Abstract
Ions released from target surfaces by impact of a high intensity and current electron beam can be accelerated and trapped in the beam potential, and further destroy the beam focus. By solving the 2D Poisson equation, we found that the charge neutralization factor of the ions to the beam under space charge limited condition is $1/3$, which is large enough to disrupt the spot size. Therefore, the ion emission at the target in a single-pulse beam/target system must be source limited. Experimental results on the time-resolved beam profile measurement have also proven that. A new focus scheme is proposed in this paper to focus the beam to a small spot size with the existence of back-streaming ions. We found that the focal spot will move upstream as the charge neutralization factor increases. By comparing the theoretical and experimental focal length of the Dragon-I accelerator (20 MeV, 2.5 kA, 60 ns flattop), we found that the average neutralization factor is about 5% in the beam/target system.
Highlights
High resolution x-ray radiography requires the production of a small spot on the surface of a bremsstrahlung converter target by a pulsed high-current relativistic electron beam [1]
A streak camera was used to record the Cherenkov radiation coming from a vertical oriented quartz fiber impacted by the incident electron beam
The influences of the ions to the beam were observed, and we proved that the ion emission in a single-pulse beam/target system is source limited
Summary
High resolution x-ray radiography requires the production of a small spot on the surface of a bremsstrahlung converter target by a pulsed high-current relativistic electron beam [1]. Different metal foils were placed upstream of the quartz fiber to generate ions when interacting with the beam. We have investigated the ion emission and beam spot size variation both experimentally and theoretically.
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