We report recent results on the performance of FLASH (Free Electron Laser in Hamburg) operating at a wavelength of 13.7 nm where unprecedented peak and average powers for a coherent EUV radiation source have been measured. In the saturation regime the peak energy approached 170 µJ for individual pulses while the average energy per pulse reached 70 µJ. The pulse duration was in the region of 10 femtoseconds and peak

In the tunneling regime of strong laser field ionization we measure a substantial fraction of neutral atoms surviving the laser pulse in excited states. The measured excited neutral atom yield extends over several orders of magnitude as a function of laser intensity. Our findings are compatible with the strong-field tunneling-plus-rescattering model, confirming the existence of a widely unexplored neutral exit channel (frustrated tunneling ionization). Strong experimental support for this mechanism as origin of excited neutral atoms stems from the dependence of the excited neutral yield on the laser ellipticity, which is as expected for a rescattering process. Theoretical support for the proposed mechanism comes from the agreement of the neutral excited state distribution centered at n = 6-10 obtained from both, a full quantum mechanical and a semiclassical calculation, in agreement with the experimental results.

We present experimental studies on ion acceleration from ultra-thin diamond-like carbon (DLC) foils irradiated by ultra-high contrast laser pulses of energy 0.7 J focussed to peak intensities of 5 × 10 19 W/cm 2 . A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell (PIC) simulations reveal, that those C 6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."