Limitations of emittance and source size measurement of laser-accelerated electron beams using the pepper-pot mask method

TL;DR

This paper investigates the limitations of the pepper-pot method for measuring emittance of laser-accelerated electron beams, revealing potential overestimations and validating results through experiments and simulations.

Contribution

It identifies the constraints of the pepper-pot technique in LWFA regimes and compares experimental measurements with simulations and independent methods.

Findings

Measured an upper bound of 26.2 ± 7.3 μm mrad for beam emittance.

Simulation results agree with experimental data and other measurement techniques.

The method may overestimate emittance in certain high-brightness regimes.

Abstract

The pepper-pot method is a widely used technique, originally proposed for measuring the emittance of space-charge-dominated electron beams from radio-frequency photoinjectors. With recent advances in producing high-brightness electron beams via laser wakefield acceleration (LWFA), the method has also been applied to evaluate emittance in this new regime. Here, the limitations of this method for measuring the emittance of LWFA electron beams are investigated, particularly in parameter regimes where the true beam emittance is overestimated. Conducting an experiment at the JETi200 laser system, we measured an upper bound for the geometric beam emittance of $(26.2 \pm 7.3)$ $\mu$m mrad using the pepper-pot method. This result is consistent with GEANT4 Monte Carlo simulation of the pepper-pot diagnostic, with an input beam-emittance parameter that matches both PIC simulations of the laser-plasma accelerator and an independent measurement using the transient optical grating method.