  Relativistic particle acceleration in collisionless shocks of supernova
remnants is accompanied by magnetic field amplification from cosmic ray (CR)
driven plasma instabilities. Bell's fast CR-current instability is predicted to
produce turbulence with a non-zero mean electric field in the shock precursor.
We present a Monte Carlo model of Fermi shock acceleration explicitly taking
into account an effective mean upstream electric field. Our model is nonlinear
and includes the backreaction effects of efficient Fermi acceleration on the
shock structure.
