We discuss the optimization and control of laser-driven proton beams. Specifically, we report on the dependence of high-intensity laser accelerated proton beams on the material properties of various thin-film targets. Evidence of star-like filaments and beam hollowing (predicted from the electrothermal instability theory) is observed on Radiochromic Film (RCF) and CR-39 nuclear track detectors. The proton beam spatial profile is found to depend on initial target conductivity and target thickness. For resistive target materials, these structured profiles are explained by the inhibition of current, due to the lack of a return current. The conductors, however, can support large propagating currents due to the substantial cold return current which is composed of free charge carriers in the conduction band to neutralize the plasma from the interaction. The empirical plot shows relationship between the maximum proton energy and the target thickness also supports the return current and target normal sheath acceleration (TNSA) theory. We have also observed filamentary structures in the proton beam like those expected from the Weibel instability in the electron beam. Along with the ion acceleration, a clear electron beam is detected by the RCF along the tangent to the target, which is also the surface direction of target plate.
Available at: http://works.bepress.com/donald_umstadter/50/