P. Homer

Double Lloyd’s mirror: versatile instrument for XUV surface interferometry and interferometric microscopy

We have developed a double Lloyds mirror wavefront-splitting interferometer, constituting a compact device for surface probing in the XUV and soft X-ray spectral domain. The device consists of two independently adjustable superpolished flat surfaces, operated under grazing incidence angle to reflect a diverging or parallel beam. When the mirrors are appropriately inclined to each other, the structure produces interference fringes at the required distance and with tuneable fringe period. The double Lloyds mirror may be used alone for surface topography with nanometric altitude resolution, or in conjunction with an imaging element for interferometric XUV surface microscopy. In the latter case, resolution in the plane of the probed surface is about micron, which is given by the quality of the imaging element and/or by the detector pixel size. Here, we present results obtained using the double Lloyds mirror in two separate X-ray laser and high harmonics generation (HHG) application projects. The first experiment was aimed at understanding microscopic nature of the effects involved in laserinduced optical damage of thin pellicles, exposed to sub-ns laser pulses (438 nm) producing fluence of up to 10 Jcm-2. The probing source in this case was a QSS neon-like zinc soft X-ray laser, proving a few mJ at 21.2 nm in ~100-ps pulses. The second experiment was carried out using a narrowly collimated HHG beam near 30 nm, employed to topographically probe the surface of a semiconductor chip.

Ablative microstructuring with plasma-based XUV lasers and efficient processing of materials by dual action of XUV/NIR–VIS ultrashort pulses

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a plasma-based extreme ultraviolet (XUV) laser at 21 nm. A nickel mesh with a period of 100 μ m was 10×demagnified and imprinted on poly(methyl methacrylate) (PMMA) via direct ablation. This first demonstration of single-shot projection, single-step lithography illustrates the great potential of XUV lasers for the direct patterning of materials with a resolution scalable down to the submicrometer domain. In the second part, we present a novel experimental method for improving the efficiency of surface processing of various solids achieved by simultaneous action of XUV, obtained from high-order harmonic generation, and near-infrared (NIR)–VIS laser pulses. The NIR–VIS pulse interacts with free charge carriers produced by the energetic XUV photons, so that its absorption dramatically increases. Laser-induced periodic surface structures were effectively produced using this technique.

Development of Plasma X-Ray Amplifiers Based on Solid Targets for the Injector-Amplifier Scheme

Results of experimental studies aimed at generation and diagnostics of advanced soft X-ray amplifiers, produced from solid targets, are presented. 2D profiles of electron density of short plasma columns, generated by ~300-ps laser pulses under various illuminating conditions, were investigated by near-field distribution of the plasma self-emission, and by X-ray laser backlighting at 21 nm, accessing in the given geometry electron densities of 1022 cm-3. The obtained data indicate that by employing line focus with concave intensity profile it is possible to generate laterally highly uniform plasma columns of width ~500 °m, potentially suitable as amplifiers with negligible lateral refraction. By X-ray laser backlighting we further probed the morphology and gain region of test Zn plasmas, pumped by a sequence of a loosely focused weak prepulse and tightly focused main pulse, separated by 5.5 ns. The data clearly show the beneficial role of the prepulse in lateral homogenization of the plasma, and reveal narrow ~50-°m gain region.

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.

Repetition rate target and fusion chamber systems for HiPER

We review development in the repetition-rate target area systems and technologies within the Work Package 15 of the HiPER Preparatory Phase project. The activities carried out in 2009-2010 have been involving analysis of solutions and baseline design of major elements of the repetition-rated fusion chamber, analysis of prospective injector technologies, numerical modelling of target survival during acceleration phase and during flight in the environment of fusion chamber, analysis of options of remote handling, systems of mitigation of fusion debris, and others. The suggested solutions assume operation at the repetition rate of 10 Hz and fusion yield between 20 and 100 MJ. Shock ignition is assumed as the baseline ignition scenario, although some technologies are applicable in the fast ignition; a number of the technologies identified are exploitable as well in the indirect drive. The operation of the HiPER repetition-rate chamber will contribute to technology development for the Demonstration Reactor HiPER facility.

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.

Advanced optical damage studies using x-ray laser interferometric microscopy

We present early results of an application of X-ray laser, aimed at understanding the effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses. For the purpose of the experiment, a novel interferometric microscopy technique was designed and tested. The interferometric beamline employed a double Lloyds mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8 along a plane inclined with respect to the propagation direction of the X-ray beam. The objects investigated were thin plane beamsplitters made of fused silica (SiO2), irradiated by damaging laser light at 438 nm and in situ probed by the developed technique of interferometric microscopy. The soft X-ray beam was emitted by neon-like zinc laser, delivering up to 10 mJ at 21.2 nm. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage of the rear surface of the beamsplitter occurs approximately during of much after the laser pulse. Another issue examined by the X-ray interferometric microscopy technique was whether the surface perturbation seen shortly after the impact of the damaging pulse is associated or not with the pattern of permanent surface modifications.

Development of soft x-ray lasers at PALS and their applications in dense plasma physics

We present a review of recent development and applications of soft x-ray lasers, undertaken at the PALS Centre. The applications benefit from up to 10-mJ pulses at the wavelength of 21.2 nm. We describe the pumping regimes used to produce this soft x-ray laser, and outline its emission characteristics. A significant fraction of applications carried out using this device includes probing of dense plasmas produced by IR laser pulses and high-energy-density-in-matter experiments. Results obtained in these experiments are reviewed, including x-ray laser probing of dense plasmas, measurements of transmission of focused soft x-ray radiation at intensities of up to 1012 Wcm-2, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering

Measurements of x-ray laser wavefront profile using PDI technique

The point diffraction interferometer (PDI) is a simple self-referencing interferometer, designed here to measure the wavefront profile of a soft X-ray laser emitting at the wavelength of 21.2 nm. It is a monolithic device consisting of a thin filter and a very small pinhole (~1 μm), located near the axis of the X-ray laser focal spot. The foil material around the hole is semitransparent for the X-ray radiation of interest, acting like a neutral density filter. The small pinhole is a diffraction aperture and plays a spatial filtering role, generating spherical wave that acts a reference. The interferometric pattern is produced on a detector placed a few centimeters behind the foil. The beam wavefront profile is reconstructed from the information encoded in the pattern. In this paper we discuss the overall design of the PDI wavefront sensor setup, namely its geometry, fringe contrast, level of the detected signal, size of the pinhole, and candidate materials for the PDI foil.