Abstract
A novel mechanism for injection, emittance selection, and postacceleration for laser wakefield electron acceleration is identified and described. It is shown that a laser prepulse can create an ionized plasma filament through multiphoton ionization and this heats the electrons and ions, driving an ellipsoidal blast-wave aligned with the laser-axis. The subsequent high-intensity laser-pulse generates a plasma wakefield which, on entering the leading edge of the blast-wave structure, encounters a sharp reduction in electron density, causing density down-ramp electron injection. The injected electrons are accelerated to $\ensuremath{\sim}2\text{ }\text{ }\mathrm{MeV}$ within the blast-wave. After the main laser-pulse has propagated past the blast-wave, it drives a secondary wakefield within the homogenous background plasma. On exiting the blast-wave structure, the preaccelerated electrons encounter these secondary wakefields, are retrapped, and accelerated to higher energies. Due to the longitudinal extent of the blast-wave, only those electrons with small transverse velocity are retrapped, leading to the potential for the generation of electron bunches with reduced transverse size and emittance.
Highlights
Laser wakefield acceleration of quasi-monoenergetic relativistic electron bunches [1,2,3] have the potential to reduce the scale, and cost, of future particleaccelerators as the accelerating fields are 3–4 orders of magnitude higher than those achievable in state-of-the-art radio-frequency accelerators.Key challenges for the LWFA scheme are electron injection into, and trapping by, the plasma wave
After the main laser-pulse has propagated past the blast-wave, it drives a secondary wakefield within the homogenous background plasma
In order to better understand the physics underlying the experimental observations, numerical modeling was performed of the effects of the prepulse, the subsequent plasma evolution, the high intensity laser-plasma interaction and subsequent wakefield acceleration
Summary
Laser wakefield acceleration of quasi-monoenergetic relativistic electron bunches [1,2,3] have the potential to reduce the scale, and cost, of future particleaccelerators as the accelerating fields are 3–4 orders of magnitude higher than those achievable in state-of-the-art radio-frequency accelerators. Experimental and theoretical studies [27,28,29,30] have shown that a laser prepulse aligned on-axis significantly improved the divergence of the accelerated electrons, and was able to produce quasi-monoenergetic electron beams when the blast-wave was present In these previous works, the improvement has been attributed to two mechanisms which have a density dependance: below a background number density of ∼3 × 1019 cm−3, sharp-density down-ramp injection at the leading edge is found to be the dominant mechanism [27,28]. A secondary wakefield is generated after the laser has passed the blast-wave structure, the preaccelerated electrons encounter this secondary wakefield and are accelerated to higher energies We show that this two-stage injection and acceleration process is beneficial in three respects; increasing the total charge injected, reducing the bunch transverse size, and reducing the transverse momentum of those electrons which are trapped. Due to the efficiency of the trapping process, electrons can be accelerated in multiple buckets behind the drive laser
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