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

TL;DR

This work investigates the limits of the pepper-pot mask method for measuring transverse emittance of laser wakefield accelerator (LWFA) electron beams. By combining GEANT4 Monte Carlo simulations, PIC simulations of LWFA beams, and an experimental measurement at the JETi200 system, the authors quantify regimes where the pepper-pot overestimates low emittance and underestimates large emittance, with reliable measurements only in a mid-range of emittance. They show that for LWFA beams with emittance below tens of μm mrad, the pepper-pot is inadequate, while larger emittances can be captured with appropriately designed masks, though even then cross-checks with other diagnostics are essential. The study provides design guidelines and demonstrates that, for current LWFA parameters, complementary techniques such as laser-grating diagnostics or quadrupole-based scans yield more accurate emittance determinations. Overall, the results highlight the need for tailored, geometry-aware diagnostics when characterizing high-brightness LWFA beams in compact setups.

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 $(26.2 \pm 7.3)$ $μ$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.

Figures (9)

• Figure 1: Calculated emittance from GEANT4 simulations for pepper-pot masks with pitches of (a) 120 and (b) 150 across different hole diameters. Panel (c) shows results from the penumbral deconvolution method for the 150 pitch at low emittance. Uncertainties were below 5% and thus omitted. Curves following $\epsilon_{\mathrm{rms},\,\mathrm{calc}}^2 = \Omega^2\theta_{x}^2 + \epsilon_{\mathrm{rms},\,\mathrm{in}}^2$ were fitted to the data, with the black dashed line marking the diagonal. The fitted values of $\Omega$ are shown in Appendix \ref{['appendix:fitting']}. Additionally, region I marks where the emittance is overestimated, region II where it is underestimated, and region III where it is recovered with less than 10% error from the input RMS emittance.
• Figure 2: Comparison of the fit model for the beamlets depending on the source size of the electron beam. Gaussian fit (red dashed line) and penumbral convolution (blue dotted line) between a Gaussian source and the mask holes fitted to the simulated beamlet image (solid gray line) for various beam waists $\sigma_\mathrm{rms}$ while maintaining a fixed beam divergence of $1.85$.
• Figure 3: Simulated trace space of an electron beam with a 40 source size and a beam emittance of 74 (from Fig. \ref{['fig:source scan geant4']} a)) after its interaction with a pepper-pot mask with $d=50$ and $\Lambda=120~\um$. The trace space of the beam is sampled in beamlets, and background noise arises due to electrons that are scattered at large angles.
• Figure 4: Integrated signals (summation over the y-axis to investigate $\epsilon_{x}$) from GEANT4 simulation results of the beamlet signal at the screen for a pepper-pot mask with a hole diameter of 50 and a $\Lambda=120$ (left) and 150 (right). The increased pitch size $\Lambda$ leads to a lower overall baseline.
• Figure 5: Experimental layout for emittance measurements using the pepper-pot mask.