Peer-Review Publications

Laser-produced solid density plasmas are well-known as table-top sources of electromagnetic radiation. Recent studies have shown that energetic broadband terahertz pulses (T rays) can also be generated from laser-driven compact ion accelerators. Here we report the measurement of record-breaking T-Ray pulses with energies no less than 0.7 mJ. The terahertz spectrum has been characterized for frequencies ranging from 0.1–133  THz. The dependence of T-Ray yield on incident laser energy is linear and shows no tendencies of saturation. The noncollinear emission pattern and the high yield reveal that the T rays are generated by the transient field at the rear surface of the solid target.

We report an experimental study of the charge-transfer process in collisions of Xe^q+ ions (16≤q≤20) with magnesium atoms at an energy of 5.5q keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe^q+-Mg collision also behaves very differently from “traditional” collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.

We report on the nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment. A single 320 fs pulse is divided into four replicas, spectrally broadened in a solid-core fiber, and subsequently recombined. This approach makes it possible to reduce the nonlinearities in the fiber and therefore to use total input peak power of about 13.3 MW, which is more than three times higher than the self-focusing threshold. Finally, the combined output pulse could be compressed to sub-100 fs pulse duration. This general and universal approach holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.

We present a novel approach for the amplification of high peak power femtosecond laser pulses at a high repetition rate. This approach is based on an all-diode pumped burst mode laser scheme. In this scheme, pulse bursts with a total duration between 1 and 2 ms are be generated and amplified. They contain 50 to 2000 individual pulses equally spaced in time. The individual pulses have an initial duration of 350 fs and are stretched to 50 ps prior to amplification. The amplifier stage is based on Yb3+:CaF2 cooled to 100 K. In this amplifier, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz is demonstrated. For lower repetition rates the total output energy per burst can be scaled up to 915 mJ using a longer pump duration. This corresponds to an efficiency as high as 25% of extracted energy from absorbed pump energy. This is the highest efficiency, which has so far been demonstrated for a pulsed Yb3+:CaF2 amplifier.

The development of a few-cycle optical probe-pulse for the investigation of laser-plasma interactions driven by a Ti:sapphire, 30 Terawatt (TW) laser system is described. The probe is seeded by a fraction of the driving laser's energy and is spectrally broadened via self-phase modulation in a hollow core fiber filled with a rare gas, then temporally compressed to a few optical cycles via chirped mirrors. Shadowgrams of the laser-driven plasma wave created in relativistic electron acceleration experiments are presented with few-fs temporal resolution, which is shown to be independent of post-interaction spectral filtering of the probe-beam.

The periodic time modulations, found recently in the two-body orbital electron capture (EC) decay of both, hydrogen-like 140Pr58+ and 142Pm60+ ions, with periods near to 7 s7 s and amplitudes of about 20%, were re-investigated for the case of 142Pm60+ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period T=7.11(11) s, in accordance with a modulation period T=7.12(11) s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to a_R=0.107(24) and a_P=0.134(27) for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors exactly our previous findings of modulation periods near to 7 s, though with distinctly smaller amplitudes. Also the three-body β+ decays have been analyzed. For a supposed modulation period near to 7 s we found an amplitude a=0.027(27), compatible with a=0a=0 and in agreement with the preliminary result a=0.030(30) of our previous experiment. These observations could point at weak interaction as origin of the observed 7 s-modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.

The long-term stability of optical parametric chirped-pulse amplifiers is hindered by thermal path length drifts affecting the temporal pump-to-signal overlap. A kilowatt-pumped burst amplifier is presented delivering broadband 1.4 mJ pulses with a spectral bandwidth supporting sub-7 fs pulse duration. Active temporal overlap control can be achieved by feedback of optical timing signals from cross-correlation or spectral measurements. Using a balanced optical cross-correlator, we achieve a pump-to-signal synchronization with a residual jitter of only (46 ± 2) fs rms. Additionally, we propose passive pump-to-signal stabilization with an intrinsic jitter of (7.0 ± 0.5) fs rms using white-light continuum generation.

