Plasma dynamics between laser-induced breakdown and relativistically induced transparency: An investigation of high-intensity laser-solid interactions by time-resolved off-harmonic optical shadowgraphy

Laser-plasma-based ion accelerators are becoming a versatile platform to drive different fields of applied research and life sciences, for example translational research in radiation oncology. To ensure stable accelerator performance, complete control over the ion source, i.e., the high-intensity laser-solid interaction, is required. However, idealized interaction conditions are almost impossible to reach, as the utilized high-power lasers always feature a non-negligible amount of light preceding the laser peak. This leading edge of the laser pulse usually exceeds the ionization potential of bound electrons much earlier than the arrival of the high-power laser peak and the solid-density target undergoes significant modifications even before the actual high-intensity laser-plasma interaction starts. Control over this so-called target pre-expansion is a key requirement to achieve quantitative agreement between numerical simulations and experiments of high-intensity laser-solid interactions. This thesis investigates several aspects that are relevant to improve the capability of simulations to model realistic experimental scenarios. The corresponding experiments are conducted with cryogenic hydrogen-jet targets and the DRACO-PW laser at peak intensities between 10^12 W/cm^2 and 10^21 W/cm^2 . The experimental implementation of time-resolved optical-probing diagnostics and technical innovations with respect to the technique of off-harmonic optical probing overcome the disturbances by parasitic plasma self-emission and allow for unprecedented observations of the target evolution during the laser-target interactions. The laser-induced breakdown of solids, i.e., the phase transition from the solid to the plasma state, can be considered as an heuristic starting point of high-intensity laser-solid interactions. As it is highly relevant to simulations of target pre-expansion, Chapter 3 of this thesis presents time-resolved measurements of laser-induced breakdown in laser-target interactions at peak intensities between 0.6 * 10^21 W/cm^2 and 5.7 * 10^21 W/cm^2 . By increasing the peak intensity, a lowering of the applicable threshold intensity of laser-induced breakdown well below the appearance intensity of barrier-suppression ionization occurs. The observation demonstrates the relevance of the pulse-duration dependence of laser-induced breakdown and laser-induced damage threshold to the starting point of high-intensity laser-solid interactions. To apply the results to other laser-target assemblies, we provide a detailed instruction of how to pinpoint the starting point by comparing measurements of the laser contrast with a characterization study of the target-specific thresholds of laser-induced breakdown at low laser intensity. Chapter 4 of this thesis presents an example of how optical-probing diagnostics are able to estimate target pre-expansion as a starting condition for particle-in-cell simulations. The measurement allows to restrict the surface gradient of the pre-expanded plasma density to an exponential scalelength between 0.06 um and 0.13 um. Furthermore, the plasma-expansion dynamics induced by the ultra-relativistic laser peak are computed and post-processed by ray-tracing simulations. A comparison to the experimental results yields that the formation of the measured shadowgrams is governed by refraction in the plasma-density gradients and that the observed volumetric transparency of the target at 1.4 ps after the laser peak is not caused by relativistically induced transparency but by plasma expansion into vacuum instead

