Energy scaling of laser accelerated protons, and performance of reduced mass targets

Proton acceleration from thin foils and reduced mass targets is studied with the 150 TW Ti:Sa DRACO Laser at FZD. DRACO has $\sim$30 fs pulses, with up to 5 J at 10 Hz, and a contrast of 1e-10 in the ps regime, and 1e-9 to 1e-10 in the ns regime. Proton spectra are measured in radiochromic film stacks and magnetic spectrometers. Flat metallic foils exhibit a near-linear scaling of the maximum proton energy with laser power, consistent with [1] in the limiting case of ultrashort laser pulses [2]. Despite the high laser contrast, a slight deformation of the target rear surface results in a reproducible deflection of the emission of energetic protons away from the target normal direction [2]. The mass limited targets of 2 $\mu$m thick Si, were fabricated by MEMS techniques and ranged from 20x20 $\mu$m$^2$ to 100x100 $\mu$m$^2$ lateral size. Significant influence of the target edge and supporting stalks is observed, which depending on size can both both increase or decrease the maximum proton energy in comparison to a flat foil.\\[4pt] [1] J. Schreiber \textit{et al}., \textbf{PRL 97}, 045005 (2006).\\[0pt] [2] K. Zeil \textit{et al}., \textbf{NJP 12}, 045015 (2010).