Schematic of the experimental setup. The laser pulse with a duration of about 80 fs enters the high density helium gas jet, where self-modulation shapes the laser pulse into short fragments, which trigger quasimonoenergetic electron bubble acceleration.
Profile of gas jet density , plasma frequency , and wavelength , respectively. The directly important physical quantity for wakefield acceleration is the plasma wavelength , scaling as and thus is leveling out partly the peaked density profile as well as possible local density inhomogeneities.
PIC simulation results with ILLUMINATION with a comparably low gas density of . Weak self-modulation of the laser pulse (bottom windows) leads too late to small enough pulse fragments to trigger electron bubble acceleration (top windows). Note that while the 3D simulation box had a size of , much smaller windows are depicted here in order to save space and to focus on the middle of the simulation boxes where the main action takes place.
PIC simulation results with ILLUMINATION with an especially high electron density of . Due to the faster growth rates significant self-modulation of the laser pulse (lower pictures) starts very early and leads to longitudinal and transversal filamentation and the formation of one or even two bubbles [see (c) and Fig. 5 for a zoomed view of the same picture]. Laser depletion is reached before the beam has arrived at the plasma peak density.
PIC simulation results with ILLUMINATION, depicting details from Figs. 4(c) and 4(d) with different color coding. From (a) it can be seen, that both pulse fragments are able to drive a bubble. Soon afterwards, the second bubble dies as a result of fading pulse fragment, while the leading pulse fragment is now short enough to trap and accelerate electrons.
Snapshot of hosing of the laser pulse as observed with ILLUMINATION. Depicted is the laser pulse intensity as in Fig. 4(f) with more suitable color coding and with pulse envelope and centroid (top window).
PIC simulations with ILLUMINATION. A medium electron density of such as in the experiment makes sure that self-modulation, fragmentation, and hosing are neither too strong nor too weak. Distinct but energetic laser pulse fragments are formed which lead to the generation of a comparably stable bubble with a high electron density bunch accelerated inside.
(a) Scaling law prediction from Refs. 24 and 25 for the peak quasimonoenergetic electron energy, and (b) experimentally measured, quasimonoenergetic spectrum.
Comparison of scaling law prediction for the maximum electron energies from Eqs. (4) and (3) with experimentally observed values.
Two different scenarios of generation of multiply spiked electron spectra. (a) Generation of mildly spiked spectra due to the partial decay of an electron-accelerating plasma bubble. Left top corner: measured spectrum, right: spectrum of the electrons inside the green dashed obtained from simulation in the bottom snapshot, which depicts the according plasma density distribution in the simulation. (b) Pronounced electron double peaks can be generated when two laser pulse fragments drive bubbles. Top window: measured spectrum, bottom window: situation as observed in simulations.
Transmitted laser light spectrum. (a) In addition to the peak at the central laser wavelength at 800 nm there is a broad and strong Stokes-shifted light portion. (b) Expected frequency of the Stokes peak in dependence on the local linear (solid line) and nonlinear plasma frequency (assuming ).
(a) PIC simulations [ILLUMINATION (Ref. 20)] show oscillations of the electron bunch inside the bubble in the polarization direction of the laser field (top window) while there are considerably less oscillations in the perpendicular transverse direction (bottom window). (b) Experimental electron signal as detected on an image plate in the polarization direction showing a strong transversal fine structure.
Schematic overview on the accelerating fields of SMLWFA in comparison to other types of accelerators. Due to the high electron densities with SMLWFA, the (longitudinal) accelerating fields can be considerably higher than with LWFA or PWFA, and by many orders of magnitude higher than in conventional rf cavity based accelerators. Only the (transverse) electric fields in focused laser pulses can be even higher.
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