Bubble merging in laser wakefield acceleration as a way towards high-charge 1-GeV class electron beams

Abstract

Laser wakefield acceleration offers ultra-high accelerating gradients, enabling compact sources of multi-GeV electron beams with femtosecond durations and quasi-monoenergetic spectra. In this work, we investigated the nonlinear evolution of plasma bubbles and their effect on electron injection and energy gain, using high-fidelity, multi-dimensional particle-in-cell (PIC) simulations performed with the open-source code SMILEI. A relativistically intense laser pulse ( a0 =7.7, λ0 =0.8 μm, w0 =20 μm, E = 30 J, and τ=30 fs) propagated through a helium gaseous medium with a plateau density of ne =7 × 1018 cm−3 after complete ionization. The simulations revealed complex bubble dynamics, including initial formation, disruption, destruction, and bubble merging, strongly influenced by local longitudinal electric field Ex approaching or exceeding the cold wave-breaking limit EWB. Exceeding the wave-breaking field EWB triggered bubble destabilization, while the bubble merging at the end of the plateau plasma density region extended the acceleration length, sustaining high longitudinal fields on the order of 1 TV m−1 and boosting continuous electron injection. This mechanism produced a dense, quasi-monoenergetic electron bunch with a charge of 1 nC, a peak energy of ∼1 GeV, femtosecond-scale duration, energy spread below 10%, rms angular divergence θy of 7 mrad, and a longitudinal rms beam size of σx=10 μm. Comprehensive analysis of the wakefield evolution demonstrated that exploiting nonlinear bubble evolution and interactions near the wave-breaking limit can enhance both energy gain and electron beam quality, providing actionable strategies for next-generation compact LPAs.