Fig. 3. Background-subtracted projections of consecutive electron-injection events.

Each projection (event) is a vertical integration over the central region of a background-subtracted spectrometer camera image. Brighter colours indicate regions of high charge density dQ/dx, corresponding to accelerated electrons. The quadrupoles of the spectrometer were varied to focus at energies of 460–620 MeV over the duration of the dataset. No other parameters were varied deliberately. The consistent peak around energy E ≈ 600 MeV demonstrates the stability and reliability of the electron acceleration.