M. H. Edwards

Measurement of the duration of X-ray lasing pumped by an optical laser pulse of picosecond duration

Measurements of the duration of X-ray lasing pumped with picosecond pulses from the VULCAN optical laser are obtained using a streak camera with 700 fs temporal resolution. Combined with a temporal smearing due to the spectrometer employed, we have measured X-ray laser pulse durations for Ni-like silver at 13.9 nm with a total time resolution of 1.1 ps. For Ni-like silver, the X-ray laser output has a steep rise followed by an approximately exponential temporal decay with measured full-width at half-maximum (FWHM) of 3.7 (±0.5) ps. For Ne-like nickel lasing at 23.1 nm, the measured duration of lasing is ≈10.7 (±1) ps (FWHM). An estimate of the duration of the X-ray laser gain has been obtained by temporally resolving spectrally integrated continuum and resonance line emission. For Ni-like silver, this time of emission is ≈22 (±2) ps (FWHM), while for Ne-like nickel we measure ≈35 (±2) ps (FWHM). Assuming that these times of emission correspond to the gain duration, we show that a simple model consistently relates the gain durations to the measured durations of X-ray lasing.

Comparison of simulated and experimental time resolved emission for a Ne-like nickel x-ray laser

X-ray laser output at 23.1 nm and the intensity of resonance line and continuum emission between 0.6 and 1.8 nm emitted from a nickel plasma are simulated using a fluid and atomic physics program. The simulations are undertaken for the conditions of a recent experiment using a Nd:glass laser with a ~1.2 ps pulse at 7 × 1015 W cm−2 irradiance pumping a plasma pre-formed by a 280 ps duration pulse at 2 × 1013 W cm−2 with peak-to-peak pulse separation usually set at 300 ps. The simulated duration of x-ray lasing (~12 ps) agrees with the measured laser duration and the temporal output of continuum and resonance lines is in agreement with measurements made using a streak camera.

Development and applications of multimillijoule soft X-ray lasers

We review development of multimillijoule X-ray lasers and of applications of these new laboratory sources carried out recently at the PALS facility. A backbone of this development is the neon-like zinc laser providing saturated output at 21.2 nm, with up to 10 mJ of energy per pulse. This represents currently the most energetic soft X-ray laboratory source. Recent improvements in its operation include better control of the beam shape, and more complete understanding of the prepulse pumping. The laser at 21.2 nm has been employed for a number of application experiments reviewed in this paper. They include transmission measurements of intense soft X-ray radiation, studies of fundamental processes of soft X-ray ablation, ablation micropatterning, feasibility study of soft X-ray Thomson scattering from dense plasmas, visualization of nanometric transient perturbation of optical surfaces, measurements of ablation rates of foils heated by IR pulses, and studies of 2D plasma hydrodynamics in the regime of sequential illumination.

Recent Progress on the Understanding of the Transient Ni‐like Ag X‐ray Laser at 13.9 nm at LULI facilities

This paper summarises our recent progress achieved in the characterisation and understanding of the Ni‐like Ag transient X‐ray laser pumped under traveling wave irradiation. We carried out two experiments at the LULI CPA laser facility. Several diagnostics of the plasma emission at the XRL wavelength or in the keV range indicate the presence of small‐scale spatial structures in the emitting XRL source. Single‐shot Fresnel interference patterns at 13.9 nm were successfully obtained with a good fringe visibility. For the first time we obtained plasma images with a high spatial resolution about 1 μm, showing the effects of pumping parameters on the X‐ray laser far‐field.

Characterisation of the grazing incidence pumped nickel-like molybdenum x-ray laser and experimental investigation into lasing from high Z targets

Using the grazing incidence pumping technique with a 600 mJ, 500 ps background pulse and a 250 mJ, 200 fs main pulse the lasing emission from a molybdenum target has been studied. A flat field spectrometer designed to observe the X-ray laser emission in both the first and second orders was used to record the time integrated data. Time resolved data was obtained by installing an Axis-Photonique PX1 X-ray streak camera to observe the first order output from the spectrometer whilst retaining the time integrated second order observation. In this paper both time integrated and time resolved data are presented for a range of grazing angles, target lengths, delays between pumping pulses and pumping energy. Comparisons are also drawn with simulations from the Ehybrid and Medusa codes. An additional experiment is also described in which a two colour pumping method is used to investigate lasing at short wavelength from high Z targets (Z ≥ 62).

X-ray lasers as probes to measure plasma ablation rates

The rate of laser ablation at irradiances of ~2x1014 Wcm-2 of solid iron and aluminum has been measured using the transmission of a neon-like zinc X-ray laser at 21.2 nm through thin iron and aluminum targets. It is shown that the opacity of ablated material falls rapidly with increasing temperatures and decreasing density from the solid value. As ablated plasma becomes transparent to the X-ray laser flux, the thickness of solid, unablated material and hence the rate of ablation can be measured from time resolved X-ray laser transmission. A self-regulating model of laser ablation and fluid code simulations with absorption to thermal plasma of 5-10% show agreement with our measured ablation rates.

Plasma Opacity and Laser Ablation Measurements Using X-Ray Lasers

The use of x-ray lasers as probes of the opacity of hot dense plasma and rates of laser ablation is considered. It is shown that x-ray lasers are sufficiently bright to overcome plasma emission and enable plasma opacity to be measured. A demonstration experiment is presented where the temporal evolution of the opacity of a thin iron plasma at high temperature (30 – 250 eV) formed from an initially 50 nm thick solid tamped with a plastic overlay after heating by a laser pulse has been measured using the transmission of a nickel-like silver x-ray laser at 13.9 nm. The experimental results are compared to transmission calculations based on the iron opacity evaluated in a post-processor from predictions of the plasma conditions using a fluid and atomic physics code (EHYBRID). In another experiment, it is shown that laser ablation of a solid iron layer that is not tamped can be determined by the change in transmission of a 21.2 nm x-ray laser.

Hot dense plasma opacity measurements using x-ray lasers

Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.

X-Ray Lasers as Probes of Plasma Parameters

General issues relevant to the probing of plasmas with backlighters are first discussed. It is shown that soft x-ray wavelengths (> 10 nm) can generally probe thicker plasmas than harder radiation ( 100 eV). Demonstration experiments are discussed where the transmission of x-ray lasers through other laser-heated solid targets have been used (i) to measure the rate of laser ablation of solid targets and (ii) to probe the opacity of a plasma material. It is shown that narrow bandwidth radiation, such as from an x-ray laser, offers the ability to probe much more optically thick plasmas than backlighters composed of moderate to wide bandwidth spectral lines.

Ablation Measurements Using Ni-Like Ag X-Ray Laser Transmission

Abstract. Results of a recent experiment to measure laser ablation rates from solid targets consisting of thin (50 nm) Fe at irradiances of ~ 2×1013 Wcm−2 are presented. A Ni-like Ag X-ray laser at 13.9 nm was utilised to probe the targets and measurements of the transmission of the X-ray laser through the target were recorded by a flat-field spectrometer. The laser ablation rate is determined by the transmission of the X-ray laser through the remaining, unablated, iron layer. A self-regulating model of laser ablation is fitted to the experimental data to give a value of the absorption, A, of the heating laser in the target plasma of A = 0.01.