Optimisation of Thin Plastic Foil Targets for Production of Laser-Generated Protons in the GeV Range

In order to realistically simulate the interaction of a femtosecond laser pulse with a nanometre-thick target it is necessary to consider a target preplasma formation due to the nanosecond long amplified-spontaneous-emission pedestal and/or prepulse. The relatively long interaction time dictated that hydrodynamic simulations should be employed to predict the target particles' number density distributions prior the arrival of the main laser pulse. By using the output of the hydrodynamic simulations as input into particle-in-cell simulations, a detailed understanding of the complete laser-foil interaction is achieved. Once the laser pulse interacts with the preplasma it deposits a fraction of its energy on the target, before it is either reflected from the critical density surface or transmitted through an underdense plasma channel. A fraction of hot electrons is ejected from the target leaving the foil in a net positive potential, which in turn results in proton and heavy ion ejection. In this work protons reaching ~25 MeV are predicted for a laser of ~40 TW peak power and ~600 MeV are expected from a ~4 PW laser system.