Yield Optimization and Time Structure of Femtosecond Laser PlasmaKαSources

The generation of femtosecond $K\alpha$ x rays from laser-irradiated plasmas is studied with a view to optimizing photon number and pulse duration. Using analytical and numerical models of hot electron generation and subsequent transport in a range of materials, it is shown that an optimum laser intensity $I_{\mathrm{opt}}=7\mathrm{}\times {10}^9{Z}^{4.4}$ exists for maximum $K\alpha$ yield. Furthermore, it is demonstrated that bulk targets are unsuitable for generating sub-ps x-ray pulses: instead, design criteria are proposed for achieving $K\alpha$ pulse durations $\le 100\mathrm{fs}$ using foils of $\approx 2\mu \mathrm{m}$ thickness.