E. Wolfrum

Gain saturation of the Ni-like X-ray lasers

Abstract Driven by a double 75 ps pulse with 2.2 ns separation, saturated operation of nickel-like Ag, In, Sn, and Sm X-ray lasers have been demonstrated with only 75 J drive energy on target. The variation of X-ray laser output with target length is found to fit well to a simple model for an amplified spontaneous emission (ASE) laser including saturation. Small signal gains of ∼10 cm −1 , effective gain length products of ∼18, and saturation irradiance of (1–5)×10 10 W/cm 2 are measured for these lasers using a fitting procedure.

Thomson scattering measurements of heat flow in a laser-produced plasma

Measurements of the electron distribution and heat flow between the critical and ablation surfaces in a laser-produced plasma have been obtained using Thomson scattering. A frequency-quadrupled probe beam was used to obtain Thomson spectra at above-critical densities in a plasma produced by irradiation of solid targets with the fundamental laser light at irradiances of 3 × 1014 W cm−2. Comparison of Thomson spectra at the ion acoustic frequency (sensitive to the cold return current) with simulated spectra shows that the data are consistent with Fokker–Planck simulations of the electron distribution function, providing the first direct information on the electron distribution function.

Measurement of single mode imprint in laser ablative drive of a thin Al foil by extreme ultraviolet laser radiography

The temporal development of laser driven single mode perturbations in thin Al foils has been measured using extreme ultraviolet (XUV) laser radiography. 15, 30, 70 and 90 μm single modes were imprinted on 2 μm thick Al foils with an optical driver laser at 527 nm for intensities in the range 5×1012 to 1.5×1013u2009Wu2009cm−2. The magnitude of the imprinted perturbation at the time of shock break out was determined by fitting to the data estimated curves of growth of the Rayleigh–Taylor instability after shock break out. The efficiency of imprinting is independent of perturbation wavelength in the parameter range of this experiment, suggesting little influence of thermal conduction smoothing. The results are of interest for directly driven inertially confined fusion.

Measurements of the XUV transmission of aluminium with a soft x-ray laser

We have used XUV lasers to make absolute measurements of the photoabsorption coefficient of Al at energies just below that of the L3 absorption edge at 72.7 eV. Transmission measurements at photon energies of 53.7 and 63.3 eV have been made using Ne-like Ni and Ge XUV lasers. The XUV laser output was recorded in first and second orders using a flat-field spectrometer. Al foils with steps of various thicknesses were placed over the first order diffracted signal, while the second order diffraction was used to monitor the beam profile at each position. The transmission data agree extremely well with the original measurements at these wavelengths made by Henke and co-workers (Henke B L, Gullikson E M and Davis J C 1993 At. Data Nucl. Data Tables 54 181), but are in conflict with subsequent measurements which are currently in common use (Gullikson E M, Denham P, Mrowka S and Underwood J H 1994 Phys. Rev. B 49 16 283). The exact values of the absorption coefficients in this region of the spectrum have significant implications for the diagnosis of the energy and intensity output of XUV lasers.

Optimization of double pulse pumping for Ni-like Sm x-ray lasers

We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 A, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model.

X-ray line reabsorption in a rapidly expanding plasma

Abstract We present high-resolution spectroscopic measurements of the optically thick hydrogen-like Al Ly-α line shape from a cylindrically expanding plasma. The cylindrical expansion is produced by symmetrically irradiating a 120 μm diameter coated wire target with six beams of the VULCAN Ndxa0:xa0glass laser at an irradiance of 2×10 14 W cm −2 . Small shifts in the line position and changes in the line shape can be attributed to radiation emitted from different regions of the plasma and passing through different gradients in plasma density, temperature, and velocity. The experimental profiles are compared to a time-dependent hydrodynamics code incorporating a Sobolev escape probability treatment of the radiative transport.

Detailed hydrodynamic and X-ray spectroscopic analysis of a laser-produced rapidly-expanding aluminium plasma

Abstract We present a detailed analysis of K-shell emission from laser-produced rapidly-expanding Al plasmas. This work forms part of a series of experiments performed at the Vulcan laser facility of the Rutherford Appleton Laboratory, UK. 1-D planar expansion was obtained by over-illuminating Al-microdot targets supported on CH plastic foils. The small size of the Al-plasma ensured high spatial and frequency resolution of the spectra, obtained with a single crystal spectrometer, two vertical dispersion variant double crystal spectrometers, and a vertical dispersion variant Johann Spectrometer. The hydrodynamic properties of the plasma were measured independently by spatially and temporally resolved Thomson scattering, utilizing a 4ω probe beam. This enabled sub- and super-critical densities to be probed relative to the 1ω heater beams. The deduced plasma hydrodynamic conditions are compared with those generated from the 1-D hydro-code Medusa, and the significant differences found in the electron temperature discussed. Synthetic spectra generated from the detailed term collisional radiative non-LTE atomic physics code Fly are compared with the experimental spectra for the measured hydrodynamic parameters, and for those taken from Medusa. Excellent agreement is only found for both the H- and He-like Al series when careful account is taken of the temporal evolution of the electron temperature.

Optimisation of drive pulse configuration for a Ni-like Sn X-ray laser at 12 nm☆

Abstract The current saturated operation of X-ray lasers at wavelengths > 15 nm requires at least kilojoule drive energy, which is only available at the largest laser installations in the world. Using a specially designed drive pulse configuration, saturated operation of a Ni-like Sn X-ray laser at 12 nm has been achieved with only 75 J drive energy. An efficiency as high as 9 × 10 6 in converting laser energy from the 1 eV optical spectral range to the 100 eV soft X-ray range has been reached. This paves the way for applications of saturated X-ray lasers at 12 nm at many other smaller laboratories.

Progress with saturated soft X-ray lasers pumped by the Vulcan laser

Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.

A Thomson scattering post-processor for the MEDUSA hydrocode

Abstract In order to understand the physical processes that occur in laser-produced plasmas it is necessary to diagnose the time-dependent hydrodynamic conditions. Thomson scattering is, in principle, an ideal diagnostic as it provides a non-intrusive method of measuring ion and electron temperature, electron density, plasma velocity, and heat flow. We describe here a post-processor for the MEDUSA hydrocode that simulates streak camera images of the Thomson spectra. The post-processor can be used in three ways: (1) creating simulated streak camera images that can be compared directly with experimental data, (2) evaluating experimental designs to determine the viability of the Thomson scattering diagnostic, and (3) as an automated data analysis routine for extracting hydrodynamic parameters from a calibrated experimental streak camera image.