Computational and experimental studies of the beam-target interaction for high-dose, multi-pulse radiography

The conversion of an intense relativistic electron beam into x-rays for radiographic imaging is achieved through the bremsstrahlung process of electrons in a target of optimal thickness. To achieve desirable resolution for thick objects, an extremely high-brightness electron beam is used, and a significant amount of beam energy can be deposited in a small area of the target. Vaporization of the target material and expansion of the resultant plasma can occur. In a multi-pulsing design, which will resolve dynamic behavior of the object, the expanding plasma can have an effect on the quality of subsequent electron beam pulses. The evolution of the plasma was investigated using a two-dimensional Eulerian magnetohydrodynamic code. The driving, or initial, condition for the plasma is the energy deposited in the target by the electron beam. Because the spatial and temporal beam energy deposition profiles can affect the plasma dynamics several deposition models were tested. Experiments at Los Alamos' Integrated Test Stand (ITS) have characterized the expanding plasma for several target materials using a 5.25-MeV, 3.8-kA, 4-mm-diameter electron beam. Measurements such as axial expansion velocity helped benchmark the code and validate the deposition modes. Using the models that showed best agreement with the ITS experiments, calculations were done for a planned upgraded facility (20-MeV, 4.5-kA beam, spot size reduced by 1/4) to evaluate a multi-pulsing scheme in which the beam is moved to clean sections of the target for subsequent pulses. To minimize electron beam steering, means of confining the target plasma were also explored