  An ultra-bright high-intensity X- and gamma-radiation source is proposed. A
high-density thin plasma slab, accelerating in the radiation pressure dominant
regime by a co-propagating ultra-intense electromagnetic wave, reflects a
counter-propagating relativistically strong electromagnetic wave, producing
strongly time-compressed and intensified radiation due to the double Doppler
effect. The reflected light contains relativistic harmonics generated at the
plasma slab, all upshifted with the same factor as the fundamental mode of the
incident light.
