Intrinsic beam emittance of laser-accelerated electrons measured by x-ray spectroscopic imaging

Abstract

The recent combination of ultra-intense lasers and laser-accelerated electron beams is enabling the development of a new generation of compact x-ray light sources, the coherence of which depends directly on electron beam emittance. Although the emittance of accelerated electron beams can be low, it can grow due to the effects of space charge during free-space propagation. Direct experimental measurement of this important property is complicated by micron-scale beam sizes, and the presence of intense fields at the location where space charge acts. Reported here is a novel, non-destructive, single-shot method that overcame this problem. It employed an intense laser probe pulse, and spectroscopic imaging of the inverse-Compton scattered x-rays, allowing measurement of an ultra-low value for the normalized transverse emittance, 0.15 (±0.06) π mm mrad, as well as study of its subsequent growth upon exiting the accelerator. The technique and results are critical for designing multi-stage laser-wakefield accelerators, and generating high-brightness, spatially coherent x-rays.

Measured Emittance Dependence on the Injection Method in Laser Plasma Accelerators

“…While the measured emittances are higher than what has been reported elsewhere [10,11,13], the measurements presented in this Letter were carried out at sig-nificantly higher spectral charge densities. Furthermore, the space charge effect, which was not relevant in Ref.…”

“…Furthermore, the space charge effect, which was not relevant in Ref. [10,11,13], can be effectively suppressed by operating at higher energies (as in Ref. [13]), or by using a magnetic chicane placed close to the source to longitundally stretch the electron beam and reduce the peak current.…”

“…Although a key critical parameter, to date very few direct emittance measurements have been carried out, perhaps owing to the typical percent-level energy spreads and challenges of LPA stability and tunability. Besides indirect techniques based on spectral analysis of X-ray betatron or Compton radiation [9][10][11] (which rely on various assumptions, simulations, and complex analysis to unravel the e-beam source properties), traditional knife scans and pepperpot measurements are limited in their utility [12]. Only Weingartner et al [13] performed direct post-LPA emittance characterization by combining conventional quadrupole focusing techniques and the energy-dispersed plane of a magnetic spectrometer (a technique also recently applied in electron beam driven plasma wakefield acceleration experiments [14]).…”

Control and optimization of a staged laser-wakefield accelerator

“…The motivation for using LWFA electron sources is that the acceleration gradient is two to three orders of magnitude higher than conventional radio frequency linear accelerators; therefore, high-energy electrons can be accelerated with a much smaller footprint. Other useful characteristics of laser-accelerated electrons include extremely short fs-level duration [5], and ultra-small bunch emittance [6,7].…”

Long-distance characterization of high-quality laser-wakefield-accelerated electron beams

“…Upon entering the vacuum region without the focusing force, the e beam is no longer restricted [12] and quickly diverges. Furthermore, the transverse beam emittance tends to increase rapidly during the transition from the plasma field to the vacuum drift stage, because the increasing transverse beam correlation between position and momentum rapidly becomes the predominant term in the emittance due to the transverse phase-space rotation of the electrons [13] and energy deviation [5] . To theoretically describe the emittance evolution in the plasma-vacuum transition stage as a function of the propagation distance, we first present the evolution of the transverse beam size and beam divergence based on Ref.…”