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

Abstract

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.

A new diagnostic for the measurement of the ionic emission generated by an electron-photon conversion target

“…Heavy Ta+ ions probably exist, but are not observed, because they are expected to reach the foil #2 after the end of the beam pulse. These results confirm the ionic emission hypothesis [1][2][3][4].…”

“…The X-ray spot size broadening effect was observed during the pulse time duration on the prototype facilities ITS at Los Alamos [1] and PIVAIR at CESTA. It was assigned to the emission of both heavy ions issued from the target material [1], [2] and light ions H+ issued from hydrogen atoms contaminating the target [3], [4].…”

“…It was assigned to the emission of both heavy ions issued from the target material [1], [2] and light ions H+ issued from hydrogen atoms contaminating the target [3], [4]. They are produced by the vaporization of the target.…”

Numerical simulations and experiments of beam-target interaction for multipulse bremsstrahlung converter applications

“…Linear induction accelerators (LIA), such as AIRIX, DARHT-I, and DRAGON-I, produce intense x-ray pulse by focusing a short pulse of high current electron beam onto a high Z material target (bremsstrahlung converter target) [1][2][3][4][5][6]. The electrons deposit enormous amounts of energy into the target in a small volume during a very short time period ( $ 60 ns), raise the temperature of the material up to more than 2 eV [7], and lead to a violent hydrodynamic motion inside the target.…”

Intense electron beam disruption due to ion release from surfaces

“…Others have proposed that ions from the foil material are generated after the foil material melts [6]. In the former model, the foil temperature rise leads to thermal desorption of impurities absorbed on the surface that are subsequently ionized [7].…”