Fig. 1.

Energy spectra of 15 consecutive shots. a Raw energy electron spectra. The color map represents the charge density (pC mm⁻²) on the detector. b Energy spectrum of the first shot from a. The filled area represents the charge within the FWHM, the yellow dashed line represents the mean peak energy and the black dashed line represents the maximum attained energy (E _max) at 0.1 pC MeV⁻¹. Obtained with a supersonic gas jet with a 1.6 mm-long plasma density plateau of 3.1 × 10¹⁸ cm⁻³, 1% nitrogen doping and 2.5 J laser energy in 30 fs FWHM duration. Line graphs of all shots shown in (a) can be found in Supplementary Fig. 2