Optical injection dynamics in two laser wakefield acceleration configurations

Abstract

The injection and acceleration dynamics of electron bunches generated by two different optical injection mechanisms, the injection by an orthogonally crossing pulse with perpendicular polarization and injection by a copropagating preceding pulse, are studied by means of 2D numerical particle-in-cell (PIC) simulations. The effect of the ion cavity (bubble) shape variations induced by injection pulses on the electron bunch formation and observable parameters is explored for early injection and acceleration phases. Even if both schemes have three different injection regions, from which three independent electron sub-bunches emerge, the final merged electron bunch does not exhibit a significant substructure in studied parameters as transverse and longitudinal emittance. The 2D PIC simulations also reveal that the final electron bunch parameters are mainly affected by the spatial charge distribution of individual subbunches. Further, the model of the electric and magnetic fields within the slowly evolving ellipsoidal bubble is derived. The electron trajectories in acceleration later stages are analyzed by employing this model for the dynamic changes in the bubble size observed in PIC simulations.

Attosecond betatron radiation pulse train

“…Increase of betatron light by localized injection of a group of electrons in the shape of an annulus was also reported [34]. The X-ray flux can also be increased due to shortening of the betatron oscillation wavelength during the natural longitudinal expansion of bubble [35].…”

Electron self-injection threshold for the tandem-pulse laser wakefield accelerator

“…Notice how electrons from different lateral positions relative to the laser axis can still end up phase-synchronised after injection -a feature studied in some detail in Ref. [48]. Here, electrons are injected and accelerated up to 140 MeV.…”

“…Such a collinear double-pulse scheme was recently examined by Horný et al 48 , using a lower-intensity preceding laser pulse as a means of gaining more control over the self-injected bunch charge and dynamics, potentially leading to higher energy gain and/or bunch charge for electrons and enhanced betatron emission in the nonlinear blowout regime 49,50 . This tandem-pulse scheme, which relies on a double-cavity, has two advantages: first, the leading pulse ensures that the plasma encountered by the 2nd driver is fully pre-ionized; second, the accumulation and recycling of free electrons at the back of the first cavity provides a concentrated source for the second pulse to act on, ultimately enhancing the accelerating field behind it, resulting in higher electron beam energy and current.…”

Laser wakefield accelerator driven by the super-Gaussian laser beam in the focus

“…In order to address this issue, it has been shown that it is possible to harness the transverse injection process by controlled injection mechanisms. For instance, previous particlein-cell (PIC) simulations showed that the bubble expansion can be reached by optical injections, either by the collision of the drive pulse with a less intense counter-propagating pulse [25] or by a less intense preceding laser pulse [26]. Moreover, it is possible to initiate electron injection with a plasma density transition from high to low density [27,28].…”