Beam-target interactions in single-and multi-pulse radiography

This report describes calculations concerning the interaction of intense electron beam pulses with a solid target. In Section 2, we treat the propagation of a beam pulse through a dense plasma plume in front of the target, resulting from material blown off from the target by prior pulses. Because of the short magnetic decay-time, the primary effect of the plasma is to shift the focal spot of the beam longitudinally by an amount which is constant over most of the beam pulse. It may be possible to compensate for this effect by changing the upstream focusing elements from one beam pulse to the next. Section 3 describes a mechanism by which lighter ion species can diffuse to the surface of a plasma plume, thereby potentially increasing the concentration of bulk contaminant species such as hydrogen at the leading edge of the plume. These ions could then become a light-ion source for subsequent beam pulses. Based on the calculations, we tentatively recommend bulk contaminant fractions be limited to 10{sup -5}10{sup 4}. In Section 4, we estimate the number of adsorbed monolayers needed to provide a space-charge-limited (SCL) ion source at the target for the initial beam pulse. We find that {approx} 10 monolayers are required for SCL emission of H{sub 2}{sup +} ions. This may explain why there was little evidence of focus disruption in ETA-II target experiments