X-ray harmonic radiation extending to 3.3 angstrom, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to similar to 3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy h nu > 1 keV is found to be into a cone angle <4 degrees, significantly less than that of the incident laser cone (20 degrees).
Dromey, B., Adams, D., Kar, S., Bellei, C., Carroll, D. C., Clarke, R. J., Green, J. S., Kneip, S., & Mckenna, P. (2009). High brightness kev harmonics from relativistically oscillating plasma surfaces. European Physical Journal - Special Topics, 175(1), 57-60. https://doi.org/10.1140/epjst/e2009-01117-4