Demonstration of ultra-low emittance beams in a kHz laser wakefield accelerator and application to electron diffraction

TL;DR

This work demonstrates a compact method to measure the emittance of few-MeV, kHz-rate laser-wakefield accelerator beams using a permanent magnet solenoid and an energy-dispersive setup, achieving a normalized emittance of approximately $ε_n ≈ 124$ nm·rad at 2.7 MeV. The authors validate the approach with General Particle Tracer simulations and a direct diffraction experiment on a silicon nanomembrane, obtaining clear Bragg peaks and linking the observed pattern to an electron wavelength of about $λ_B ≈ 1.46$ pm (≈0.5 MeV). The results show that kHz LWFA beams can reach emittance levels comparable to conventional RF guns in the MeV regime, highlighting their potential for time-resolved electron diffraction and pump-probe studies. The work also discusses temporal-resolution challenges due to energy spread and outlines strategies, such as energy-streaking or beam recompression, to approach sub-10 fs capabilities, marking a significant step toward practical, high-repetition-rate UED with laser-driven sources.

Abstract

We present a compact, cost-effective method for measuring the emittance of kHz-repetition-rate laser-wakefield accelerated electron beams using a permanent solenoid. The measured normalized emittance, $ε_n = 124\,\mathrm{nm \cdot rad}$ ($\simeq 0.04 π\,\mathrm{mm \cdot mrad}$) at $2.7\,$MeV, is comparable to that of ultra-low emittance radiofrequency guns used for electron diffraction. Leveraging this low emittance, we successfully applied the electron beam to electron diffraction. We demonstrate diffraction images obtained from a single crystal Silicon nano-membrane sample, clearly resolving diffraction peaks across multiple orders.

Figures (5)

• Figure 1: Typical spectrum (a) and beam profile (b) obtained during the experimental emittance measurement.
• Figure 2: a) Experimental set up. Electrons, accelerated by laser-wakefield acceleration, pass through the focusing solenoid and the dispersive dipole (which separates them by energy), then hit the scintillator. Without the dipole, the system has a magnification $M \simeq 8$. b) Schematic of the permanent solenoid magnet enclosed in the Teflon shell and soft iron shielding. c) Comparison of the measured on-axis magnetic field and the simulated field using COMSOL.
• Figure 3: a) Image of the focal spot with no dispersive dipole. b) Experimental data of the emittance measurement for $\delta = 70.0\,$mm. b.i. shows the focused beam registered from the GAGG emission, the orange line indicates the energy $E = 2.7\,$MeV selected fo analysis and b.ii. shows the cross-section of the beam in the $y$ direction for this energy. c) Plain lines : simulated evolution of the measured cross-section beam size for E = $2.7\,$MeV as a function of the solenoid position for various sets of initial conditions. Blue dots : experimental data for E = $2.7\,$MeV. d) Best fit of the experimental data with simulated data using initial parameters $\sigma_r = 3.5\,$µ m and $\epsilon_g = 20\,\mathrm{nm \cdot rad}$. The error bars account both for the RMS fluctuations of the experimental data and the resolution limits of the imaging system.
• Figure 4: a) Experimental setup for electron diffraction. Electrons are accelerated via laser-wakefield acceleration and focused by the solenoid onto a scintillator. The silicon sample, on which the electrons diffract, is positioned immediately after the solenoid. The resulting diffraction pattern is observed on the scintillator. b) Diffraction pattern with the sample in place. The image is acquired with a $10\,$s exposure time, corresponding to $10^4$ shots. c) Cross-sectional analysis of the diffraction pattern for $y = 3.41\,$mm and $y = -4.16\,$mm.
• Figure 5: A) Review of the emittance reported by various conventional accelerator facilities with energies below $10\,$MeV. a) liu_experimental_2018. b) setiniyaz_beam_2016. c) li_experimental_2009. d) desy_regae_2025. e) mev-ued_2025. B) Corresponding electron beam charges reported.