Mechanisms for electron trapping and acceleration in magnetic islands from lab to space

Understanding how magnetic islands (toroidally reconnected magnetic flux tubes) trap and accelerate electrons can improve models for space weather forecasting and disruption mitigation strategies in fusion devices. Recent experiments at the DIII-D tokamak demonstrated that electrons of energies up to 20MeV can be trapped in magnetic islands located in the core plasma. As 20MeV-electrons travel at relativistic velocities (~99.967% of the speed of light), it can be expected that the resulting particle drifts are large enough to prevent the electrons from following smaller features in the magnetic field topology, such as islands. Here we propose that suprathermal, but nonrelativistic, electrons are initially trapped in islands and subsequently accelerated to relativistic speeds through a Fermi-type process. The electrons must be already suprathermal and close-to-collisionless for efficient Fermi acceleration. We calculate the characteristic island width needed to confine suprathermal electrons and the characteristic trapping time needed for acceleration to the electron energies measured in the DIII-D experiments.