Three electron beams from a laser-plasma wakefield accelerator and the energy apportioning question

Abstract

Laser-wakefield accelerators are compact devices capable of delivering ultra-short electron bunches with pC-level charge and MeV-GeV energy by exploiting the ultra-high electric fields arising from the interaction of intense laser pulses with plasma. We show experimentally and through numerical simulations that a high-energy electron beam is produced simultaneously with two stable lower-energy beams that are ejected in oblique and counter-propagating directions, typically carrying off 5–10% of the initial laser energy. A MeV, 10s nC oblique beam is ejected in a 30°–60° hollow cone, which is filled with more energetic electrons determined by the injection dynamics. A nC-level, 100s keV backward-directed beam is mainly produced at the leading edge of the plasma column. We discuss the apportioning of absorbed laser energy amongst the three beams. Knowledge of the distribution of laser energy and electron beam charge, which determine the overall efficiency, is important for various applications of laser-wakefield accelerators, including the development of staged high-energy accelerators.

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

“…Therefore, the angular distribution and spectrum of CTR also change little. As a representative example, we show results obtained for parameters similar to the experiments on the characterisation of wide-angle electron beams reported in [36]. A horizontally polarized laser with 0.8μm wavelength, normalized vector potential a 0 =3, spot radius w 0 =7 μm and 20fs pulse duration interacts with a pre-ionized plasma with density 2×10 19 cm −3 and 500μm length.…”

“…These wide-angle electrons form a cone containing up to 10s-nC total charge in a bunch with duration of the order of 1ps, based on PIC simulations. Experiments and simulations show that wide-angle beams are emitted with high stability and little variation in energy and ejection angle for a wide range of laser and plasma parameters [36,37].…”

“…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]. We show that these obliquely emitted beams, with nC-level charge and 1-2 MeV energy, can produce 10sμJ to mJ-level THz radiation when passed through a foil or at the plasma-vacuum boundary of the accelerator.…”

“…High-energy electron bunches with typically 1%-10% energy spread can also be accelerated along the laser axis. The LWFA is modelled in three-dimensional geometry using the particle-in-cell (PIC) code Object-oriented Simulation Rapid Implementation System (OSIRIS) [38] and details of these simulations are presented in [36]. A selection of the phase-space distribution of wide-angle electrons obtained with OSIRIS is loaded into a code based on the vector model described in [39], which calculates transition radiation (TR) from a perfectly conducting screen of arbitrary shape and orientation for arbitrary electron energies and particle numbers.…”

Wide-angle electron beams from laser-wakefield accelerators

“…[20][21][22] At even lower energies, high-charge beams can be obtained by exploiting the little known property of laser-wakefield accelerators that intrinsically leads to MeV electrons at wide angles as part of the bubble formation process. [23][24][25] These oblique electrons typically have 1-2 MeV mean energy and nC-level total charge, E-mail: enrico.brunetti@strath.ac.uk, d.a.jaroszynski@strath.ac.uk and are distributed in a broad cone of 30…”