Electron beam-target interaction and spot size stabilization in flash x-ray radiography
- Applied Physics Division, Los Alamos National Laboratory, X-PA, MS B259, P.O. Box 1663, Los Alamos, New Mexico 87545 (United States)
A high current relativistic electron beam incident on a high-Z target to produce bremsstrahlung photons for radiographic applications can be subjected to charge neutralization by target plasma ion production due to energy deposition by the electron beam. This partial charge neutralization can lead to premature focusing of the electron beam at a distance away from the target and subsequent radial divergence. Furthermore, as the ion column continues 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 radiographic images. The ion effects were confirmed via particle-in-cell simulations and analysis, and methods were investigated to suppress the growth of the electron beam spot size in single- and multiple-pulse radiographic applications. The concept of a self-biased target was proposed and validated by computer simulation showing that the electron beam can be used in a configuration 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 serves as a barrier to the ions but is essentially transparent to the incoming electron beam. Our study also showed that optimized confinement of plasma ions with the electromagnetic or the mechanical method can provide an additional ion-focusing effect which leads to a desirable further reduction of the beam spot size. (c) 2000 American Institute of Physics.
- OSTI ID:
- 20216083
- Journal Information:
- Physics of Plasmas, Vol. 7, Issue 5; Other Information: PBD: May 2000; ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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