Experiment vs. theory on electric inhibition of fast electron penetration of targets

A dominant force of inhibition of fast electrons in normal density matter is due to an axially directed electrostatic field. Fast electrons leave the critical density layer and enter the solid in an assumed relativistic Maxwellian energy distribution. Within a cycle of the solid density plasma frequency, the charge separation is neutralized by a background return current density j{sub b} = en{sub b}v{sub b} equal and opposite to the fast electron current density j{sub f} = en{sub f}v{sub f} [1] where it is assumed that the fast electron number density is much less than the background number density, n{sub f} << n{sub b} [2]. This charge and current neutralization allows the forward moving fast electron current to temporarily exceed the Alfven limit by many orders of magnitude [3]. During this period the cold return current, in passing through the material resistivity, ohmically generates an electric field in opposition to the fast current. As a result, the fast electron current loses its energy to the material, via the return current, in the form of heat [4]. So, although the highly energetic electrons suffer relatively little direct collisional loss of energy (owing to the inverse relation of the Coulomb cross section to velocity), their motion is substantially damped by ohmic heating of the slower return current. The equation for the ohmically generated electric field, E, is given by Ohm's law, E = j{sub c}{eta} where {eta} is the material resistivity