Abstract
Emittance posts limits on the key requirements of final pulse length and spot size on target in heavy ion fusion drivers. In this paper, we show studies on the effect of nonlinear space charge on longitudinal emittance growth in the drift compression section. We perform simulations, using the 3D PIC code WARP, for a high current beam under conditions of bends and longitudinal compression. The linear growth rate for longitudinal emittance turns out to depend only on the peak line charge density, and is independent of pulse length, velocity tilt, and/or the pipe and beam size. This surprisingly simple result is confirmed by simulations and analytic calculations.
Highlights
Heavy ion fusion driver designs require, in general, multiple intense ion beams with small spots and short durations to impinge on the target with precise timing
To generate the illumination pattern required by the target design, multiple drift compression beam lines with bends are usually required to transport beams from the multibeam linear accelerators to the target
The pulse length and focal spot sizes at target are fundamentally limited by beam emittances just before the focus
Summary
Heavy ion fusion driver designs require, in general, multiple intense ion beams with small spots and short durations to impinge on the target with precise timing. In the presence of bends, the transverse dynamics is greatly affected by longitudinal momentum dispersion. Head-to-tail velocity tilt required for beam compression, which will lead to centroid offset [1]. Another is the local velocity spread, which will contribute to transverse emittance growth. Both the local velocity spread as well as head-to-tail distortion of the longitudinal phase space can lead to longitudinal emittance growth. The results are compared to an analytic model using an approximation to the problem of the longitudinal space charge self-field of an intense, long beam (‘‘g-factor’’ model) [3,4,5]
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