Miscellaneous publications

We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF2. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps. The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5 J per burst with a repetition rate of 10Hz. Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%. Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst. To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.

Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.

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Using state-of-the-art high-power laser systems, we are able to routinely generate extreme energy densities and focused light intensities in a controlled laboratory environment. During the interaction of these laser pulses with matter a plasma is generated that can both sustain and support huge electric fields. These can be used as a novel type of accelerator structure for electrons and ions having properties favorable for a large number of future applications.

Energy-loss straggling measurements performed at the high-momentum-resolution magnetic spectrometer FRS with bare and highly charged O-8, Xe-54, (79)AU, and U-92 ions with specific kinetic energies (700-1000) MeV/u are reported. The results are in good agreement with rigorous calculations recently reported by Lindhard and Sorensen and reveal systematic deviations from the well-known relativistic Bohr formula, which was obtained within the framework of the first-order Born approximation.

A possibility of using the Cherenkov technique to measure the average energy of collimated relativistic nuclear Au-79 beams is investigated in the article for specific beam-radiator interaction (strong deceleration, fragmentation) produces a considerable effect of the spreading of the Cherenkov one. Two Cherenkov methods - achromatic (E, alpha) and combined ("combi")- were employed for this purpose. At an energy about 1 GeV/u the accuracy of measurement by the "combi" method was +/-1.4MeV/u. The accuracy of measurement by the (E,alpha) method was +/-0.9MeV/u at the energy E(Au) approximate to 640MeV/u. These results turned out to be in good agreement with initial E(Au)(0) data from the SIS accelerator with allowances made for energy losses over the beam line (the maximum discrepancy was less than or equal to 0.4%). With all the set of these data, the direction condition was for the first time checked for VChR from heavy nuclei. It is shown to be fulfilled to an accuracy above +/-5' in the angle range 27 degrees < Theta(Ch) < 38.5 degrees.

Resonant transfer and excitation in highly charged uranium ions has been investigated by applying the X-ray/particle coincidence technique. This complete detection method for the RTE process in combination with the large L-shell fine structure splitting in uranium allows, for the first time, an L-subshell differential as well as angular differential study of the excitation and radiative stabilization of different intermediate electronic states. Measured L-subshell differential cross sections for the KLL-RTE in U90+—>C collisions compare well with full relativistic calculations. For the radiative stabilization of the KL(1/2)L(3/2) intermediate states a significant anisotropy was found.

Total cross sections for radiative double electron capture (RDEC) in collisions of fast highly charged ions with light target atoms were estimated. The applied method is based on the principle of detailed balance using the cross sections for the double photoionization process. Preliminary results of a dedicated RDEC experiment aiming at the observation of photons with twice the single REC photon energy, associated with double charge exchange, are discussed. Additionally, results of complementary measurements of double capture (without photon registration) by fully stripped U ions in collisions with C atoms at 295 MeV/u are presented. Here, a surprisingly enhanced cross section for double electron capture is found in accordance with observations reported in the literature.

Based on systematic experimental studies and on exact relativistic calculations of the REC process in relativistic heavy-ion atom collisions, the limits of applicability of the nonrelativistic dipole approximation are discussed. In particular, direct comparison between a general REC scaling law and all available total electron-capture cross sections related to REC into bare and H-like ions is given. Similar to K-REC, a fortuitous agreement between the nonrelativistic approach and the experimental data is found. Moreover, the results of a dedicated L-REC experiment are presented, where this process was investigated for energy-resolved subshells. The measured angular distributions for REC into the j = 1/2 and j = 3/2 levels are in excellent agreement with exact relativistic calculations and manifest a complete breakdown of the nonrelativistic theory.

Resonant transfer and excitation (RTE) followed by X-ray emission in highly charged uranium ions colliding with carbon atoms has been investigated by applying the X-ray-particle-coincidence technique. This renders possible a subshell differential investigation of the population and radiative stabilization of different excited intermediate states. Preliminary results for the measured RTE excitation function for U90+ —> C collisions are presented.