Annual Report 2023 - Institute of Ion Beam Physics and Materials Research

The year 2023 was highly successful, marked by significant high-level publications and the acquisition of new projects. The latter is increasingly crucial given the tight budget in 2023, which is expected to become even tighter in 2024 due to rising costs and well-deserved salary increases for our employees. As a result, we must reduce the number of our non-permanent scientific staff, which will impact future productivity. Despite these challenges, our performance in 2023 remained outstanding with a total of 178 refereed publications and an average impact factor of 8.1. Notable publications include 8 from the Nature Publishing Group, 7 from Advanced (Functional) Materials, 4 from ACS Nano, and 2 from Angewandte Chemie. Our excellence was further recognized by the HZDR Research Award, which again went to our Institute, this time awarded to Dr. Oleksii Volkov and Dr. Oleksandr Pylypovskyi from the Department of Intelligent Materials and Devices for their theoretical and experimental investigations into chiral symmetry breaking in magnetic 3D textures. Furthermore, Dr. Lukas Körber, who completed his PhD in 2023 with summa cum laude, was the recipient of both the Helmholtz Doctoral Award in the research field of Matter and the HZDR Doctoral Award. Prof. Manfred Helm was honored as an APS Fellow. In 2023, the majority of newly approved projects are financed by the Saxonian Ministry of Science, Culture and Tourism and the Helmholtz Initiative and Networking Fund which is a confirmation of the application relevance of our research. Our infrastructure upgrades are progressing as planned. The new AMS (Accelerator Mass Spectrometry) building was handed over to us in fall 2023. This year, we anticipate the arrival of our new 1 MV accelerator, which will be a dedicated AMS system. We aim to achieve full user operation by 2025. Finally, we extend our heartfelt thanks to all partners, friends, and organizations who supported our progress in 2023. We are particularly grateful to the Executive Board of the Helmholtz-Zentrum Dresden-Rossendorf, the Ministry of Science, Culture and Tourism of the Free State of Saxony, and the Federal Ministries of Education and Research, and of Economic Affairs and Climate Action. Many partners from universities, industry, and research institutes worldwide have been essential to our development. Lastly, the directors wish to thank all members of our institute for their exceptional efforts and contributions in these extraordinary times

Wege zum effizienten Rückbau von Reaktorkomponenten und Betonabschirmung: Berechnung des Aktivitätsinventars und deren Validierung an Bohrkernen sowie Mobilitätsuntersuchungen von Radionukliden – WERREBA

Das Ziel des Vorhabens war es, genaue Kenntnisse über die entstandenen radioaktiven Nuk-lide während des Leistungsbetriebs eines Kernkraftwerkes, die zeitliche Veränderung der Ak-tivität und die daraus resultierende Verteilung der Aktivität in den einzelnen Phasen des Rück-baus zu erhalten. Die Aktivitätsverteilungen sollten dabei anlagenspezifisch für den Reaktor-druckbehälter (RDB), dessen Einbauten, den Reaktordeckel und die erste Betonabschirmung (biologisches Schild) bestimmt werden. Dabei lag der Schwerpunkt besonders auf der expe-rimentellen Bestimmung der Nuklidzusammensetzung sowie deren Aktivität und chemischen Bindung im Material. Die Untersuchungen wurden an Originalmaterial sowohl aus dem RDB als auch aus dem Beton durchgeführt und dienen der Validierung und Verifizierung der durchgeführten Rechnungen. Im Fall der stark aktivierten Reaktorkomponenten könnten den Behörden und Betreibern In-formationen bereitgestellt werden, ob neben der direkten Zerlegung die Methode der Abkling-lagerung als eine ökologische und wirtschaftliche Alternative in Betracht kommt. Mit einer möglichen Zwischenlagerung könnten sowohl die endzulagernde aktive Abfallmenge reduziert als auch wertvolle Metalle wieder recycelt werden. Zusätzlich wird die Strahlenbelastung für das Rückbaupersonal verringert. Im Fall der Betonabschirmung wurden Aussagen zur möglichen chemischen Mobilität der Radionuklide getroffen, welche direkten Einfluss auf die Rückbaustrategie und die Endlage-rung hat. Denn für beides ist nicht nur die absolute Menge, sondern auch die strukturelle Ein-bindung der Radionuklide im Beton wichtig. Diese ist entscheidend für die Stabilität der Bin-dung der Radionuklide im Beton und damit für den Umfang und die Kinetik möglicher Auflö-sungen mit Übergang in die wässrige Phase während des Rückbaus und im Endlager

Optimisation strategies for proton acceleration from thin foils with petawatt ultrashort pulse lasers

