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Passive Focusing of Intense Ion Beams


The radial control of high current ion beams from induction accelerators or generated by short pulsed intense laser-plasma interactions is generally limited by Coulomb repulsion of the ion beam. In this dissertation, we investigate a novel focusing technique for the radial control of intense ion beams. The concept envisions using a stack of thin, closely spaced conducting foils to mitigate defocusing self-electric forces to enable self-magnetic forces to focus.

The ion beam must be energetic enough to penetrate the stack of foils with limited beam degradation from scattering and energy loss. We study beam focusing and scattering and find constraints on the design and performance of such a passive lens.

We further investigate the effects of secondary electrons generated by the impact of ions on foils and show the importance of secondary electrons whose velocity is higher than the ion beam velocity. In this case, current neutralization could exceed charge neutralization, i.e., the self-magnetic focusing would be reduced more than the self-electric defocusing, producing a net defocusing force on the ion beam.

A statistical envelope model is developed and its predictions are compared to those of the PIC simulation code WARP.

An ion beam focused by the stack of thin foils may have potential applications to research fields requiring intense beams, e.g., nuclear fusion, tumor therapy or high energy density laboratory physics. We provide in the appendix an analysis of the expansion of a thin foil heated to the warm dense matter (WDM) regime which can be reached by such beams.

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