  We investigate the heating of the quasi-free electrons in large rare gas
clusters (N exceeding 10^5 atoms) by short laser pulses at moderate intensities
(I~10^{15} Wcm^{-2}). We identify elastic large-angle backscattering of
electrons at ionic cores in the presence of a laser field as an efficient
heating mechanism resembling the Fermi shuttle. Its efficiency as well as the
effect of collective electron motion, electron-impact ionization and cluster
charging, are studied employing a mean-field classical transport simulation.
Results for the absolute x-ray yields are in surprisingly good quantitative
agreement with recent experimental results.
