155 154 Principal Scientist Profiles Thomas Stöhlker Thomas Stöhlker Principal Scientist Profiles PROFESSOR OF THE PHYSICS OF HIGHLY CHARGED IONS, INSTITUTE OF OPTICS AND QUANTUM ELECTRONICS AND HELMHOLTZ INSTITUTE JENA Prof. Stöhlker is the director of the Helmholtz Institute Jena and the representative of the APPA research branch (Atomic Physics, Plasma Physics, Materials Research) at GSI, Helmholtz Centre for Heavy Ion Research in Darmstadt. He also serves as speaker of the research programme “From Matter to Materials and Life” of the Helmholtz Association. In addition, he is a member of the research programme “From Matter to Materials and Life” of the Helmholtz Association. THOMAS STÖHLKER RESEARCH AREAS Prof. Stöhlker’s research interests are focused on electron dynamics in strong and extreme fields, with particular emphasis of the effects of quantum electrodynam-ics (QED): • Experiments on bound-state QED and the atomic structure of few-electron ions at high-Z • Radiative processes in collisions of relativistic particles (ions and electrons) • Collision dynamics involving heavy ions • Light-matter interaction in the strong-field regime • Application of advanced x-ray and electron-detector and spectrometer concepts TEACHING FIELDS Prof. Stöhlker’s teaching is focused on the physics of simple atomic systems, including the atomic structure, atomic collisions, and fundamental aspects such as QED and parity violation. He gives courses and seminars in: • Key experiments in modern atomic physics • The interaction of high-energy radiation with matter • Photonic processes in highly ionized matter RESEARCH METHODS Prof. Stöhlker runs sophisticated setups for photon, x-ray, electron, and ion spectroscopy which are used for the experiments at storage rings, traps and synchrotrons, including: • Energy, time and spatially resolving detectors for x-ray imaging and polarimetry of hard x-rays • X-ray spectrometers of transmission and reflection type • Micro calorimeters • Dense-cluster targets for H2, He, N2, Ne, Ar, Xe RECENT RESEARCH RESULTS One- and two-electron ions provide an ideal testing ground for fundamental atomic structure theories, for the investigation of QED (self energy and vacuum polarization) as well as relativistic and correlation effects [1]. A new experiment at CRYRING@ESR, the first installation to be made available at the FAIR accelerator and storage ring complex currently under construction near Darmstadt, aims to determine the ground-state Lamb shift in hydrogen-like uranium with an accuracy that substantially exceeds 1%. This will provide the most stringent test of bound-state QED for one-electron systems in the strong field regime approaching the Schwinger limit (1016 V/cm). At extreme magnetic fields, meanwhile, substantial progress was made in 2017 when an experiment conducted on 209Bi82+,80+ improved the experimental precision of the socalled specific difference between the hyperfine splittings by more than an order of magnitude while finding a stillunexplained 7-σ deviation from theoretical predictions [2]. Current activities in these fields focus on the application of novel low-temperature bolometers for the hard X-ray regime. For polarization studies of hard x and γ radiation, the application of Compton scattering is another promising approach. Experiments utilizing novel 2D strip or pixel detectors focus on the study of photonic electron transitions and scattering processes in the strong-field domain (characteristic radiation, Rayleigh scattering, electron bremsstrahlung and recombination radiation) where spin effects are important and provide detailed information about the quantum dynamics in the strong-field domain [3]. HIGH-PRECISION CRYSTAL SPECTROSCOPY FOR STRONG FIELD QUANTUM ELECTRODYNAMICS A breakthrough has been achieved very recently during the investigation of the Lamb shift in the largely unexplored regime of extremely strong electric fields. In an experiment performed at the ion storage ring ESR in Darmstadt, the Lyman-α transitions of H-like gold (Au78+) have been measured with substantial statistical significance by applying the highresolution transmission crystal-spectrometer system FOCAL [Gassner et al., New J. Phys. 20, 073033 (2018)]. In the figure, the FOCAL spectrometer setup used in this experiment is shown. The resolution of this setup outperforms conventional solid-state germanium detectors by almost one order of magnitude. Up until the present, such detectors have been used in all previous experiments, addressing the topic of strong-field QED via the spectroscopy of the ground-state transitions in high-Z ions. In order to achieve the desired goal of a critical benchmark of the present theory of QED for the realm of extreme fields, such a high-resolution setup like FOCAL is mandatory for an accurate determination of the transition energies and for a further substantial improvement in experimental results. A further refinement of the results is, however, hampered by the systematic accuracy of the setup. The latest efforts of the large international collaboration team, including the Institute of Optics and Quantum Electronics at the Friedrich Schiller University Jena and the Helmholtz Institute Jena, therefore strive to commission an Electron Beam Ion Trap (EBIT) as a compact X-ray source for tests and characterizations of novel detectors. These will facilitate a high-profile experiment currently being installed at CRYRING@ESR, and expected to be performed in 2021. [1] Gassner et al., New J. Phys. 20, 073033 (2018). [2] Ullmann et al., Nat. Commun. 8, 15484 (2017). [3] Blumenhagen et al., New J. Phys. 18, 103034 (2016). Contact: Phone: + 49 3641 9-47600 Email: t.stoehlker@uni-jena.de
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