A 200-keV super electron beam ion trap is being used to study the few electron ions of heavy elements. This device can produce and trap any highly charged ion at rest in the laboratory, including bare U92+ ions. Measurements of the ground state binding energies of hydrogenlike and heliumlike ions, and the 2s-2p transition energies in lithiumlike through fluorinelike ions are being used to determine the QED energies in high-Z ions. The ionization cross sections for hydrogenlike and heliumlike high-Z ions are being directly measured for the first time. There are future opportunities for several new kinds of experiments.

The photon angular distributions for radiative electron capture (REC) into the j=1/2 and j=3/2 L subshell levels were measured and calculated for U90+ —> C collisions at 89 MeV/u. The experiment provides first study of the photon angular distribution of REC into projectile p state (j=3/2) which was found to be exhibit a slight backward peaking in the laboratory frame. For radiative capture to the j=1/2 states, the measured angular distribution deviates considerably from symmetry around 90°. The results demonstrate that the usual sin^2(θlab) distribution is not valid in the high-Z regime.

The SIS accelerator facilities at GSI in combination with a magnetic spectrometer allow the direct measurement of stopping powers for relativistic heavy ions up to uranium with high precision for the first time. Here, we report on representative results obtained for projectiles up to xenon in the energy range from 700 to 1000 MeV/u. Systematic deviations from the Bethe stopping-power theory are observed, whereas the data are in good agreement with theory once Mott and Bloch corrections and the Fermi density effect are included.

Relativistic heavy-ion collisions of few-electron projectiles ranging from argon up to uranium have been investigated in solid and gaseous media. Electron-loss and electron-capture cross sections, charge-state distributions, as well as energy loss and energy deposition have been measured and are compared with theoretical predictions. Especially fully-ionized heavy projectiles represent a unique possibility to test atomic-collision theories.

Projectile X-ray emission and charge-exchange processes are studied for bare and one-electron Bi, Pb, and U ions in collisions with gaseous and solid targets at relativistic energies. The experimental data for both the total and subshell differential cross sections are discussed and compared with theoretical predictions. Moreover, the experiments were extended to atomic structure studies. The result for the groundstate Lamb-shift for hydrogenic uranium is presented, which compares very well with theoretical prediction.

The heavy ion synchrotron/storage ring facility at GSI, SIS/ESR, provides intense beams of cooled, highly-charged ions up to naked uranium (U92+). By electron capture during ion-atom collisions in the gas target of the ESR or by recombination at ion-electron encounters in the 'electron cooler" excited states are populated. The detailed structure of very heavy one-, two- and three-electron ions is studied. The different mechanisms leading to the excited states are described, as well as the new experimental tools now available for a detailed spectroscopy of these interesting systems. Special emphasis is given to X-ray transitions to the groundstates in H- and He-like systems. For the heaviest species the groundstate Lambshift can now be probed on an accuracy level of better than 10% using solid-state X-ray detectors. Applying dispersive X-ray analysing techniques, this accuracy will certainly be improved in future. However, utilizing the dielectronic recombination resonances for a spectroscopy, the structure in Li-like heavy ions can already be probed now on the sub eV level.

Charge state distributions for 3-16 MeV/u Ni ions penetrating C-stripper foils measured during UNILAC operation over the past decade are summarized. The results agree reasonably well with calculations including excitation, stabilization and charge changing processes. Special emphasis is given to the fraction of emerging few-electron ions, i.e. from naked to Li-like ions. The results are applied to a beam foil spectroscopy experiment investigating intra-L shell and Rydberg transitions in He- and Li-like Ni ions.

The Lyman a transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare U92+ ions with gaseous targets at the storage ring ESR. By applying x-ray-particle coincidence techniques the ground-state transition energies in the rest frame could be determined with a precision of about 600 ppm. The experimental result is in excellent agreement with the theoretical prediction for the ground-state Lamb shift in hydrogenlike uranium and is already sensitive to the 2s Lamb shift.

Bound-state beta- decay was observed for the first time by storing bare Dy-163(66)66+ ions in a heavy-ion storage ring. From the number of Ho-163(67)66+ daughter ions, measured as a function of the storage time, a half-life of 47+/-5/4 d was derived.