Abstract
Recent advances in laser-plasma accelerators, including the generation of GeV-scale electron bunches, enable applications such as driving a compact free-electron laser (FEL). Significant reduction in size of the FEL is facilitated by the expected ultrahigh peak beam currents (10--100 kA) generated in laser-plasma accelerators. At low electron energies such peak currents are expected to cause space-charge effects such as bunch expansion and induced energy variations along the bunch, potentially hindering the FEL process. In this paper we discuss a self-consistent approach to modeling space-charge effects for the regime of laser-plasma-accelerated ultracompact electron bunches at low or moderate energies. Analytical treatments are considered as well as point-to-point particle simulations, including the beam transport from the laser-plasma accelerator through focusing devices and the undulator. In contradiction to non-self-consistent analyses (i.e., neglecting bunch evolution), which predict a linearly growing energy chirp, we have found the energy chirp reaches a maximum and decreases thereafter. The impact of the space-charge induced chirp on FEL performance is discussed and possible solutions are presented.
Highlights
Since the demonstration in 2004 of high-quality electron beams generated by laser-plasma accelerators [1,2,3], there has been a growing interest in this field and the potential applications of these beams
For large t > t0, x scales approximately linearly, while in the initial stage for t < t0 the ‘‘debunching’’ has not developed yet. This result is important for the interpretation of the temporal behavior of the energy chirps found in the particle simulations, as discussed
A large energy variation along the bunch implies that the radiation emitted by electrons will not be in resonance with electrons further ahead in the beam, if the energy chirp is larger than the free-electron laser (FEL) bandwidth
Summary
Since the demonstration in 2004 of high-quality electron beams generated by laser-plasma accelerators [1,2,3], there has been a growing interest in this field and the potential applications of these beams. Recent publications on laser-plasma-accelerator-based FELs [6] have discussed the benefits of such ultrahigh peak currents, such as drastically shrinking the length of the FEL undulator by 1 or 2 orders of magnitude, allowing for socalled ‘‘tabletop’’ FELs. On the other hand, such ultracompact electron bunches are subject to large space-charge effects. Previous work on space-charge dynamics [32,33] has focused on large energy spread beams generated in the self-modulated regime of laser-plasma accelerators. Recent publications [34,35] on space-charge effects in laser-plasma-accelerators considered non-self-consistent, perturbative approaches to examine the space-charge induced energy variation along the bunch and found that the energy chirp grows linearly with propagation distance.
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