Analyzing the filamentation of MeV-range proton bunches in a laser-driven ion beamline and optimizing their peak intensity

In this article, we report on the latest investigations and achievements in proton beam shaping with our laser-driven ion beamline at GSI Helmholtzzentrum für Schwerionenforschung GmbH. This beamline was realized within the framework of the Laser Ion Generation, Handling, and Transport (LIGHT) collaboration to study the combination of laser-driven ion beams with conventional accelerator components. At its current state, the ions are accelerated by the high-power laser PHELIX via target normal sheath acceleration, and two pulsed high-magnetic solenoids are used for energy selection, transport, and transverse focusing. In between the two solenoids, there is a rf cavity that gives the LIGHT beamline the capability to longitudinally manipulate and temporally compress ion bunches to sub-nanosecond durations. To get optimal results, the rf cavity has to be synchronized with the PHELIX laser and therefore a reliable measurement of the temporal ion beam profile is necessary. In the past, these measurements showed unexpected correlations between the temporal beam profile and the phase as well as the electric field strength of the cavity. In this article, we present a numerical simulation of the beam transport through the LIGHT beamline which explains this behavior by a beam filamentation. We also report on our latest experimental campaigns, in which we combined transverse and longitudinal focusing for the first time. This led to proton bunches with a peak intensity of (3.28±0.24)×10^{8} protons/(ns mm^{2}) at a central energy of (7.72±0.14) MeV. The intensity refers to a circle with a diameter of (1.38±0.02) mm that encloses 50% of the protons in the focal spot at the end of the beamline. The temporal bunch width at this position was (742±40) ps (FWHM).