Space-charge effects in high brightness electron beam emittance measurements

TLDR

In low‑energy, high‑brightness photoinjector beams, self‑field collective motion dominates transverse dynamics, rendering traditional phase‑space diagnostics ineffective. The study investigates emittance measurement in space‑charge dominated, high‑brightness beam systems, focusing on the role of space‑charge forces in quadrupole‑scan diagnostics. The authors analyze quadrupole‑scan emittance measurements, explain mitigation via multislit or pepper‑pot techniques, interpret data with envelope and multiparticle simulations, and outline methods to detect systematic errors. Experimental comparison of quadrupole scanning and slit‑based emittance measurement on a 5 MeV beam shows that the β‑function to transverse plasma wavelength ratio governs quadrupole‑scan outcomes.

Abstract

The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit- or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam's $\ensuremath{\beta}$ function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.