Laser-driven plasma accelerators can produce high-energy, high peak current ion beams by irradiating solid materials with ultra-intense laser pulses. This innovative concept attracts a lot of attention for various multidisciplinary applications as a compact and energy-efficient alternative to conventional accelerators. The maturation of plasma accelerators from complex physics experiments to turnkey particle sources for practical applications necessitates breakthroughs in the generated beam parameters, their robustness and scalability to higher repetition rates and efficiencies. This thesis investigates viable optimisation strategies for enhancing ion acceleration from thin foil targets in ultra-intense laser-plasma interactions. The influence of the detailed laser pulse parameters on plasma-based ion acceleration has been systematically investigated in a series of experiments carried out on two state-of-the-art high-power laser systems. A central aspect of this work is the establishment and integration of laser diagnostics and operational techniques to advance control of the interaction conditions for maximum acceleration performance. Meticulous efforts in continuously monitoring and enhancing the temporal intensity contrast of the laser system, enabled to optimise ion acceleration in two different regimes, each offering unique perspectives for applications. Using the widely established target-normal sheath acceleration (TNSA) scheme and adjusting the temporal shape of the laser pulse accordingly, proton energies up to 70 MeV were reliably obtained over many months of operation. Asymmetric laser pulses, deviating significantly from the standard conditions of an ideally compressed pulse, resulted in the highest particle numbers and an average energy gain ≥ 37 %. This beam quality enhancement is demonstrated across a broad range of parameters, including thickness and material of the target, laser energy and temporal intensity contrast. To overcome the energy scaling limitations of TNSA, the second part of the thesis focuses on an advanced acceleration scheme occurring in the relativistically induced transparency (RIT) regime. The combination of thin foil targets with precisely matched temporal contrast conditions of the laser enabled a transition of the initially opaque targets to transparency upon main pulse arrival. Laser-driven proton acceleration to a record energy of 150 MeV is experimentally demonstrated using only 22 J of laser energy on target. The low-divergent high-energy component of the accelerated beam is spatially and spectrally well separated from a lower energetic TNSA component. Start-to-end simulations validate these results and elucidate the role of preceding laser light in pre-expanding the target along with the detailed acceleration dynamics during the main pulse interaction. The ultrashort pulse duration of the laser facilitates a rapid succession of multiple known acceleration regimes to cascade efficiently at the onset of RIT, leading to the observed beam parameters and enabling ion acceleration to unprecedented energies. The discussed acceleration scheme was successfully replicated at two different laser facilities and for different temporal contrast levels. The results demonstrate the robustness of this scenario and that the optimum target thickness decreases with improved laser contrast due to reduced pre-expansion. Target transparency was found to identify the best-performance shots within the acquired data sets, making it a suitable feedback parameter for automated laser and target optimisation to enhance stability of plasma accelerators in the future. Overall, the obtained results and described optimisation strategies of this thesis may become the guiding step for the further development of laser-driven ion accelerators

Annual Report 2023 - Institute of Resource Ecology

The IRE is one of the ten institutes of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Our research ac-tivities are mainly integrated into the program “Nuclear Waste Management, Safety and Radiation Research (NUSAFE)” of the Helmholtz Association (HGF) and fo-cus on the topics “Safety of Nuclear Waste Disposal” and “Safety Research for Nuclear Reactors”. The program NUSAFE, and therefore all work which is done at IRE, belong to the research field “Energy” of the HGF

Temporal contrast-dependent modeling of laser-driven solids - studying femtosecond-nanometer interactions and probing

