E. Förster

Short-pulse laser - plasma interactions

Recent theoretical and experimental research with short-pulse, high-intensity lasers is surveyed with particular emphasis on new physical processes that occur in interactions with low- and high-density plasmas. Basic models of femtosecond laser - solid interaction are described including collisional absorption, transport, hydrodynamics, fast electron and hard x-ray generation, together with recently predicted phenomena at extreme intensities, such as gigagauss magnetic fields and induced transparency. New developments in the complementary field of nonlinear propagation in ionized gases are reviewed, including field ionization, relativistic self-focusing, wakefield generation and scattering instabilities. Applications in the areas of x-ray generation for medical and biological imaging, new coherent light sources, nonlinear wave guiding and particle acceleration are also examined.

Monochromatic focusing of subpicosecond x-ray pulses in the keV range

An effective x-ray optical method to focus keV x-ray pulses shorter than one picosecond by using spherically or toroidally bent crystals is presented. The spectral, spatial, and time-dependent properties of focusing by two-dimensional bent crystals are calculated by considering geometrical effects, physical limitation in high performance crystal optics, and reflectivities obtained by x-ray diffraction theory. These properties are compared with first experimental results of focusing x rays from a plasma created by a laser pulse with 4.5 mJ energy and 100 fs pulse length. The x-ray signals, simultaneously obtained from a von Hamos spectrometer and two-dimensional bent crystals are compared and found in good agreement with theoretical data. The possibilities and aspects of laser pump x-ray probe experiments using this type of x-ray optics system and currently available laser systems are discussed.

Microfocus Cu K ? source for femtosecond x-ray science

We demonstrate a subpicosecond 1 kHz femtosecond x-ray source with a well-accessible quasi-point size (10??m diameter) providing Cu K? emission with a maximum flux of 6.8×1010? photons?s for continuous operation of 10 h. A new geometry that essentially facilitates the adjustment and diminishes the temporal jitter between the x-ray probe and the laser pump pulse is implemented for time-resolved diffraction experiments.

X-ray microscopy of laser-produced plasmas with the use of bent crystals

X-ray spectroscopical and microscopical methods are used for the determination of the spectral and spatial distribution of X-ray intensity of laser-produced plasmas. The use of Bragg reflections of two-dimensionally bent crystals enables the X-ray microscopical imaging in narrow spectral ranges (Δλ/λ=10 −4 to 10 −2 ) with wavelengths 0.1 nm

Novel method for characterizing relativistic electron beams in a harsh laser-plasma environment.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

X-ray reflection properties of elastically bent perfect crystals in Bragg geometry

Reflection curves of bent crystals were calculated using the Takagi–Taupin theory of dependence on bending radius, wavelength, crystal thickness and Bragg angle. The reflection properties of bent crystals were measured for bending radii down to 90 mm. Usually, for the measurement of rocking curves of crystals with large bending radii, the double-crystal diffractometer was used in parallel position (n, −n). A special achromatic diffractometer consisting of a plane and a bent crystal is proposed. It is used to measure rocking curves of bent crystals with small bending radii (R < 1 m). Experimental values show close agreement with the theory. The reflection properties are important for X-ray microscopy with two-dimensionally bent crystals and for X-ray spectroscopy with bent crystals.

Ablation of solids using a femtosecond extreme ultraviolet free electron laser

The ablation of solids by high energy femtosecond pulses from an extreme ultraviolet (XUV) free electron laser has been investigated using picosecond optical imaging. The time-resolved measurements are supplemented by an analysis of the permanent structural surface modifications. Compared with femtosecond optical excitation, distinct differences in the material response are found which are attributed to the increased penetration depth of the XUV radiation and the absence of any absorption nonlinearities.

Flat and Spherically Bent Muscovite (Mica) Crystals for X-ray Spectroscopy

Essential parameters for the application of crystals to quantitative X-ray spectroscopy are the upper wavelength limit and the quantitative reflection properties (intrinsic resolving power, luminosity) of the crystal. Due to the large lattice constant, muscovite, a mica group mineral, can be used in the wavelength range up to about 2nm. Muscovite crystals can be bent to small radii of curvature due to their favourable cleavage and elastic properties. Characteristic reflection properties at reflections 002 – 00 24 were investigated theoretically and experimentally. The integrated reflectivity was calculated for various reflections of perfect flat as well as spherically bent muscovite crystals with curvature radii R = 100 and R = 186mm. It was measured for flat crystals in the reflections 00 10 – 00 26 using CuKα- and MoKα-radiation from X-ray tubes and compared with calculations for both perfect and mosaic crystals. Available high-quality muscovite crystals have a mosaic structure with a mosaic spread of about 1 arcmin. This mosaic spread limits the spectral resolving power for high reflection orders.

High-luminosity, high-resolution, x-ray spectroscopy of laser-produced plasma by vertical-geometry Johann spectrometer

Successful applications of the vertical-geometry Johann spectrometer (VJS) in advanced plasma spectroscopy are reported. Different experimental configurations are discussed, and a complete quantitative analysis of the spectrometer function including the transfer of the spectral lines is presented. The method for reconstruction of the spectra emitted from extended, quasilinear sources is described; the precision attainable and possible sources of errors are discussed. Due to the combination of high collection efficiency, and spectral and one-dimensional spatial resolution, the instrument is particularly suitable for high-precision measurements of the spectral line profiles and positions in nonhomogeneous plasmas. The examples of experimental results, which are superior to those obtained in earlier measurements, demonstrate the VJS performance and suggest a broad field of possible applications.

Kinetic to thermal energy transfer and interpenetration in the collision of laser-produced plasmas

An experimental and numerical analysis of the collision of two plasmas produced from laser-exploded Al/Al and Al/Mg pairs of foils is presented. Various imaging and spectroscopic x-ray techniques have been used to diagnose the collision over a broad range of intertarget distances and laser intensities. Ion temperatures in the 10 keV range have been measured from Doppler broadening. Electron temperatures and densities have been deduced from line ratios and interpenetration distances have been determined by the spatial extent of Mg and Al x-ray lines. Eulerian multifluid simulations have been developed and coupled to atomic physics postprocessing. The comparison of the measurements with these simulations shows that interpenetration prevails at large intertarget distances and high laser intensities; kinetic to thermal energy transfer then takes place on a ∼200-μm wide region and during ∼150 ps.