Plasma suppression and spot size stabilization in single and multiple pulse flash x-ray radiography
Next generation x-ray radiography machines must use high electron beam current to provide the necessary dose and very small spot size to achieve the desired optical resolution. However, this combination of high current in a small area leads to undesirable side effects that were not important in old machines. Specifically, the intense local energy deposition from the high intensity electron beam causes vaporization of the bremsstrahlung target. The hot plasma thus generated provides a copious source of positive ions that are rapidly accelerated into the negative potential well of the incoming electron beam. As the ions propagate upstream, they partially charge neutralize the electron beam, causing its spot size to increase. The authors have studied the electron beam spot stabilization for the Dual Axis Radiographic Hydrotest Facility (DARHT). in 1997 the concept of an electrically self-biased target was developed to limit concept of an electrically self-biased target was developed to limit the length of the charge neutralizing ion column through the use of the electron beam to charge the target negatively. Shortly thereafter, a target chamber based on the self-biased target concept was designed and fielded on the Los Alamos Integrated Test Stand for DARHT. The experiments clearly confirmed the validity of the theoretical concept and the utility of the design to achieve a stable radiographic spot throughout the electron beam pulse. The negatively charged target creates a bias potential large enough to trap the ions in a small spatial region near the target, resulting in a stable radiographic spot. The self-bias target has obvious application for single pulse radiography. However, for multiple pulses, one has to contain the plasma plume evolving from the target. This can be best achieved through the use of a mechanical barrier constructed of low atomic weight, high temperature material, such as beryllium, special alloys, or carbides. This thin barrier is placed about 1-cm upstream from the converter target and allows the electron beam to pass through with minimal scattering and energy degradation. The plasma plume is confined between the target and the barrier. Computer simulation has shown stability of the electron beam spot throughout the pulse. The plasma plume expansion is spatially confined by this design to retain stable spot size for subsequent pulses. Detailed simulation results with experimental validation will be presented.
- Research Organization:
- Los Alamos National Lab., NM (US)
- OSTI ID:
- 20067597
- Resource Relation:
- Conference: 1999 IEEE International Conference on Plasma Science, Monterey, CA (US), 06/20/1999--06/24/1999; Other Information: PBD: 1999; Related Information: In: The 26th IEEE international conference on plasma science, 342 pages.
- Country of Publication:
- United States
- Language:
- English
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