During the last two decades, ion storage-cooler rings have been proven as powerful devices for addressing precision experiments in the realm of atomic physics, nuclear physics and nuclear astrophysics. Most important, in particular for stored unstable nuclides, is the unrivalled capability of ion cooler-rings to generate brilliant beams of highest phase–space density owing to sophisticated cooling techniques, and to store them for extended periods of time by preserving their charge state. This report focuses on nuclear physics and nuclear astrophysics experiments with in-flight produced exotic ions that were injected into storage-cooler rings. Those experiments were conducted within the last decade mainly at the only operating facilities that are capable to provide and to store exotic ions, namely the ESR in Darmstadt, Germany and the CSRe-ring in Lanzhou, China. The majority of nuclear physics experiments performed at these equipments concerns ground-state properties of nuclei far from stability, such as masses and lifetimes. The rich harvest of these measurements is presented. In particular it is shown that storage-cooler rings are ideal, if not the only, devices where two-body beta decays of exotic highly-charged ions, such as bound-state beta decay and orbital electron capture, can be studied in every detail, based on “single-ion decay spectroscopy”. Furthermore, experiments at the border between atomic and nuclear physics are discussed which render valuable information on nuclear properties by exploiting one of the most precise tools of atomic spectroscopy on stored ions, the “dielectronic recombination”. Ion storage rings with cooled exotic beams and equipped with thin internal gas targets deliver also unrivalled opportunities for addressing with highest precision key reactions in the fields of nuclear astrophysics and nuclear structure. First very promising experiments exploring the potential of ion cooler-rings in this realm have been already conducted. However, in view of the small nuclear cross sections, many of fervently desired experiments in this field will still suffer from the insufficient number of exotic ions that can be delivered and stored at the time being. The realistic hope on a breakthrough in this field is based on the ion storage rings to come, with their estimated improvements in the intensity of exotic ion beams by many orders of magnitude.

Plasmon measurements hold great promise for providing highly accurate data on the physical properties of plasmas in the high-energy density physics regime. To this end we demonstrate in recent experiments at the Linac Coherent Light Source the first spectrally-resolved measurements of plasmons using a seeded 8-keV x-ray laser beam. Forward x-ray Thomson scattering spectra from isochorically heated solid aluminum show a well-resolved plasmon feature that is down-shifted in energy by 19 eV from the incident 8 keV elastic scattering feature. In this spectral range, the simultaneously measured backscatter spectrum shows no spectral features indicating observation of collective plasmon oscillations on a scattering length comparable to the screening length. This technique is a prerequisite for Thomson scattering measurements in compressed matter where the plasmon shift is a sensitive function of the free electron density and where the plasmon intensity provides information on temperature.

This paper reports on the effect of the chemical composition on the glass structure, the coefficients of thermal expansion and the fluorescence properties of Sm3+-doped La2O3–Al2O3–SiO2-glasses. The silica concentration was varied between 50 and 70 mol% and the La2O3:Al2O3 ratio between 50:50 and 30:70. The glass formation and the densities are evaluated and FTIR reflectance spectra, coefficients of thermal expansion and fluorescence lifetimes are determined. It is shown that high SiO2 concentrations and low La2O3:Al2O3 ratios result in relatively high fluorescence lifetime (2.19 ms, 4G5/2) and low coefficients of thermal expansion (4.6 × 10^−6 / K). The coefficients of thermal expansion and the fluorescence lifetimes show a linear dependency on the ratio LaO3/2/(AlO3/2 + SiO2).

We investigate the angular distribution of electrons that are emitted in the ionization of hydrogen-like ions by twisted photons. Analysis is performed based on the first-order perturbation theory and the non-relativistic Schrödinger equation. Special attention is paid to the dependence of the electron emission pattern on the impact parameter b of the ion with respect to the centre of the twisted wave front. In order to explore such a dependence, detailed calculations were carried out for the photoionization of the 1s ground and 2 p_y excited states of neutral hydrogen atoms. Based on these calculations, we argue that for relatively small impact parameters, the electron angular distributions may be strongly affected by altering the position of the atom within the wave front. In contrast, if the atom is placed far from the front centre, the emission pattern of the electrons is independent of the impact parameter b and resembles that observed in the photoionization by plane wave photons.

