First proton acceleration experiments at DRACO - January 6, 2009
News from the Forschungszentrum Dresden-Rossendorf
Ion density plot of an expanding ultrathin solid foil (initial density 1.1*1022 / cm³ = 10 times critical density nc) about 700 fs after interaction with a 1020 W/cm² 30 fs laser pulse. The pronounced 2D structure reveals the interaction of protons with the ponderomotive force of the laser as the target became transparent during the interaction.
The 150 TW laser system Draco (Dresden laser acceleration source) represents a new generation of chirped-pulse-amplification Ti:Sapphire lasers where pulse durations of below 30 fs can be obtained at a pulse energy of several Joule and a pulse repetition rate of 10 Hz.
Focusing the laser beam with a short focal length off-axis parabolic mirror a focal spot diameter of only 4.5 micrometer has been reached allowing for a focal intensity of the order of 5x1020 W/cm2 on target. At such intensities ions could be accelerated from the back side of laser irradiated thin foils. A very first test with a 13 micrometer Al foil led to maximum proton energies of about 5.5 MeV
Experimental development is accompanied by numerical simulations of the interaction process. As such an approach has proven to be crucial for successful target development, two particle-in-cell codes have been installed at FZD in collaboration with partners in Belfast, Munich and Reno (USA). First studies aim for the improvement of the absorption efficiency of the laser light in extremely thin and especially stacked targets as well as for the micro-structuring of the target.