Pulses of 30 fs duration, 800 nm center wavelength from the DRACO laser at Helmholtz-Zentrum Dresden-Rossendorf (HZDR)66,67 were focused to spot size 20 μm (FWHM) with typical energy 2 J onto the entrance plane of a 3-mm or 5-mm-long He gas jet doped with 1% Nitrogen. The laser pulse fully ionized the helium, creating plasma of electron density in the range 4<ne<6×1018 cm-3, and drove a LWFA in the self-truncated ionization-injection regime66,68. A magnetic electron spectrometer67 with its entrance plane at z=30 cm downstream of the gas jet exit determined the electron energy distribution for each shot. The spectrometer records the dispersed electron beam using a Konica Minolta OG 400 scintillating screen that is converted to charge per unit energy per pixel69 (see Fig. 1b, left panel for an example from the 3 mm jet) using the methods described in Section IV. of Kurz et al.69. The absolute charge calibration uncertainty for our system is 19% and is shown with vertical error bars at the quasi-monoenergetic peak in presented electron spectra, however the relative uncertainty from shot-to-shot variations in charge are much smaller than this. Errors in electron energy measurement >200 MeV arise primarily from pointing and divergence fluctuations of LWFA electrons entering the magnetic spectrometer11 and is 2% for electrons in the range of 200–350 MeV. Electrons with energy Ee<200 MeV are recorded near the spectrometer’s focal plane and have <1% uncertainty determined by the accuracy of the magnetic field measurement. The electron spectra presented here consist of a quasi-monoenergetic peak with central energy in the range 200<Ee<350 MeV (Lorentz factor 390<γe<685), energy spread 20 MeV (FWHM), rms divergence 2 mrad and charge in the range 200<Q<300 pC, which is responsible for most X-ray production, and a weak poly-energetic, low-energy background. The 2% error in electron energy is indicated as horizontal error bars at the peak or average electron energy for quasi-monoenergetic electron spectra. The 5-mm jet yielded electrons with energy up to 550 MeV, with a stronger poly-energetic background.

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