An Induction Accelerator of Cosmic Rays on the Axis of an Accretion Disk
  The structure and magnitude of the electric field created by a rotating accretion disk with a poloidal magnetic field is found for the case of a vacuum approximation along the axis. The accretion disk is modeled as a torus filled with plasma and the frozen-in magnetic field. The dimensions and location of the maximum electric field are found, as well as the energy of the accelerated particles. The gravitational field is assumed to be weak. 
A.A. Shatskiy & N.S. Kardashev