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Restoring betatron phase coherence in a beam-loaded laser-wakefield accelerator

Köhler, A.; Pausch, R.; Bussmann, M.; Couperus Cabadağ, J. P.; Debus, A.; Krämer, J. M.; Schöbel, S.; Zarini, O.; Schramm, U.; Irman, A.

Matched beam loading in laser wakefield acceleration (LWFA), characterizing the state of flattening the accelerating electric field along the bunch, leads to the minimization of energy spread at high bunch charges. Here, we experimentally demonstrate by independently controlling injected charge and accelerating gradients, using the self-truncated ionization injection scheme, that minimal energy spread coincides with a reduction of the normalized beam divergence. With the simultaneous confirmation of the micrometer-small beam radius at the plasma exit, deduced from betatron radiation spectroscopy, we attribute this effect to the minimization of chromatic betatron decoherence. These findings are supported by rigorous three-dimensional particle-in-cell simulations tracking self-consistently particle trajectories from injection, acceleration until beam extraction to vacuum. We conclude that beam-loaded LWFA enables highest longitudinal and transverse phase space densities.

Keywords: laser-wakefield acceleration; beam loading; beam coherence restoration; beam decoherence; betatron radiation; ionization injection; particle-in-cell simulations

Permalink: https://www.hzdr.de/publications/Publ-33103
Publ.-Id: 33103