Spatial characteristics of Kα radiation from weakly relativistic laser plasmas
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The spatial dependence of Kα emission generated from laser-produced hot electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of Kα production as a function of laser intensity is measured and compared with modeling results. We use the terawatt Ti:sapphire laser at MPQ and vary the peak intensity from 1015 to 1018 W/cm2 with a pulse duration of 200 fs. A solid Cu target is placed at various positions in the laser focus, which allows one to vary the intensity but keep the total energy on the target constant. When the target is near best focus, the FWHM of the Kα emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of Kα production using the fundamental wavelength of the laser, a clear maximum of Kα emission is observed at a position away from best focus, where the peak intensity is down by more than an order of magnitude from the value at best focus. When the second harmonic of the laser is used, the Kα emission is peaked near best focus. The Kα emission from layer targets is used to obtain an estimate of the temperature of the hot electrons. Modeling of Kα production, using a Monte Carlo electron/photon transport code, shows the relationship between incident electron energy and the emitted Kα emission. Efficient Kα generation from the low-intensity wings of the laser pulse contributes to the large spot size of the Kα emission. The lower electron temperatures that are expected for the second harmonic explain the differences in the location of maximum Kα emission for the two wavelengths. We discuss the use of Kα emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths.
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