Effect of Plasma Scale Length on Multi-MeV Proton Production by Intense Laser Pulses

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at $I{\lambda }^2=5\mathrm{}\times {10}^{19}\mathrm{W}\mathrm{cm}{}^{-2\mathrm{}}$ has been studied. With an unperturbed foil, protons with energy $>20\mathrm{MeV}$ were formed in an exponential energy spectrum with a temperature of $2.5\pm 0.3\mathrm{MeV}$. When a plasma with a scale length of $100\mu \mathrm{m}$ was preformed on the back of the foil, the maximum proton energy was reduced to $<5\mathrm{MeV}$ and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.