Hot surface ionic line emission and cold K-inner shell emission from petawatt-laser-irradiated Cu foil targets

Abstract

A hot, $2\text{to}3\mathrm{keV}$ electron temperature surface plasma was observed in the interaction of a $0.7\mathrm{ps}$ petawatt laser beam with solid copper-foil targets at intensities $>{10}^{20}\mathrm{W}∕{\mathrm{cm}}^2$ . Copper K-shell spectra were measured in the range of $8\text{to}9\mathrm{keV}$ using a single-photon-counting x-ray charged-coupled-device camera. In addition to ${\mathrm{K}}_{\alpha }$ and ${\mathrm{K}}_{\beta }$ inner-shell lines, the emission contained the Cu ${\mathrm{He}}_{\alpha }$ and ${\mathrm{Ly}}_{\alpha }$ lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than $3\times {10}^{18}\mathrm{W}∕{\mathrm{cm}}^2$ , only the ${\mathrm{K}}_{\alpha }$ and ${\mathrm{K}}_{\beta }$ inner-shell emissions are detected. Measurements of the absolute ${\mathrm{K}}_{\alpha }$ yield as a function of the laser intensity are in general agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume.