Imaging laser-wakefield-accelerated electrons using miniature magnetic quadrupole lenses

Abstract

The improvement of the energy spread, beam divergence, and pointing fluctuations are some of the main challenges currently facing the field of laser-wakefield acceleration of electrons. We address these issues by manipulating the electron beams after their generation using miniature magnetic quadrupole lenses with field gradients of $500 T=m. By imaging electron beams the spectral resolution of dipole magnet spectrometers can be significantly increased, resulting in measured energy spreads down to 1.0% rms at 190 MeV. The focusing of different electron energies demonstrates the tunability of the lens system and could be used to filter out off-target energies in order to reduce the energy spread even further. By collimating the beam, the shot-to-shot spatial stability of the beam is improved by a factor of 5 measured at a distance of 1 m from the source. Additionally, by deliberately transversely offsetting a quadrupole lens, the electron beam can be steered in any direction by several mrad. These methods can be implemented while still maintaining the ultrashort bunch duration and low emittance of the beam and, except for undesired electron energies in the energy filter, without any loss of charge. This reliable and compact control of laser-wakefield accelerated electron beams is independent of the accelerator itself, allowing immediate application of currently available beams.

Physics of Quasi-Monoenergetic Laser-Plasma Acceleration of Electrons in the Blowout Regime

“…Poor collimation of the continuously injected electrons, ∆α cont ≈ 3 ∆α mono ,t o g e t h e r with their distribution over the energy range 10-15 times broader than the absolute energy spread of the leading bunch, reduce dramatically the brightness of the energy tail. These poorly collimated beam components can be dispersed in vacuum using miniature magnetic quadrupole lenses, further improving the beam collimation and reducing the energy spread (Weingartner et al, 2011). More details on electron acceleration in the wake of negatively chirped pulse are shown in Fig.…”

“…Even though complete elimination of dark current is hard to achieve in high-density plasmas (γ g = 10 -15), strong reduction of the charge in the poorly collimated, continuous low-energy tail is useful for applications. Subsequent manipulations with the beam using permanent magnets may further improve its quality (Weingartner et al, 2011).…”

Transport and Non-Invasive Position Detection of Electron Beams from Laser-Plasma Accelerators

“…In order to deploy such magnets for beam focusing in an LPA setup with characteristic energies of several hundred MeV and focusing distances on the order of a few meters, the magnetic quadrupole lenses would need to have lengths on the order of several ten centimeters. These dimensions allow for electron trajectory path differences, which will ultimately result in a significant temporal elongation of femtosecond electron beams [7]. Owing to the inherently short duration of laser-plasma accelerated beams on a scale of ∼ 10 fs, small magnetic quadrupole lenses with gradients on the order of several hundred T/m are required for conservation of the initial temporal pulse structure.…”

High-gradient microelectromechanical system quadrupole electromagnets for particle beam focusing and steering

“…In electron microscopes, stronger magnetic lenses can be used to reduce the beam size at the sample and/or increase the magnification of the system while higher order magnetic elements correct the effect of aberrations on the instrument spatial resolution [9,10]. In plasma wakefield accelerator applications, matching the beam to the extreme strong focusing of a plasma channel necessitates very small spots at injection [11,12] using very strong, very short focal length quadrupoles, and the large angular divergence leaving the laser plasma wakefield accelerator necessitates high-gradient focusing over a short distance to minimize bunch elongation and retain high peak current [13]. In multistage laser-dielectric accelerator and undulator structures, strong magnetic optics matched in size-scale to the sub-mm accelerator or undulator gap are required to realize a full scale demonstration of a light source system [14][15][16][17][18].…”