Abstract

Abstract A relativistic electron probe has been developed in the DIII-D tokamak, capable of simultaneously
resolving pitch angles and energies of runaway electrons (REs) for the first time. Due to relativistic
speeds of REs, their gyro-orbit size becomes comparable to those of fast deuterium with energies in
tens of keV range. This allows for the measurement of RE strike images on a phosphor plane, with
their orbits being deflected by the Lorentz force as they pass through a pinhole aperture. The strike
positions correspond to the energies and pitch of the incident REs. Monte Carlo N-Particle Transport
Code has shown that an ultra-thin phosphor coating significantly reduces energy deposition from
γ-rays, while allowing much greater deposition from REs, minimizing the background noise. The
novel system, developed for DIII-D tokamak, is expected to provide unprecedented insights into the
phase-space dynamics of REs.