes these spurious currents to significant momentum transfer targets, and can effectively quench dangerous electron flows.
Even with the tremendous time and resources devoted to magnetic confinement in tokamak devices, this current research highlights the continuing difficulties tokamaks have containing high temperature plasma for fusion reactions. Devices whose design accounts for, and works with, the inherent instability of plasma are simpler and can be made more inexpensively.