  We analyze the dynamics of fast electrons in plasmas containing partially
ionized impurity atoms, where the screening effect of bound electrons must be
included. We derive analytical expressions for the deflection and slowing-down
frequencies, and show that they are increased significantly compared to the
results obtained with complete screening, already at sub-relativistic electron
energies. Furthermore, we show that the modifications to the deflection and
slowing down frequencies are of equal importance in describing the runaway
current evolution. Our results greatly affect fast-electron dynamics and have
important implications, e.g. for the efficacy of mitigation strategies for
runaway electrons in tokamak devices, and energy loss during relativistic
breakdown in atmospheric discharges.
