Referierte Publikationen

We report the observation of an interference between the electric dipole (E1) and the magnetic quadrupole (M2) amplitudes for the linear polarization of the Ly-alpha(1) (2p(3/2) -> 1s(1/2)) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-alpha(1) radiation due to the E1-M2 amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the E1 and the M2 transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the 2p(3/2) state.

We lay out a new approach to solving the two-centre Dirac eigenvalue problem. It is based on the application of B-spline basis sets constructed in Cassini coordinates. The approach provides a very promising way for the theoretical description of various atomic processes such as charge transfer, excitation, ionization and electron-positron pair production that accompany slow collisions of heavy ions. Moreover, yielding a (quasi-) complete set of eigensolutions, the finite basis set method allows a systematic analysis of the quantum electrodynamics (QED) corrections to the energy levels of heavy quasi-molecules formed in these collisions.

The high-harmonic generation (HHG) yield depends on several important experimental parameters and can be described with models including single atom response and propagation effects. In our recent paper we extended this description by adding a stimulated emission process and named it self-seeded X-ray parametric amplification (XPA).

The nonperturbative electron-positron pair production (Schwinger effect) is considered for space- and time-dependent electric fields E(x,t). Based on the Dirac-Heisenberg-Wigner formalism, we derive a system of partial differential equations of infinite order for the 16 irreducible components of the Wigner function. In the limit of spatially homogeneous fields the Vlasov equation of quantum kinetic theory is rediscovered. It is shown that the quantum kinetic formalism can be exactly solved in the case of a constant electric field E(t)=E_0 and the Sauter-type electric field E(t)=E_0 sech^2 (t/τ). These analytic solutions translate into corresponding expressions within the Dirac-Heisenberg-Wigner formalism and allow to discuss the effect of higher derivatives. We observe that spatial field variations typically exert a strong influence on the components of the Wigner function for large momenta or for late times.

We present a novel, cascaded acceleration scheme for the generation of spectrally controlled ion beams using a laser-based accelerator in a 'double-stage' setup. An MeV proton beam produced during a relativistic laser-plasma interaction on a thin foil target is spectrally shaped by a secondary laser-plasma interaction on a separate foil, reliably creating well-separated quasi-monoenergetic features in the energy spectrum. The observed modulations are fully explained by a one-dimensional (1D) model supported by numerical simulations. These findings demonstrate that laser acceleration can, in principle, be applied in an additive manner.

We present experimental results from an all-optical diagnostic method to directly measure the evolution of the hot-electron distribution driving the acceleration of ions from thin foils using high-intensity lasers. Central parameters of laser ion acceleration such as the hot-electron density, the temperature distribution and the conversion efficiency from laser pulse energy into hot electrons become comprehensively accessible with this technique.

We study ALP-photon-conversion within strong inhomogeneous electromagnetic fields as provided by contemporary high-intensity laser systems. We observe that probe photons traversing the focal spot of a superposition of Gaussian beams of a single high-intensity laser at fundamental and frequency-doubled mode can experience a frequency shift due to their intermittent propagation as axion-like-particles. This process is strongly peaked for resonant masses on the order of the involved laser frequencies. Purely laser-based experiments in optical setups are sensitive to ALPs in the eV mass range and can thus complement ALP searches at dipole magnets.

We show that the peak powers of ytterbium-doped fiber chirped pulse amplification (CPA) can be scaled by at least 1 order of magnitude (in the transform limit) as compared to current systems by using a different spectral region of operation. A simple and fast model for saturated broadband fiber CPA systems is developed and applied to study the impact of the interplay between the spectrally dependent small signal gain and the saturation on the output bandwidth. The influence of self-phase modulation on the recompression of the pulse is discussed. It can be shown that the novel operation regime exhibits superior performance even if nonlinear effects are considered. The numerical results are significant for the design of the next generation of ultrafast high power fiber lasers.

A method for solving the time-dependent two-center Dirac equation is developed. The time-dependent Dirac wave function is represented as a sum of atomiclike Dirac-Sturm orbitals, localized at the ions. The atomic orbitals are generated by solving numerically the one-center Dirac and Dirac-Sturm equations by means of a finite-difference approach with the Coulomb potential taken as the sum of the exact reference-nucleus potential and of the other nucleus within the monopole approximation. An original procedure for calculating the two-center integrals with these orbitals is proposed. As a first test of the approach developed here, calculations of the charge-transfer and ionization cross sections for the H(1s)-proton collisions at proton energies from 1 to 100 keV are performed. The obtained results are compared with related experimental and other theoretical data. To investigate the role of the relativistic effects, the charge-transfer cross sections in collisions of Ne9+(1s)-Ne10+ (at energies from 0.1 to 10 MeV/u) and U91+(1s)-U92+ (at energies from 6 to 10 MeV/u) are calculated for both relativistic and nonrelativistic cases.

