Fig. 4. Measurement of the highest peak energies μ_E achieved at different plasma densities n_pe, with and without a gradient in the plasma density.

The error bars arise from the position–energy conversion. The gradients chosen are those that were observed to maximize the energy gain. Acceleration to 2.0 ± 0.1 GeV is achieved with a plasma density of 6.6 × 10¹⁴ cm⁻³ with a density difference of +2.2% ± 0.1% over 10 m.