  It is discovered that the tokamak field geometry generates a pitch-angle
scattering effect for runaway electrons. This neoclassical pitch-angle
scattering is much stronger than the collisional scattering and invalidates the
gyro-center model for runaway electrons. As a result, the energy limit of
runaway electrons is found to be larger than the prediction of the gyro-center
model and to depend heavily on the background magnetic field.
