Ulrich Andiel

K-shell spectra from hot dense aluminum layers buried in carbon and heated by ultrashort laser pulses

Ultrashort laser pulses allow for the generation of hot plasmas near solid state densities. For this purpose a Ti:Sapphire laser was used, which delivers after frequency doubling, pulses of high contrast with an energy of about 60mJ and a duration of 150fs at 395nm. The typical intensity on the target was a few 1017W/cm2. To achieve a high degree of uniformity we used targets consisting of a 25nm thin Al tracer layer buried at different depths up to 400nm in solid carbon. Time-integrated Al K-shell spectra are presented. Characteristic features of the spectra are significant high-order satellite line emission, strong line broadening and a center-of-mass line shift to the red, which was observed in transitions from principal quantum number n=2 or 3 to 1. Accurate measurement of the shift was made possible by using the cold Si Kα line as an absolute wavelength calibration. In addition to time-integrated measurements, we used an ultrafast X-ray streak camera to obtain time and spectrally resolved spectra. Typical durations of the Lyα and Heα lines are in the range 2–4 ps. The experimental results are compared with a time-dependent model, which combines hydrodynamic simulations, time-dependent atomic kinetics, detailed spectral line shapes including line shifts, and radiation transport.

Comparative study of time-resolved K-shell spectra from aluminum plasmas generated by ultrashort laser pulses at 395 and 790 nm

A comparative study of temporally and spectrally resolved K-shell emission from aluminum targets heated with 150 fs Ti:sapphire laser pulses at 790 and 395 nm is presented at an intensity of 5×1017 W/cm2. Whereas at 395 nm spectrally broad intense plasma lines and a weak Kα line with durations (full width at half maximum) of 1–2 ps are observed, the spectrum at 790 nm shows weak narrow plasma lines and an intense Kα line with durations of 3–5 ps.

Generation of hot plasma at solid density by high-contrast ultra-short laser pulses

Abstract We have generated a high-density plasma by focusing frequency-doubled Ti-Sapphire laser pulses at λ=395 nm and a duration of 150 fs on flat solid targets at an intensity of ≈10 17 W / cm 2 . Using solid Al targets tamped by a thin surface layer of MgO we measured the Al K-shell emission. The measured resonance lines ( Ly α , He α and Heβ) and their Li-like and He-like satellites are extremely broad. In addition, we observed clear center of gravity red shifts of the main lines by using the cold Kα line as a reference. The temporal duration of the K-shell emission, measured by an X-ray streak camera, is ≈2.5 ps. Analysis of the spectra yields an effective electron temperature of ≈300 eV and an electron density of ≈(7–10)×10 23 cm −3 . A very weak emission of cold Al Kα indicates a low level of energy deposited by fast electrons in the solid. Essential for achieving these results is the suppression of early time expansion of the Al target by using both, a high-contrast laser pulse and a tamped target.

Conical x-ray crystal spectrometer for time integrated and time resolved measurements

We present a new x-ray spectrometer setup based on a conically curved mica crystal in the von Hamos configuration. An x-ray sensitive charge coupled device or streak camera can be easily coupled in a perpendicular orientation to the spectrally resolved x-ray line focus. Shifting the crystal along its axis of symmetry allows one to change the dispersion and the signal flux density on the detector. A large spectral range at high resolution and adjustable signal intensity is accessible with this setup.

Influence of the density gradient on the duration of Si-Kα pulses generated by a double femtosecond-laser pulse

We have studied the duration of Si-Kα x-ray pulses generated by two subsequent femtosecond-laser pulses (λ=395 nm, pulse duration=180 fs) separated by 20 ps, which were focused on a glass target to intensities of a few 1017 W/cm2. A toroidally bent crystal was used to image the Si-Kα pulses emitted by the target on the cathode of an ultrafast streak camera. While the duration of the first Kα pulse is 1.1 ps, the duration of the second Kα pulse is considerably longer by ≈0.7 ps. This effect is attributed to the preplasma created by the first laser pulse and seen by the second one.

