# Also in the Article

Laser wakefield electron acceleration
This protocol is extracted from research article:
Compact spectroscopy of keV to MeV X-rays from a laser wakefield accelerator
Sci Rep, Jul 13, 2021;

Procedure

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 $4 cm$-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 MeV (Lorentz factor $390<γe<685$), energy spread $∼$ 20 MeV (FWHM), rms divergence 2 mrad and charge in the range $200 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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# Also in the Article

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