X-ray betatron radiation is produced by oscillations of electrons in the intense focusing field of a laser-plasma accelerator. These hard x-rays show promise for use in femtosecond-scale time-resolved radiography of ultrafast processes. However, the spectral characteristics of betatron radiation have only been inferred from filter pack measurements. In order to achieve higher resolution spectral information about the betatron emission, we used an x-ray charge-coupled device to record the spectrum of betatron radiation, with a full width at half maximum resolution of 225 eV. In addition, we have recorded simultaneous electron and x-ray spectra along with x-ray images that allow for a determination of the betatron emission source size, as well as differences in the x-ray spectra as a function of the energy spectrum of accelerated electrons.

At the Institute of Optics and Quantum Electronics in Jena, Germany, the currently most powerful diode-pumped solid-state laser system with 25-TW peak power Polaris is in operation. In this paper we give an overview about the dispersion management of the chirped pulse amplification in order to minimize the pulse duration and thus to maximize the intensity available for experiments. A detailed description of the stretcher and compressor design with a novel alignment routine is given as well as measurements for the pulse duration and the temporal contrast. The far field measurement of the beam focussed by an off-axis parabola yields a nearly diffraction limited focal spot.

A study of the contribution of refluxing electrons in the production of K-alpha radiation from high-intensity laser irradiated thin targets has been performed. Thin copper foils both freestanding, and backed by a thick substrate were irradiated with laser pulses of energies around 100 J at intensities ranging from below 10^17 to above 10^19 W/cm2. At high laser intensities we find a strong reduction in the K-alpha yield from targets backed by the substrate. The observed yield reduction is in good agreement with a simple model using hot electron spectra from particle-in-cell simulations or directly inferred from the measured bremsstrahlung emission and can therefore be interpreted as due to the suppression of hot electron refluxing. The study shows that refluxing electrons play a dominant role in high-intensity laser driven K- alpha generation and have to be taken into account in designing targets for laser driven high-flux K-alpha sources.

Electron capture processes of heavy ions, like Ge(q+), Xe(q+). Pb(q+), and U(q+), respectively, with the charge q approximate to 10-40, occurring in collisions with gaseous targets are considered in the E = 0.1-100 MeV/u projectile energy range. Calculations of single-electron capture cross sections are performed using the CDW and the CAPTURE computer codes These are compared with available experimental data and CTMC (Classical-Trajectory Monte Carlo) calculations. Although the overall agreement is found to be within a factor of two, In some cases of heavy many-electron projectiles, e g. U(28+) + N(2), Ar collisions, experimental cross sections at high energies are far smaller than theoretical predictions Moreover, for these collision systems the observed energy dependencies are quite different from each other. Possible reasons for this behavior and how the theoretical models can be improved are discussed.

Significant progress in high repetition rate ultrashort pulse sources based on fiber technology is presented. These systems enable operation at a high repetition rate of up to 500 kHz and high average power in the extreme ultraviolet wavelength range via high harmonic generation in a gas jet. High average power few-cycle pulses of a fiber amplifier pumped optical parametric chirped pulse amplifier are used to produce µW level average power for the strongest harmonic at 42.9 nm at a repetition rate of 96 kHz.

Experimental measurements of magnetic fields generated in the cavity of a self-injecting laser-wakefield accelerator are presented. Faraday rotation is used to determine the existence of multimegagauss fields, constrained to a transverse dimension comparable to the plasma wavelength similar to lambda(p) and several lambda(p) longitudinally. The fields are generated rapidly and move with the driving laser. In our experiment, the appearance of the magnetic fields is correlated with the production of relativistic electrons, indicating that they are inherently tied to the growth and wave breaking of the nonlinear plasma wave. This evolution is confirmed by numerical simulations, showing that these measurements provide insight into the wakefield evolution with high spatial and temporal resolution.

We report here more than 50% coverage of the XUV spectral range between 18 nm and 35 nm by tuning the high-order harmonics generated by a fixed frequency Nd:Glass laser system. The tuning range achieved is suitable to seed Ni-like Y, Zr and Mo soft X-ray lasers.

