Multiple pulse electron beam converter design for high power radiography

The typical response of the x-ray converter material to the passage of a hi gh-powered relativistic electron beam is vaporization and rapid dispersal. The effect of this dispersal on subsequent pulses for multi-pulse radiograp hy is the collective effects on the propagation of the electron beam throug h the expanding plasma and the reduced number of electron to photon interac tions. Thus, for the dual-axis radiographic hydrodynamic test facility, the converter material must either be replaced or confined long enough to acco mmodate the entire pulse train. Typically the 1-mm-thick high Z and full de nsity converter material is chosen to give peak dose and minimum radiograph ic spot. For repeated pulses we propose a modified converter, constructed o f either low density, high Z material in the form of foam or of foils space d over ten times the axial thickness of the standard 1 mm converter. The co nverter material is confined within a tube to impede outward motion in radi us outside the beam interaction region. We report single-pulse experiments which measure the dose and spot size produced by the modified converter and compare them to similar measurements made by the standard converter. For m ultiple pulses over a microsecond time scale, we calculate the radial and a xial hydrodynamic flow to study the material reflux into the converter volu me and the resultant density decrease as the electron beam energy is deposi ted. Both the electron transport through the expanding low density plasma a nd beam in the higher density material are modeled. The x-ray source dose a nd spot size are calculated to evaluate the impact of the changing converte r material density distribution on the radiographic spot size and dose. The results indicate that a multiple-pulse converter design for three or four high-power beam pulses is feasible. (C) 2001 American Institute of Physics.