Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

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Ultrafast x‐ray sources*

High-order harmonic generation from a solid target surface has been investigated using femtosecond laser pulses focused to intensities greater than ${10}^{18}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The experiments show that the harmonics are very intense, with a conversion efficiency that is one or two orders of magnitude larger than that of harmonics generated in gases. Beside the observation of presently the shortest wavelength harmonics from femtosecond-laser solid target interaction, i.e., down to 22 nm, an anomaly has been observed in the harmonic spectrum. In contrast to the expected well-known continuous ``roll off'' of the high-harmonic orders, the harmonic intensity decreases with the increase of harmonic order, but in between shows minima which are significantly less intense than the neighboring harmonics. Furthermore, the order of the harmonic minima depend on target material. Additional calculations using numerical kinetic particle simulations and a simpler oscillating mirror model show that the physical origin of these modulations is an intricate interplay of resonance absorption and ponderomotive force which leads to a complex electron density profile evolution. Furthermore, this is emphasized by a spectral line analysis of the harmonics. In agreement with the theory, broad lines have been observed and, in particular for the harmonics in the minima, a complex interference structure is present.

Anomalies in high-order harmonic generation at relativistic intensities

A set of materials—titanium, copper, and germanium—has been experimented with at the OMEGA laser facility [Boehly, Opt. Commun. 133, 495 (1997)] by irradiating thin foils with a prepulse prior to a main pulse with variable delay, in order to design efficient x-ray laser-sources for backlighting, material testing, and code validation. This concept led to increasing factors from 2 to 4 comparing to cases without prepulse, in the experimental conditions. As a result, high multi-keV x-ray conversion rates have been obtained: 9% for titanium around 4keV, 1% for copper around 8keV, and 2.5 to 3% for germanium around 10keV, which places these pre-exploded metallic targets close to the gas with respect to their performance, with wider energy range. A good agreement with hydroradiative code FCI2 [Schurtz, Phys. Plasmas 7, 4238 (2000)] calculations is found for titanium and copper on all diagnostics, with nonlocal-thermal-equilibrium atomic physics and, either nonlocal thermal conduction taking self-generated B-fields into account, or limited thermal conduction with intensity-dependent factor f. The results for germanium indicate that dielectronic processes could play a more significant role when higher irradiation intensity on higher Z material.

Efficient multi-keV X-ray sources from laser-exploded metallic thin foils

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about $38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.

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Bremsstrahlung and line spectroscopy of warm dense aluminum plasma heated by xuv free-electron-laser radiation.

The interaction of high‐intensity contrast, picosecond, 1‐μm laser pulses with solid metal targets is studied with Kα emission from multilayer targets, fast ion blowoff, and other diagnostics. It is found that the characteristics of the interaction are determined by the intensity of the p‐polarized component of the incoming laser field, rather than the total intensity. Consistent with resonance absorption, 20%–30% of the laser energy is deposited into suprathermal electrons that have temperatures from 2–10 keV.

Resonance absorption in high‐intensity contrast, picosecond laser–plasma interactions*

Soft x‐ray spectra generated on flat Al targets by a 500 fs KrF* laser pulse (intensity 5.3×1015 W/cm2), preceded by a prepulse of the same pulse duration, have been measured as a function of the pulse separation and the prepulse intensity. It was found that not only the total x‐ray emission was much stronger when the prepulse was present but, in particular, lines in the shorter wavelength region had a higher intensity than without prepulse, and were strongly dependent on the time separation between the prepulse and the main pulse.

F. P. Schaefer

Papers

Soft x‐ray spectra produced by subpicosecond laser‐double‐pulses