The properties of two strongly bent Highly Annealed Pyrolytic Graphite (HAPG) crystals with different thicknesses of 40 μm and 100 μm are studied at all possible reflection orders using x-rays at 4.5 keV and 8 keV photon energies. Typical reflecting areas within 50% reflectivity drop boundaries have sizes of about ≤ 1 mm. These domains are mis-oriented by ≤ 1 minutes of arc to each other. The mosaicity was measured to be ~ 0.06° on a 1 × 1 mm 2 scale, whereas it amounts to ~ 0.14° when the probed area becomes > 2 × 1 mm 2 . We find that the integrated reflectivity of the reflection (004) is in good agreement with the kinematical diffraction theory, while a maximum value of 2.3 mrad is achieved for 8 keV and reflection (002). The highest spectral resolution is obtained with an x-ray source of ≤ 50 μm size and a 40 μm thin graphite coating, which amounts to E /Δ E ≥ 1000 for 4.5 keV and 8 keV. In the case of 8 keV and reflection (008), the resolving power exceeds E /Δ E = 2000. In von-Hámos geometry, it was found that > 60% of the reflected photons are confined in a central 500 μm wide profile where high spectral resolution is pertained. Ray tracing simulations reveal that in order to pertain a certain resolution, a larger mosaicity would result in less contributing photons. Thus the efficiency of the crystal drops significantly when the mosaicity is increased and could not be increased by large crystal opening angles.

Results of the calculations of differential L−shell radiative recombination (RR) rate coefficients for bare uranium ions colliding with free electrons using the nonrelativistic dipole approximation and fully relativistic calculations are reported. The rate coefficients were obtained for very low, in the range of meV, relative electron-ion energies. We demonstrate that even for such low relative ion-electron energies the relativistic effects significantly modify the differential RR rate coefficients for the L−subshells and, as a result, the measurements of the relative electron energy dependence of the L-RR rates could be used for studying of the relativistic effects. These effects are strongest for the L_3-subshell, which is discussed here in more details.

Laser ion acceleration provides for compact, high-intensity ion sources in the multi-MeV range. Using a pulsed high-field solenoid, for the first time high-intensity laser-accelerated proton bunches could be selected from the continuous exponential spectrum and delivered to large distances, containing more than 109 particles in a narrow energy interval around a central energy of 9.4 MeV and showing ≤ 30  mrad envelope divergence. The bunches of only a few nanoseconds bunch duration were characterized 2.2 m behind the laser-plasma source with respect to arrival time, energy width, and intensity as well as spatial and temporal bunch profile.

Fibre lasers are now associated with high average powers and very high beam qualities. Both these characteristics are required by many industrial, defence and scientific applications, which explains why fibre lasers have become one of the most popular laser technologies. However, this success, which is largely founded on the outstanding characteristics of fibres as an active medium, has only been achieved through researchers around the world striving to overcome many of the limitations imposed by the fibre architecture. This Review focuses on these limitations, both past and current, and the creative solutions that have been proposed for overcoming them. These solutions have enabled fibre lasers to generate the highest diffraction-limited average power achieved to date by solid-state lasers.

Incorporation of coherent combination into a state-of-the-art fiber-chirped pulse amplification system obtains 1.1 mJ, 340 fs pulses with up to 280 W of average power at 250 kHz repetition rate. Propagation of this laser pulse inside a krypton-filled hollow-core fiber results in significant spectral broadening. Chirped mirrors are used to compress the pulses to 26 fs, 540 μJ (135 W) leading to a peak power of more than 11 GW. This unprecedented combination of high peak and average power ultrashort pulses opens up new possibilities in multidimensional surface science and coherent soft x-ray generation.

We revisit the photon polarization tensor in a homogeneous external magnetic or electric field. The starting point of our considerations is the momentum space representation of the one-loop photon polarization tensor in the presence of a homogeneous electromagnetic field, known in terms of a double parameter integral. Our focus is on explicit analytical insights for both on- and off-the-light-cone dynamics in a wide range of well-specified physical parameter regimes, ranging from the perturbative to the manifestly nonperturbative strong field regime. The basic ideas underlying well-established approximations to the photon polarization tensor are carefully examined and critically reviewed. In particular, we systematically keep track of all contributions, both the ones to be neglected and those to be taken into account explicitly, to all orders. This allows us to study their ranges of applicability in a much more systematic and rigorous way. We point out the limitations of such approximations and manage to go beyond at several instances.

