PPPS-2013: Laser-driven proton acceleration with two ultrashort laser pulses

We present results of the first laser-driven proton acceleration experiment with two ultrashort laser pulses at the Arcturus laser facility in Düsseldorf. The Ti:sapphire laser provides two laser pulses (30fs, 2.3J; 30fs, 3.7J) which reaches focused intensities up to 10²⁰ W/cm². The second pulse has a contrast of 10⁸ on a picosecond timescale. A plasma mirror enhances the contrast of the first pulse by three orders of magnitude on a picosecond timescale. The scheme of the experiment is as follows. The first laser pulse irradiates a thin foil and accelerates ions according to the target normal sheath acceleration mechanism. 10s of femtoseconds after the first pulse the second pulse is interacting with the expanding plasma and enhances the acceleration process of the ions. In the experiment the pulses were spatially overlapping on a 5μm thin Titanium foil. The delay between the pulses was varied from 10s to 100s of femtoseconds. As a main diagnostic a Thomson Parabola was employed in combination with a micro-channel-plate. In a second part of the experiment a time-and-space-resolved-interferometer (TASRI) was used to determine the electron density and temperature at the rear side of the foil with a 3μm spatial and 3ps temporal resolution. Two regimes were observed, which were only present in the two-beam configuration. One regime is dominated by a large number of different (up to 13) ion species - namely carbon, nitrogen, oxygen and titanium ions. But in this regime only a relatively low maximum proton energy was observed. In contrast to this is the other regime, where only a few weak ion species were present. In return protons with relatively high energy could be measured. The proton maximum cut-off energy in this regime was distinctly higher compared to the single-beam configuration. First simulations with the particle-in-cell code EPOCH indicate the enhanced acceleration mechanism is driven by a second push- of the electrons from the second laser pulse.