A composite target concept for multi-pulse radiography

Multi-pulse radiographic accelerators, such as the DARHT-II facility under construction at Los Alamos National Laboratory, generate X-rays by striking a solid, high-Z “bremsstrahlung converter target” with a tightly focused, high-current, relativistic electron beam. In the process, the converter target is heated to a plasma state and significant thermal expansion can occur during the interval between pulses, depleting the beam focus region of target material and reducing the X-ray output. Beam scatter and various other technical constraints prevent using thicker targets, moving the electron beam to a fresh target region, or moving fresh target material into a fixed beam location. The remaining possibility is to optimize the choice of target material to maximize the effective heat capacity while maintaining dose and minimizing scatter. A kinematic model of a “composite target” of high-Z material in a low-Z matrix is presented here as a candidate for such an optimized target. The thermal destruction of the target cannot be avoided but can be slowed sufficiently to maintain the necessary output dose. Methods of manufacturing such targets, and their radiographic performance, are being tested at the ETA-II accelerator facility at Lawrence Livermore National Laboratory.