Crossings of opposite-parity levels in He-like ions for particular values of the atomic number Z offer excellent conditions for precise experimental tests of parity-nonconservation (PNC). Such tests may be performed by inducing, through intense lasers, transitions that are only allowed in presence of PNC. Several spectroscopy experiments have been proposed in the past, but only now, with the advent of high-intensity laser facilities (e.g. PHELIX at GSI or POLARIS at Jena), their feasibility is within reach. In this paper, we use relativistic many-body perturbation theory (RMBPT) to calculate accurately the crossing conditions of the excited levels (1s2s) 1S0 and (1s2p) 3P0 and of (1s2s) 1S0 and (1s2p) 3P1 in intermediate and heavy He-like ions. Moreover, we show the dependence of the energy splittings upon the choice of the nuclear isotope.

We report the first experimental observation of a long-wavelength hosing modulation of a high-intensity laser pulse. Side-view images of the scattered optical radiation at the fundamental wavelength of the laser reveal a transverse oscillation of the laser pulse during its propagation through underdense plasma. The wavelength of the oscillation lambda(hosing) depends on the background plasma density n(e) and scales as lambda(hosing) similar to n(e)(-3/2). Comparisons with an analytical model and two-dimensional particle-in-cell simulations reveal that this laser hosing can be induced by a spatiotemporal asymmetry of the intensity distribution in the laser focus which can be caused by a misalignment of the parabolic focusing mirror or of the diffraction gratings in the pulse compressor.

At the Helmholtz center GSI, PHELIX (Petawatt High Energy Laser for heavy Ion eXperiments) has been commissioned for operation in stand-alone mode and, in combination with ions accelerated up to an energy of 13 MeV/u by the heavy ion accelerator UNILAC. The combination of PHELIX with the heavy-ion beams available at GSI enables a large variety of unique experiments. Novel research opportunities are spanning from the study of ion-matter interaction, through challenging new experiments in atomic physics, nuclear physics, and astrophysics, into the field of relativistic plasma physics.

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

We report on a novel two-dimensional position sensitive Si(Li) detector dedicated to Compton polarimetry of x-ray radiation arising from highly-charged ions. The performance of the detector system was evaluated in ion-atom collision experiments at the ESR storage ringe at GSI, Darmstadt. Based on the data obtained, the polarimeter efficiency is estimated in this work.

We demonstrated a 7.36 nm Ni-like samarium soft-x-ray laser, pumped by 36 J of a neodymium:glass chirped-pulse amplification laser. Double-pulse single-beam non-normal-incidence pumping was applied for efficient soft-x-ray laser generation. In this case, the applied technique included a single-optic focusing geometry for large beam diameters, a single-pass grating compressor, traveling-wave tuning capability, and an optimized high-energy laser double pulse. This scheme has the potential for even shorter-wavelength soft-x-ray laser pumping.

We report on high-energy ultrashort pulse generation from an all-normal-dispersion large-mode-area fiber laser by exploiting an efficient combination of nonlinear polarization evolution (NPE) and a semiconductor-based saturable absorber mode-locking mechanism. The watt-level laser directly emits chirped pulses with a duration of 1 ps and 163 nJ of pulse energy. These can be compressed to 77 fs, generating megawatt-level peak power. Intracavity dynamics are discussed by numerical simulation, and the intracavity pulse evolution reveals that NPE plays a key role in pulse shaping.

We report on a high power optical parametric amplifier delivering 8 fs pulses with 6 GW peak power. The system is pumped by a fiber amplifier and operated at 96 kHz repetition rate. The average output power is as high as 6.7 W, which is the highest average power few-cycle pulse laser reported so far. When stabilizing the seed oscillator, the system delivered carrier-envelop phase stable laser pulses. Furthermore, high harmonic generation up to the 33th order (21.8 nm) is demonstrated in a Krypton gas jet. In addition, the scalability of the presented laser system is discussed.

Heavy He-like ions are considered to be promising candidates for atomic parity-nonconservation (PNC) studies, thanks to their relatively simple atomic structure and the significant mixing between the almost degenerate (for the atomic numbers Z~64 and Z~91) opposite-parity levels 1s2s 1S0 and 1s2p 3P0. A number of experiments exploiting this level mixing have been proposed, and their implementation requires a precise knowledge of the 2 3P0–2 1S0 energy splitting for different nuclear charges and isotopes. In this paper we performed a theoretical analysis of the level splitting, employing the relativistic many-body perturbation theory and including QED corrections for all isotopes in the intervals 54⩽Z⩽71 and 86⩽Z⩽93. Possible candidates for future experimental PNC studies are discussed.