Miscellaneous publications

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In modern laser-based ion acceleration systems, the field distribution of the focused laser beam at the position of the target strongly influences the overall characteristics of the resulting ion beam. To obtain an unidirectional and quasi mono-energetic ion beam, a flat-top field distribution of the focused laser beam is optimal. This can only be achieved, by using a beam-profiling system that reshapes the incident laser beam into an Airy-shaped field distribution in the far field. Here, we present an extensive design study of such a beam-profiling system based on two free-form mirrors. In order to realize the rings of zero intensity, corresponding to the roots of the Airy-function, strong curvature peaks on the first mirror are necessary. Additionally, the alternating phase in between these rings can only be achieved with grooves on the second mirror. These aspects actually raise the question, if the used purely geometric optical modeling approach is still valid. Therefore, our design study is entirely accompanied with wave-optical simulations to identify influences of diffraction within the beam profiling system. We find that especially the grooves on the second mirror are mandatory, not only to ensure the alternating phase, but also to realize the roots of zero intensity of the Airy-function. On the other hand, these grooves cause diffraction effects in the beam-profiling system that slightly degrade the at-top focal field. These influences are in the range of a few percent and cannot be further avoided.

The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model, where light particles (photons) must have a minimum energy (frequency) to ionize atoms. The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength - the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field. Isolated attosecond pulses recently enabled studies of the dynamics of tunneling ionization of atoms in gases. On page 1348 of this issue, Schultze et al. experimentally show that atoms in a solid are also excited via the tunneling process.

Hochleistungslaser ermöglichen es inzwischen, relativistische Elektronenpulse mit bemerkenswerten Eigenschaften zu erzeugen. Neben der äußerst kurzen Beschleunigungslänge sind vor allem die kleine Quellgröße und auch die kurze Pulsdauer interessant. Mit diesen Elektronenpulsen lässt sich zudem elektromagnetische Sekundärstrahlung im Kiloelektronenvolt-Bereich erzeugen. Mit dieser Röntgenstrahlung würden auch Universitäten relativ kompakte Röntgenquellen zur Verfügung stehen, mit denen sich Effekte beobachten lassen, die auf äußerst kurzen räumlichen und zeitlichen Skalen ablaufen. Momentan ist diese Art von Forschung nur an großen, konventionellen Synchrotron-Beschleunigern möglich.

In cancer treatment it is highly desirable to identify and /or classify individual cancer cells in real time. Nowadays, the standard method is PCR which is costly and time-consuming. Here we present a different approach to rapidly classify cell types: we measure the pattern of coherently diffracted extreme ultraviolet radiation (XUV radiation at 38nm wavelength), allowing to distinguish different single breast cancer cell types. The output of our laser driven XUV light source is focused onto a single unstained and unlabeled cancer cell, and the resulting diffraction pattern is measured in reflection geometry. As we will further show, the outer shape of the object can be retrieved from the diffraction pattern with sub-micron resolution. For classification it is often not necessary to retrieve the image, it is only necessary to compare the diffraction patterns which can be regarded as a spatial fingerprint of the specimen. For a proof-of-principle experiment MCF7 and SKBR3 breast cancer cells were pipetted on gold-coated silica slides. From illuminating each single cell and measuring a diffraction pattern we could distinguish between them. Owing to the short bursts of coherent soft x-ray light, one could also image temporal changes of the specimen, i.e. studying changes upon drug application once the desired specimen is found by the classification method. Using a more powerful laser, even classifying circulating tumor cells (CTC) at a high throughput seems possible. This lab-sized equipment will allow fast classification of any kind of cells, bacteria or even viruses in the near future.

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.