Abstract
Beam degradation is examined in a laser-plasma accelerator stage with a parabolic plasma channel when the laser pulse and/or the electron beam enters the channel off axis. Betatron oscillations in the beam become incoherent, resulting in a net increase of beam emittance through phase mixing. A quantitative model for transverse emittance growth due to misalignment in multistage accelerators, valid in the linear regime, is presented and compared with particle-in-cell simulations. The model is applied to a chain of laser-plasma accelerator stages, and tolerances are derived on the initial energy spread of the electron beam and misalignment in the multistage structure, with repercussions in high-energy physics applications of laser-plasma accelerators.
Highlights
Progress made in the past decades allowed laser-plasma accelerators (LPAs) to create electron beams with increasing energy and brightness [1,2,3,4,5]
We present a model of transverse emittance growth due to transverse misalignment of the electron beam and/or the laser pulse driver in a plasma channel with a parabolic transverse density profile, where electron acceleration is taken into account
Several mechanisms responsible for emittance growth are present when both the laser pulse and the electron beam are off axis: (i) The energy spread in the electron beam causes betatron oscillations to decohere, which leads to emittance growth. (ii) Accelerated electrons follow the laser, so the electron beam performs the same slow transverse oscillations as the laser pulse, with period λ
Summary
Progress made in the past decades allowed laser-plasma accelerators (LPAs) to create electron beams with increasing energy and brightness [1,2,3,4,5]. The present article focuses on emittance growth due to laser or beam transverse misalignment at the entrance of an LPA stage. This model is, in principle, applicable to any type of accelerator provided the assumptions in Section II are satisfied. Connections between this work and the study of emittance growth in conventional accelerators are briefly discussed in the conclusion
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