Spatial characteristics of K-alpha radiation from weakly relativistic laser plasmas

The spatial dependence of K-alpha emission generated from laser-produced ho t electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of K-alpha production as a function of laser inte nsity is measured and compared with modeling results. We use the terawatt T i:sapphire laser at MPQ and vary the peak intensity from 10(15) to 10(18) W /cm(2) with a pulse duration of 200 fs. A solid Cu target is placed at vari ous positions in the laser focus, which allows one to vary the intensity bu t keep the total energy on the target constant. When the target is near bes t focus, the FWHM of the K-alpha emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of K-alpha production using the fundamental wavelength of the la ser, a clear maximum of K-alpha emission is observed at a position away fro m best focus, where the peak intensity is down by more than an order of mag nitude from the value at best focus. When the second harmonic of the laser is used, the K-alpha emission is peaked near best focus. The K-alpha emissi on from layer targets is used to obtain an estimate of the temperature of t he hot electrons. Modeling of K-alpha production, using a Monte Carlo elect ron/photon transport code, shows the relationship between incident electron energy and the emitted K-alpha emission. Efficient K-alpha generation from the low-intensity wings of the laser pulse contributes to the large spot s ize of the K-alpha emission. The lower electron temperatures that are expec ted for the second harmonic explain the differences in the location of maxi mum K-alpha emission for the two wavelengths. We discuss the use of K-alpha emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths.