Fig. 4: LPWFA using a drive-witness bunch pair.

a Averaged electron spectra with 95% confidence intervals for LWFA-only reference shots (200 consecutive shots, blue solid line) and with the second stage operated at lower plasma density than the LWFA stage, at 1.4 × 10¹⁸ cm⁻³ (200 consecutive shots, orange solid line) and 2.1 × 10¹⁸ cm⁻³ (200 consecutive shots, gray line), respectively. Blue and orange dashed lines correspond to a representative single shot from the associated data set in which a clear distinction of the driver and the witness electron spectrum is even more pronounced. For the higher density PWFA case, a representative single shot cannot be shown due to the onset of spectral overlap between the drive and witness beam, and large shot-to-shot fluctuations. The detection range of the spectrometer starts at 25 MeV. b Simulated electron density in the LWFA stage, illustrating that two bunches are injected into the first and second wakefield cavity, subsequently acting as a driver-witness pair in the PWFA stage. ζ represents the co-moving coordinate parallel to the laser propagation direction z. c, d Simulation snapshots in the PWFA stage, corresponding to the lower and higher plasma density setting in the experiment, respectively. The red lines correspond to the longitudinal on-axis electric field, showing that the plasma wave is driven by the electron bunch originating from the first LWFA cavity, whereas the contribution of the laser remnant to the wakefield formation is negligible. The accelerating phase witnessed by the electron bunch from the second LWFA cavity is controlled by tuning the plasma density. Further information regarding the pair bunch parameters and simulation details can be found in Supplementary Note 4.