Abstract
Laser wakefield acceleration (LWFA) and its particle-driven counterpart, plasma wakefield acceleration (PWFA), are commonly treated as separate, though related branches of high-gradient plasma-based acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we report on experiments covering a wide range of parameters by using nanocoulomb-class quasi-monoenergetic electron beams from LWFA with a 100-TW-class laser. Based on a controlled electron injection, these beams reach record-level performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10 pC/MeV) and divergence (1 mrad full width at half maximum divergence). The impact of charge fluctuations on the energy spectra of electron bunches is assessed for different laser parameters, including a few-cycle laser, followed by a presentation of results on beam loading in LWFA with two electron bunches. This scenario is particularly promising to provide high-quality electron beams by using one of the bunches to either tailor the laser wakefield via beam loading or to drive its own, beam-dominated wakefield. We present experimental evidence for the latter, showing a varying acceleration of a low-energy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. With the increasing availability of petawatt-class lasers the access to this new regime of laser-plasma wakefield acceleration will be further facilitated, thus providing new paths towards low-emittance beam generation for future plasma-based colliders or light sources.
Highlights
Plasma-based high-gradient wakefield accelerators have attracted significant interest in recent years due to their potential for a significant reduction in the size and cost of future accelerators [1]
Using datasets of hundreds of shots, we demonstrate the influence of beam loading on the spectral shape of electron bunches
While the exact parameters may vary, a similar behavior regarding the spectral shape is expected in all kinds of laser wakefield accelerators with localized injection after entering beam loading, be they driven by fewcycle or petawatt lasers
Summary
Plasma-based high-gradient wakefield accelerators have attracted significant interest in recent years due to their potential for a significant reduction in the size and cost of future accelerators [1]. Indicative signs for a so-called “self-mode transition” [60] from beam-loaded LWFA to pure PWFA have been reported [57,61,62,63,64] In these experiments, the drive laser either depletes in a long plasma target [61,62,64] or diffracts due to ionization defocusing [57], leaving the electron beam as sole driver of the wakefield. While earlier studies on shock-front injection [67,68,69] show only moderate beam charges of up to few tens of pC, our recent experiments using the ATLAS-300 laser system provide unprecedented performance regarding the stability, total charge, spectral charge density, and divergence of the accelerated electron beams In this experiment, the system provides laser pulses with 2 J energy at 27 fs duration, corresponding to a peak power of 75 TW. This result is why, in the following, we deliberately concentrate on data with higher shot-to-shot charge variations, which we attribute to a less stable laser performance
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