We show that the properties of the electron beam and bright x rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimized when the plasma length is matched to the laser depletion length. With a 200 TW laser pulse, this results in an x-ray beam with a median photon energy of 20 keV, $>6\times {10}^8\mathrm{photons}$ above 1 keV per shot, and a peak brightness of $3\times {10}^{22}\text{photons}{\mathrm{s}}^{-1}\mathrm{mra}{\mathrm{d}}^{-2}{\mathrm{mm}}^{-2}$ $(0.1\%\mathrm{BW}{)}^{-1}$.

• Received 6 November 2017
• Revised 23 October 2019
• Accepted 6 May 2020

DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.061301

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Published by the American Physical Society