Near-threshold electron injection in the laser–plasma wakefield accelerator leading to femtosecond bunches

Abstract

The laser-plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator.

An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator

“…1). Experimental measurements of the duration that detect transition radiation generated by the beam passing through a metal foil perturb the beam too strongly for simultaneous use with the undulator, however, other studies on ALPHA-X 19 and elsewhere 20 show that the duration of the electron beam within 1 m of the accelerator is $1-2 fs, and this is the basis for the beam transport simulations. Beam profile monitors at positions L1, L2, L3, and L4 comprise phosphor Lanex screens and 12-bit charge-coupled device (CCD) cameras.…”

High-energy coherent terahertz radiation emitted by wide-angle electron beams from a laser-wakefield accelerator

“…The longitudinal diameter of the bubble structure driving LWFAs is about 10-30 μm, depending on laser and plasma parameters [41]. Electrons injected into the bubble typically occupy a small fraction of this size and the electron bunch duration is about 1-10fs [34,35]. CTR produced by such short bunches has a broad spectrum, extending from THz frequencies or longer to the near infrared.…”

“…Leemans et al [31] reported the observation of THz radiation with energies of 3-5μJ sr −1 when forward electron beams with 5MeV energy, with a Maxwellian velocity-distribution and nC-level charge from a laser-wakefield accelerator (LWFA) cross the plasma-vacuum boundary. CTR with spectra extending from microwaves to the near infrared is also produced when high-energy electron beams from both conventional and LWFAs pass through a metallic foil [32][33][34][35]. This paper investigates the production of THz radiation from very high-charge, wide-angle electron beams from a LWFA that are not injected into the wake [36,37].…”

Characterization of non-relativistic attosecond electron pulses by transition radiation from tilted surfaces

“…A prerequisite for applying few-femtosecond or attosecond electron pulses to studies of light-matter interaction is a proper characterization of temporal shape and duration. A widespread method in electron-pulse metrology is a spectrally resolved measurement of coherent transition radiation (CTR) [39][40][41][42][43][44][45][46][47][48][49], i.e. recording and analyzing the electromagnetic fields that are emitted by a multi-electron bunch that hits an interface between media of different refractive index [40,50].…”