Macroscopic aspects of relativistic x-ray assisted high-order harmonic   generation
  A theoretical model is developed describing high-order harmonic generation (HHG) from a gas of multiply-charged ions driven by a laser field of relativistic intensity. Macroscopic propagation of harmonics is investigated in a relativistic HHG setup where the relativistic drift is suppressed by means of x-ray field assistance of the driving laser field. The possibility of phase-matched emission of the harmonics is shown. The laser field geometry is optimized to maximize the HHG yield with the corresponding phase-matching schemes. Crucial issues determining the macroscopic HHG yield are discussed in detail. 
Markus C. Kohler and Karen Z. Hatsagortsyan
physics.atom-ph