Evidence for high-energy and low-emittance electron beams using ionization injection of charge in a plasma wakefield accelerator

Abstract

Ionization injection in a plasma wakefield accelerator was investigated experimentally using two lithium plasma sources of different lengths. The ionization of the helium gas, used to confine the lithium, injects electrons in the wake. After acceleration, these injected electrons were observed as a distinct group from the drive beam on the energy spectrometer. They typically have a charge of tens of pC, an energy spread of a few GeV, and a maximum energy of up to 30 GeV. The emittance of this group of electrons can be many times smaller than the initial emittance of the drive beam. The energy scaling for the trapped charge from one plasma length to the other is consistent with the blowout theory of the plasma wakefield.

Measured Emittance Dependence on the Injection Method in Laser Plasma Accelerators

“…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]). While impressively demonstrating 0.21 mm-mrad, little was done to explore parametric dependences of the emittance.…”

Theory and measurements of emittance preservation in plasma wakefield acceleration

“…There are some significant advantages to the technique proposed here. This emittance measurement method has been used by others [39], but the approach of using curve fitting has made error analysis difficult. The proposed application of linear least squares allows for a rigorous error analysis of the measured parameters.…”

“…Emittance measurements of PWFA have been nonexistent until experiments at FACET 3 ; we present a proof-of-concept for emittance measurement which presents the possibility of robust error analysis, a feature not present in other PWFA emittance measurements (e.g. Vafaei-Najafabadi et al [39]). These improvements on the state-of-the-art theory and practice of PWFA are the main impact of this dissertation.…”

“…While the single-bunch and two-bunch PWFA experiments exhibited a high degree of instability, ionization injection experiments [39] showed a much more stable profile.…”

“…Advanced beam sources have been designed to generate beams with small emittances, although there may be potential for using PWFA regimes to produce low-emittance beams [39]. The focus of PWFA as an acceleration method, then, is to ensure that emittance growth is minimized.…”