J. Nejdl

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

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.

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.

Image Plane Holographic Microscopy With a Table-Top Soft X-Ray Laser

We demonstrate image plane holographic microscopy in the soft X-ray (SXR) spectral region, combining the coherent output from a 46.9-nm wavelength table-top SXR laser and two Fresnel zone plates. Phase and amplitude maps of the object are simultaneously obtained from holograms created at the image plane by the superposition of a reference and object beam originating from the zero and first diffraction order of the zone plates. We have used the microscope to record holograms of nanometer-scale periodic Si elbow patterns with 30% absorption contrast at the laser wavelength. The measured phase shift of 2.3 rad accurately predicts the Si dense line step height of 100 nm. The scheme is scalable to shorter wavelengths and allows for simultaneous high spatial and temporal resolution.

High-order harmonic generation using a multi-jet gas puff target

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Temporal profile of betatron radiation from laser-driven electron accelerators

The temporal profile of X-ray betatron radiation was theoretically studied for the parameters available with current laser systems. Characteristics of the betatron radiation were investigated for three different configurations of laser wakefield acceleration: typical self-injection regime and optical injection regime with perpendicularly crossed injection and drive beams, both achievable with 100 TW class laser, and ionization injection regime with a sub-10 TW laser system that was experimentally verified. Constructed spectrograms demonstrate that X-ray pulse durations are in the order of few tens of femtoseconds and the optical injection case reveals the possibility of generating X-ray pulses as short as 2.6 fs. The X-ray pulse duration depends mainly on the length of the trapped electron bunch as the emitted photons copropagate with the bunch with nearly the same velocity. These spectrograms were calculated using a novel simplified method based on the theory of Lienard-Wiechert potentials. It takes adva...

Characterization of Zn X-Ray Laser at PALS Centre, Its Applications in Dense Plasma Probing and Astrophysics

This report presents the results from experiments at PALS Centre using a Zn X-ray laser with the pulse length of 0.15 ns and the wavelength of 21.2 nm, working in single or double pass regime with the output energy of 0.4 mJ or 4 mJ per pulse, respectively. The current X-ray laser was experimentally examined to obtain its temporal coherence and spectral width using a path-difference interferometer. The double pass regime shows that QSS plasma based source-amplifier is promising for “short” fs soft X-ray pulses. The X-ray laser is commonly used for user’s experiments. Its advantages can be shown in applications such as probing of dense plasmas (up to 2.5×1024 cm−3) or single shot experiments (4×1014 photons/pulse). The simple technique based on Talbot effect was used to obtain the gradients of electron densities of line plasmas produced under conditions corresponding to XRL’s amplifiers operating in TCE and QSS regime. To investigate radiative shock wave in laboratory is challenging in aspects of the optimization of experimental parameters. Due to the high electron density (1022 cm−3) produced in the gas medium propagated by the shock wave, the velocity of the shock wave, and the absorption losses on optical elements, it is necessary to use the energetic single shot probe.

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.

Bessel spatial profile of a soft x-ray laser beam

We report far-field profile measurements of an optical-field-ionized high-order harmonic-seeded soft x-ray laser. We show that the beam transverse profile can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the infrared laser pump intensity. These experimental data are supported by a complete numerical modeling including a two-dimensional plasma amplifier simulation and a two-level soft x-ray amplification using a Maxwell–Bloch treatment. This model takes into account the experimental high-order harmonic wavefront and intensity before it is numerically amplified.

Electron Temperature Measurements of Model Ne-Like X-Ray Laser Plasmas From Resonance keV Spectra

We present results of temperature measurements of Ne-like Zn plasmas created by an intense laser pulse, using slab targets. Pulses with typical energy of 50 J in 300 ps were focused to a line or point foci, in which the pumping pulse energy was varied from shot to shot. The electron temperature in dependence on the pulse energy and irradiation intensity is studied. The temperature is inferred spectroscopically from a set of selected Ne-like spectral lines. The experimental results are compared with the numerical simulations made by the hydrodynamic code EHYBRID.