M. Kozlová

Experimental study of radiative shocks at PALS facility

We report on the investigation of strong radiative shocks generated with the high energy, sub-nanosecond iodine laser at PALS. These shock waves are characterized by a developed radiative precursor and their dynamics is analyzed over long time scales (50 ns), approaching a quasi-stationary limit. We present the first preliminary results on the rear side XUV spectroscopy. These studies are relevant to the understanding of the spectroscopic signatures of accretion shocks in Classical T Tauri Stars.

Beam properties of a deeply saturated, half-cavity zinc soft-x-ray laser

We report what is to our knowledge the first experimental study of beam quality and prepulse effect for a deeply saturated neonlike Zn soft-x-ray laser at 21.2 nm, comparing two distinct regimes of operation: amplified spontaneous emission (in single pass) and double pass. The single-pass x-ray laser output emitted by a 30-mm-long plasma column delivers a smooth, highly symmetric ellipsoidal beam with horizontal and vertical divergence of 3(±0.5) and 5(±0.5) mrad, respectively. With a half-cavity consisting of a Mo:Si flat multilayer mirror, the x-ray laser output is boosted by factor of ∼11, providing a narrowly collimated beam with horizontal and vertical divergence of 3.8(±0.5) and 5.8(±0.5) mrad, respectively. The dependence of the beam parameters on the half-cavity setup and on the level of prepulse is described and discussed.

Single-shot soft x-ray laser-induced ablative microstructuring of organic polymer with demagnifying projection

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a Ne-like Zn 21.2 nm soft x-ray laser. A nickel mesh with a period of 100 microm was approximately 10x demagnified and imprinted on poly(methyl methacrylate) via direct ablation. The quality of the ablated microstructure was found to be mainly dependent on the quality of the projected mask. This first demonstration (to our knowledge) of single-shot projection, single-step lithography illustrates the potential of soft x-ray lasers for the direct patterning of materials with a resolution scalable down to submicrometer domain.

Multi-millijoule, deeply saturated x-ray laser at 21.2 nm for applications in plasma physics

We report on the development of a multi-millijoule, ultra high-brightness Ne-like soft x-ray laser (XRL) at the wavelength of 21.2 nm, recently undertaken at the PALS Centre. The device has been implemented as a routine radiation source delivering about 4 mJ of energy in pulses with a duration of ~100 ps, in a narrowly collimated beam exhibiting high spatial quality and possessing high spatial coherence. The peak power of about 40 MW, generated by this device, is the largest value attained by an XRL to date. The same holds true for the peak spectral brightness, amounting to 1027 photons s−1 mm−2 mrad−2 0.1%BW−1. The active medium consists of a 3 cm long plasma column, sequentially generated from a slab target by an energy of ~500 J delivered by the iodine 1.315 μm driving laser. The population inversion is produced by focusing tightly down to the line ~500 J of energy into a much wider preplasma column, which makes it possible to create a lasing medium with a reduced lateral density gradient and thereby to minimize the lateral refraction of the x-ray beam. The multi-millijoule output is generated in double-pass amplification regime, achieved by a carefully designed half cavity using a flat multilayer Mo : Si mirror positioned a few millimetres near the plasma end. We describe the measured characteristics of the output beam generated by double-pass amplification, and illustrate its spatial beam coherence obtained using a Fresnel wavefront splitting interferometer. Given the available energy, peak power and brightness in a single pulse, the demonstrated XRL constitutes a tool for novel applications in both solid state and plasma physics. Examples of such applications are the generation of high-energy density, low-temperature plasmas relevant to laboratory astrophysics, x-ray interferometry of plasmas and solids with spatial resolution ranging from a micron to a few nanometres according to the specific experimental arrangement, or study of soft x-ray photoionization.

Measuring the electron density gradients of dense plasmas by deflectometry using short-wavelength probe

A new and simple experimental technique for the measurement of electron density gradients in dense laser-produced plasmas using an electromagnetic wave probe is presented. The main advantage of this method is the low requirements on coherence of the probing beam. The method is based on measuring the deformation of the Talbot pattern of a two-dimensional grating that stems from the distortion of the probe beam wave-front caused by the gradients of the index of refraction. The compromise between spatial resolution and sensitivity for the given wavelength of the probe beam is set by the experimental design. The proposed technique was experimentally verified on plasmas that were created by either a point focus or a line focus of a laser interacting with various solid targets. In the experiments reported here, all plasmas were probed by a Ne-like Zn x-ray laser beam at 21.2 nm, but the technique is applicable for any wavelength of the probe.

Scaling stellar jets to the laboratory: The power of simulations

Advances in laser and Z-pinch technology, coupled with the development of plasma diagnostics, and the availability of high-performance computers, have recently stimulated the growth of high-energy density laboratory astrophysics. In particular, a number of experiments have been designed to study radiative shocks and jets with the aim of shedding new light on physical processes linked to the ejection and accretion of mass by newly born stars. Although general scaling laws are powerful tools to link laboratory experiments with astrophysical plasmas, the phenomena modeled are often too complicated for simple scaling to remain relevant. Nevertheless, the experiments can still give important insights into the physics of astrophysical systems and can be used to provide the basic experimental validation of numerical simulations in regimes of interest to astrophysics. We will illustrate the possible links between laboratory experiments, numerical simulations, and astrophysics in the context of stellar jets. First we will discuss the propagation of stellar jets in a cross-moving interstellar medium and the scaling to Z-pinch produced jets. Our second example focuses on slab-jets produced at the Prague Asterix Laser System laser installation and their practical applications to astrophysics. Finally, we illustrate the limitations of scaling for radiative shocks, which are found at the head of the most rapid stellar jets.

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.

Plasma density-gradient measurement using x-ray laser wave-front distortion

We present an experimentally simple technique for the measurement of electron density gradients in dense laser plasmas (the plasma region of electron density up to 1024 cm-3 can be investigated with the use of available XRLs). The distortion of the XRL wave-front caused by the gradients of the electron density is measured using Talbot pattern deformation. The plasma probed by the XRL is imaged on the CCD plane, then a 2D grating is put in front of the chip so that the Talbot plane of this grating fits on the CCD. The compromise between the spatial resolution and the sensitivity for the given wavelength of the probe must be set within the grating design. The main advantages of this method are low requirements on spatial coherence of the probing beam as well as the simple alignment, which are the main difficulties of interferometry using radiation of XRLs.

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.