Fig. 5.

PIC simulation results illustrating beam loading effects. a–d Electron energy histograms for increasing injected charges within the energy peak (purple area). The green- and purple-shaded area is the contribution from electrons originating from nitrogen, and orange-shaded area from helium electrons. The black line indicates the cumulative spectrum. a Corresponds to the histogram seen in Fig. 4a. e Shown is how maximum attained energy E _max (left axis) and beam energy spread (right axis) depend on injected charge