Electron beam-target interaction and spot size stabilization in flash x-ray radiography

A high current relativistic electron beam incident on a high-Z target to pr oduce bremsstrahlung photons for radiographic applications can be subjected to charge neutralization by target plasma ion production due to energy dep osition by the electron beam. This partial charge neutralization can lead t o premature focusing of the electron beam at a distance away from the targe t and subsequent radial divergence. Furthermore, as the ion column continue s to expand, the focal point moves upstream along the path of the electron beam, causing the beam spot on the target to grow in time. The increase in radiation spot size is detrimental to the spatial resolution of radiographi c images. The ion effects were confirmed via particle-in-cell simulations a nd analysis, and methods were investigated to suppress the growth of the el ectron beam spot size in single- and multiple-pulse radiographic applicatio ns. The concept of a self-biased target was proposed and validated by compu ter simulation showing that the electron beam can be used in a configuratio n to establish an electric potential between the target and the collimator. This potential can effectively trap the ions, limit the ion column length, and thereby maintain the electron beam spot size. Another approach is the placement of a thin metallic foil at 1-2 cm in front of the target, which s erves as a barrier to the ions but is essentially transparent to the incomi ng electron beam. Our study also showed that optimized confinement of plasm a ions with the electromagnetic or the mechanical method can provide an add itional ion-focusing effect which leads to a desirable further reduction of the beam spot size. (C) 2000 American Institute of Physics. [S1070-664X(00 )93405-0].