Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact multi-MeV proton accelerators with unique bunch characteristics. Protons are accelerated in TV/m fields that are established within the micrometer-scale vicinity of the high-power laser focus. This initial acceleration phase is followed by ballistic proton bunch propagation with negligible space-charge effects over millimeters to hundreds of centimeters to the particle detector or a proton target at a dedicated irradiation site. The detected proton emission distribution can be influenced by the spatio-temporal intensity distribution in the laser focus, electron transport through the target, potential plasma instabilities, as well as local and global target geometry and surface properties.
Substantially extending this picture, our recent results show a critical influence of the milimeter scale vacuum environment on the accelerated proton bunch, where residual gas molecules are ionized by the remnant laser light that is not absorbed into the target plasma but reflected or transmitted. In an experiment with μm-sized hydrogen jet targets, this effect lead to the counter-intuitive observation of laser near-field feature imprints in the detected proton beam profiles. Our results show that the remnant laser pulse induces a quasi-static deflecting field map in the ionized residual background gas that serves as a memorizing medium and allows for asynchronous information transfer to the naturally delayed proton bunch. Occurring under typical experimental laser, target and vacuum conditions, all-optical imprinting needs to be taken into account for sensible interpretation of modulated proton beam profiles.
|Working group||Invited plenary talk|