Lifetime of primary runaway electrons estimated for high-plasma-current fusion devices

For ITER and other high-plasma-current fusion devices, runaway electrons are a matter of concern. These highly accelerated electrons, produced in great numbers during plasma disruptions, can form a runaway beam that hits and damages the wall of the machine.

A recent U.S. initiative called SCREAM (Simulation Center for Runaway Electron Avoidance and Mitigation) combines theoretical models with advanced simulation and analysis to address the runaway problem. As part of SCREAM, two physicists used kinetic analysis to predict the lifetime of primary runaway electrons, reporting the results in Physics of Plasmas.

The authors wanted to understand the distribution of primary runaway electrons by taking into account the interplay of three factors: acceleration by electric field, collisions with plasma electrons and ions, and synchrotron losses. Their analysis dealt with the kinetic equation for relativistic electrons in a straight and homogeneous magnetic field, which they were able to simplify and rescale to highlight its similarity features.

They found that the lifetime of seed runaways increases exponentially with the electric field, with the rate depending on a combination of parameters collectively called “alpha,” that includes the effects of ion charge and synchrotron time scale. For alpha much less than one, the lifetimes can be long when the electric field is only slightly about the renowned Connor-Hastie critical value, when the friction, or drag, on the relativistic electrons from ion collisions becomes energy independent and the electrons can be accelerated continuously. For alpha much larger than one, significantly stronger electric fields are necessary for runaway seed electron survival.

Long-lived runaway electrons have greater opportunity to multiply via an avalanche effect. Knowing the parameter range that creates long lifetimes will inform ITER researchers about what regimes to avoid in planned experiments.

Source: “Lifetime and universal distribution of seed runaway electrons,” by Adrian K. Fontanilla and Boris N. Breizman, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5001931 .