High power commissioning of BELLA iP2 up to 17 J

The new short focal length experimental beamline at the BELLA PW, called iP2, was commissioned up to 17 J laser pulse energy, corresponding to a peak intensity of 1.2 × 1021 W/cm2 on target, based on a measured focal spot size with FWHM 2.7 μm and Gaussian equivalent pulse length of 40 fs. The ion acceleration performance was measured under variation of the laser pulse energy and length, and the laser spot size on target. A maximum proton energy of ∼ 40 MeV was observed in the target normal sheath acceleration regime using 13 μm thick Kapton foil targets. Surveys outside the radiation shielded accelerator cave showed very low radiation levels and there was no measurable activation of experimental installations after performing several tens of shots on target. Back reflections of the laser pulse from the target interaction were monitored and partially mitigated, but ultimately caused damage in the laser frontend. This prohibited further increase of the laser pulse energy beyond 17 J. Implementation of a double plasma mirror is expected to sufficiently suppress back reflections to allow for iP2 experiments at the full BELLA PW pulse energy.