Abstract
Matched beam loading in laser wakefield acceleration, 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 laser wakefield acceleration enables highest longitudinal and transverse phase space densities.
Highlights
The concept of laser wakefield acceleration (LWFA) exploits ultrahigh accelerating field gradients of up to a few hundred Gigavolt-per-meter generated in the wake of a high-intensity laser pulse as it propagates through an optically transparent plasma [1,2]
We experimentally demonstrated that matched beam loading in a laser-wakefield accelerator, identified via its characteristic charge-dependent minimum in beam energy spread, yields a minimum in normalized beam divergence
The experiment relied on control over the injected charge in the self-truncated ionization injection regime and on monitoring the beam diameter inside the plasma via betatron x-ray spectroscopy
Summary
The concept of laser wakefield acceleration (LWFA) exploits ultrahigh accelerating field gradients of up to a few hundred Gigavolt-per-meter generated in the wake of a high-intensity laser pulse as it propagates through an optically transparent plasma [1,2]. Recently it was demonstrated that laser-plasma accelerators can be tailored for minimum energy spread at highbunch charges by reshaping the local accelerating field via matched beam loading [9,10,11,12]. This combination of high charge, essential for the beam loading regime, and the short bunch duration in the range of 10 fs [13,14,15,16,17] results in high peak–current beams exceeding 10 kA. Future applications, such as high-field THz sources [18], laboratory-size beam-driven plasma accelerators [19,20,21] and compact
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