Coherent X-Ray Generation with Laser Driven Ions
  Small parts of the electronic wavepacket of a multiple charged ion may still tunnel through the high Coulomb barrier for a sufficiently intense laser field of $10^{15}W/cm^2$ and higher. Solving numerically the corresponding Schr\"odinger equation we find that the periodic recollisions of such a wavepacket in the oscillating high power laser field will give rise to coherent X-ray radiation in the multiple keV regime, i.e. substantially higher than predicted or observed before. 
Massimo Casu, Christoph Keitel