Based on the second-order perturbation theory, we investigate the two-photon decay of K-shell vacancies in heavy atoms. The many-electron transition amplitude that occurs in the theory is evaluated by means of the independent particle approximation (IPA). By using this approach, computations are performed for the decay of neutral gold and are directly compared with recent experimental data, not relying on any scaling assumptions. The obtained results confirm previously identified discrepancies between the IPA theory and the experiment for the 2s→1s transition, and an apparent “resonance” region of the 3s→1s transition, but they show a moderate agreement with the measured data for the 3d→1s and 4s+4d→1s cases. Moreover, with the help of the IPA we discuss the validity of the nonrelativistic scaling that was employed in the past to estimate the relative two-photon transition probabilities P in heavy atoms based on calculations done for lighter elements and different decay geometries. We find, in particular, that the electric-dipole angular distribution of emitted photons holds rather well even in the high-Z domain, while the assumption that the relative probability P is independent of nuclear charge may result in 10–30% inaccuracy of theoretical predictions.

By combining an x-ray laser (XRL) with a heavy-ion storage ring, precision laser spectroscopy of the fine-structure splitting in heavy Li-like ions will be possible. An initial study has been performed to determine the feasibility of a first experiment at the experimental storage ring at GSI in Darmstadt, which also has great potential for the experiments planned for FAIR. We plan to perform a unique, direct and precise measurement of a fine-structure transition in a heavy Li-like ion. Such a measurement will test state-of-the-art atomic structure calculations in strong fields. This endeavour will require that the existing infrastructure is complemented by a dedicated beamline for the XRL. In this paper, we will discuss the details of this project and outline a proof-of-principle experiment.

Radiative double electron capture is a fundamental atomic process which should be observed in collisions of bare ions with atoms, albeit with a much smaller cross-section than single radiative electron capture. A new experiment—to observe this rare process under single-collision conditions—has been performed at the internal gas jet target of the experimental storage ring at GSI in Darmstadt. X-ray spectra associated with single and double charge exchange have been observed in 30 MeV u^(−1) collisions of bare chromium ions (Cr^(24+)) with helium and nitrogen target gases.

We have performed ab initio QED calculations of the (1s)^2(2s)^22p_3/2-(1s)^2(2s)^22p_1/2 fine-structure splitting along the boron isoelectronic sequence for all ions with 17 ≤ Z ≤ 100. This level splitting was evaluated within the extended Furry picture and by making use of four different screening potentials in order to estimate the effects of interelectronic correlations. The accuracy of the predicted transition energies has been improved significantly when compared with previous computations.

We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.

We study Bessel beams of two-level atoms that are coupled to a linearly polarized laser field. For such atom beams, we construct exact Bessel-type solutions of the Schrödinger equation beyond the paraxial approximation for beam propagation. In particular, we examine the probability density for Bessel beams of neutral two-level atoms driven by a laser field but without the level damping being taken into account. We show how the radial dependence of the probability density (from the beam axis) can be affected by tuning the parameters of the atom–laser system, such as the resonant frequency and amplitude of the laser field and/or the nuclear charge and velocity of the atomic beam.

A brief overview is presented in this paper on some experiments conducted at the Experimental Storage Ring (ESR) of GSI which addressed the β decay of stored and cooled highly charged ions. Special emphasis is placed on the two-body beta decay of bare or few-electron ions: bound-state β− decay (β_b) and its time-mirrored counterpart, orbital electron capture. The former decay mode was detected experimentally 20 years ago at the ESR. The latter could be investigated there for the first time in detail for the simplest quantum systems: hydrogen- and helium-like atoms. The main results of these experiments will be presented. Also their impact on stellar nucleosynthesis, in particular the s -process, is discussed.

The leading-order positron–atom bremsstrahlung is investigated within the rigorous relativistic approach based on the partial-wave representation of the Dirac wave functions in the external atomic field. Approximating the atomic target by an effective local potential, we calculate the Stokes parameters of the emitted photon for different polarizations of the initial positron. The results for positron–atom bremsstrahlung are compared with analogous data for the electron–atom bremsstrahlung.