Spectroscopy of plasmas at solid density generated by ultra-short laser pulses

Plasma close to solid density is generated by focusing a frequency doubled Ti-Sapphire laser delivering 150 fs high contrast pulses at an intensity of ≈1017 W/cm2 on solid flat targets. We will first discuss the mechanism of dense plasma production based on hydro code and PIC simulations and on measurements of the laser absorption and the energy transport into the dense target. Then we will present spectrally and temporally resolved measurements of aluminum K-shell spectra. Using a thin layer consisting of MgO or C on top of the Al we record Al K-shell spectra with strongly broadened and shifted lines. The analysis of the measured width and the red shift of the Ly-α line with its He-like satellites and of the He-β line with its Li-like satellites indicates the emission from a plasma at solid density with a temperature in the range 250 to 300 eV. The typical duration of the emission of these lines is ≈2 ps.

X-Ray Spectroscopy of Dense Plasmas Produced by Isochoric Heating with Ultrashort Laser Pulses

Hot plasma with temperatures up to 500 eV and densities close to solid state have been generated by focusing intense ultrashort laser pulses on flat solid targets. The heating process is attributed to the energetic electrons, which are created during the laser plasma interaction. They propagate deeply into the target and heat it isochorically. The laser was a frequency‐doubled Ti:Sapphire laser with an energy of 60 mJ and a pulse duration of 150 fs. It was focused on layered plane solid targets with an incidence angle of 45° and p‐polarized. The K‐shell spectra emitted from thin Al sample layers embedded in solid carbon show features such as line broadening, line shift and strong satellite emission, which are characteristic of dense plasmas. The typical duration of the x‐ray emission is a few ps. Al targets covered by a diamond layer show that the depth (expressed through areal density) up to which the target is heated is close to 1 mg/cm2, which corresponds to the range of 20 keV electrons. This means th...

Time-resolved x-ray spectra from laser-generated high-density plasmas

We focused frequency doubled ultra short laser pulses on solid C, F, Na and Al targets, K-shell emission was systematically investigated by time resolved spectroscopy using a sub-ps streak camera. A large number of laser shots can be accumulated when triggering the camera with an Auston switch system at very high temporal precision. The system provides an outstanding time resolution of 1.7ps accumulating thousands of laser shots. The time duration of the He-(alpha) K-shell resonance lines was observed in the range of (2-4)ps and shows a decrease with the atomic number. The experimental results are well reproduced by hydro code simulations post processed with an atomic kinetics code.

Intense high-order harmonics generated with femtosecond laser pulses at relativistic intensities interacting with high-density plasmas

High-order harmonics generated by weakly relativistic femtosecond laser pulses interacting with solid and thin foil targets were studied. It was found that the conversion efficiency is one or two orders of magnitude larger than that of gas-harmonics and thus due to the high laser intensity also the harmonics are very intense. In addition, anomalies and complex interference structures have been observed in the harmonic spectra of the solid targets. They are explained by simulations. Furthermore, for the first time intense harmonics were observed from thin foil targets and, in particular, from the target rear side. This type of studies provides a promising tool for obtaining information about the laser plasma interaction itself, e.g., on the presence of large fields and oscillations inside the foil up to a penetration depth comparable to the laser wavelength.

Hot solid-state aluminum plasmas, positrons, and neutrons generated with the garching laser facility ATLAS

We report on time-integrated and time-resolved measurements of the K-shell emission from aluminum plasmas at solid-state density isochorically heated with 2-ω ATLAS pulse of high contrast. We compare the measured spectra with simulated ones. We investigate both plane aluminum and layered targets. The latter consist of a top carbon layer upon an aluminum layer of variable thickness deposited on a sigradur (glass:like carbon) substrate. The layered targets are well suited to study electron beam transport through an overdense plasma. In a different type of experiment, we have produced 106 positrons per laser shot by the interaction of an MeV-electron jet emerging from a relativistically self-focused laser channel in an underdense helium plasma whose density is close to the critical one using a 2-mm thick lead disk. We report about details of the measurement and discuss the propsects of this new table-top positron source for a variety of applications when near-future laser systems are envisaged as a driver. F...