High harmonic generation from ions in a capillary discharge*

We demonstrate a significant extension of the high harmonic cutoff observed in xenon, up to 150 eV, by generating harmonics from ions in a capillary discharge plasma. Usually, the highest harmonics that can be generated from an atom or ion are limited by ionization-induced defocusing, beam attenuation of the laser, and a lack of phase matching of the frequency conversion process. Using a pre-ionized plasma generated by the capillary discharge, we dramatically reduce ionization-induced defocusing and energy loss of the driving laser due to ionization, allowing higher photon energies to be generated from xenon ions. We also demonstrate enhancement of the harmonic flux of nearly two orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a capillary discharge plasma for high harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths.