Generation of ultra-short betatron x-rays by laser-accelerated electron beams is of great research interest as it has many applications. In this paper, we propose a scheme for obtaining bright betatron x-rays by applying external wiggler magnetic field in the laser wakefield to resonantly drive the betatron oscillations of the accelerated electrons therein. This results in a significant enhancement of the betatron oscillation amplitude and generation of bright x-rays with high photon energy. The scheme is demonstrated using two-dimensional particle-in-cell simulation and discussed using a simple analytical model.
Controlling beam loading is essential to produce large-charge high-quality electron beams in laser wakefield acceleration. The bunch shape in the longitudinal direction is a key factor to optimize beam loading. However, how to generate an electron beam with an appropriate shape is always a difficult problem to solve. In the present paper, indirect manipulation on the bunch shape is realized by tuning the laser profile. This is because that the bunch shape relies on the initial transverse position of electrons, which can be controlled by the laser profile in the optical injection scheme. Based on this indirect manipulation method, we find that by reducing the focal spot size of the injection pulse and thus effectively inhibiting off-axis electron injection, the electron beam shape gradually changes from down-ramp to flat-top, which is more favorable for optimizing beam loading. Consequently, we use a cigar-like injection pulse to generate electron beams with large charge and low energy spread. In this manner, an electron beam with 0.1% energy spread, 214 p C total charge and 0.3 π m m ⋅ m r a d emittance can be produced.
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