We revise the calibration of scintillating screens commonly used to detect relativistic electron beams with low average current, e.g., from laser-plasma accelerators, based on new and expanded measurements that include higher charge density and different types of screens than previous work [Buck et al., Rev. Sci. Instrum. 81, 033301 (2010)]. Electron peak charge densities up to 10 nC/mm² were provided by focused picosecond-long electron beams delivered by the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf. At low charge densities, a linear scintillation response was found, followed by the onset of saturation in the range of nC/mm². The absolute calibration factor (photons/sr/pC) in this linear regime was measured to be almost a factor of 2 lower than that reported by Buck et al. retrospectively implying a higher charge in the charge measurements performed with the former calibration. A good agreement was found with the results provided by Glinec et al. [Rev. Sci. Instrum. 77, 103301 (2006)]. Furthermore long-term irradiation tests with an integrated dose of approximately 50 nC/mm² indicate a significant decrease of the scintillation efficiency over time. Finally, in order to enable the transfer of the absolute calibration between laboratories, a new constant reference light source has been developed.

We report on experimental investigations of plasma wave structures in a plasma wakefield acceleration (PWFA) stage which is driven by electron beams from a preceding laser plasma accelerator. Femtosecond optical probing is utilized to allow for direct visualization of the plasma dynamics inside the target. We compare two regimes in which the driver propagates either through an initially neutral gas, or a preformed plasma. In the first case, plasma waves are observed that quickly damp after a few oscillations and are located within a narrow plasma channel ionized by the driver, having about the same transverse size as the plasma wakefield cavities. In contrast, for the latter robust cavities are recorded sustained over many periods. Furthermore, here an elongation of the first cavity is measured, which becomes stronger with increasing driver beam charge. Since the cavity length is linked to the maximum accelerating field strength, this elongation implies an increased field strength. This observation is supported by 3D particle-in-cell simulations performed with PIConGPU. This work can be extended for the investigation of driver depletion by probing at different propagation distances inside the plasma, which is essential for the development of high energy efficiency PWFAs.