Establishing precise control over the unique beam parameters of laser-accelerated ions from relativistic ultra-short pulse laser-solid interactions has been a major goal for the past 20 years. While the spatio-temporal coupling of laser-pulse and target parameters create transient phenomena at femtosecond-nanometer scales that are decisive for the acceleration performance, these scales have also largely been inaccessible to experimental observation. Computer simulations of laser-driven plasmas provide valuable insight into the physics at play. Nevertheless, predictive capabilities are still lacking due to the massive computational cost to perform these in 3D at high resolution for extended simulation times. This thesis investigates the optimal acceleration of protons from ultra-thin foils following the interaction with an ultra-short ultra-high intensity laser pulse, including realistic contrast conditions up to a picosecond before the main pulse. Advanced ionization methods implemented into the highly scalable, open-source particle-in-cell code PIConGPU enabled this study. Supporting two experimental campaigns, the new methods led to a deeper understanding of the physics of Laser-Wakefield acceleration and Colloidal Crystal melting, respectively, for they now allowed to explain experimental observations with simulated ionization- and plasma dynamics. Subsequently, explorative 3D3V simulations of enhanced laser-ion acceleration were performed on the Swiss supercomputer Piz Daint. There, the inclusion of realistic laser contrast conditions altered the intra-pulse dynamics of the acceleration process significantly. Contrary to a perfect Gaussian pulse, a better spatio-temporal overlap of the protons with the electron sheath origin allowed for full exploitation of the accelerating potential, leading to higher maximum energies. Adapting well-known analytic models allowed to match the results qualitatively and, in chosen cases, quantitatively. However, despite complex 3D plasma dynamics not being reflected within the 1D models, the upper limit of ion acceleration performance within the TNSA scenario can be predicted remarkably well. Radiation signatures obtained from synthetic diagnostics of electrons, protons, and bremsstrahlung photons show that the target state at maximum laser intensity is encoded, previewing how experiments may gain insight into this previously unobservable time frame. Furthermore, as X-ray Free Electron Laser facilities have only recently begun to allow observations at femtosecond-nanometer scales, benchmarking the physics models for solid-density plasma simulations is now in reach. Finally, this thesis presents the first start-to-end simulations of optical-pump, X-ray-probe laser-solid interactions with the photon scattering code ParaTAXIS. The associated PIC simulations guided the planning and execution of an LCLS experiment, demonstrating the first observation of solid-density plasma distribution driven by near-relativistic short-pulse laser pulses at femtosecond-nanometer resolution

Annual Report 2022 - Institute of Resource Ecology

The Institute of Resource Ecology (IRE) is one of the ten institutes of the Helmholtz-Zentrum Dresden – Rossendorf (HZDR). Our research activities are mainly integrated into the program “Nuclear Waste Management, Safety and Ra-diation Research (NUSAFE)” of the Helmholtz Association (HGF) and focus on the topics “Safety of Nuclear Waste Disposal” and “Safety Research for Nuclear Reactors”. The program NUSAFE, and therefore all work which is done at IRE, belong to the research field “Energy” of the HGF

Fracture mechanics investigation of reactor pressure vessel steels by means of sub-sized specimens (KLEINPROBEN)

The embrittlement of reactor pressure vessel (RPV) steels due to neutron irradiation restricts the operating lifetime of nuclear reactors. The reference temperature 0, obtained from fracture mechanics testing using the Master Curve concept, is a good indicator of the irradiation resistance of a material. The measurement of the shift in 0 after neutron irradiation, which accompanies the embrittlement of the material, using the Master Curve concept, enables the assessment of the reactor materials. In the context of worldwide life time extensions of nuclear power plants, the limited availability of neutron irradiated materials (surveillance materials) is a challenge. Testing of miniaturized 0.16T C(T) specimens manufactured from already tested standard Charpy-sized specimens helps to solve the material shortage problem. In this work, four different reactor pressure vessel steels with different compositions were investigated in the unirradiated and in the neutron-irradiated condition. A total number of 189 mini-C(T) samples were fabricated and tested. An important component of this study is the transferability of fracture mechanics data from mini-C(T) to standard Charpy-sized specimen. Our results demonstrate good agreement of the reference temperatures from the mini-C(T) specimens with those from standard Charpy-sized specimens. RPV steels containing higher Cu and P contents exhibit a higher increase in 0 after irradiation. The fracture surfaces were investigated using SEM in order to record the location of the fracture initiators. The fracture modes were also determined. A large number of test results formed the basis for a censoring probability function, which was used to optimally select the testing temperature in Master Curve testing. The effect of the slow stable crack growth censoring criteria from ASTM E1921 on the determination of 0 was analysed and found to have a minor effect. Our results demonstrate the validity of mini-C(T) specimen testing and confirm the role of the impurity elements Cu and P in neutron embrittlement. We anticipate further research linking microstructure to the fracture properties of materials before and after neutron irradiation and the optimization of Master Curve testing using the results from our statistical analysis

Primordial nuclides and low-level counting at Felsenkeller

Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its cross- section in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide

Annual Report 2022 - Institute of Ion Beam Physics and Materials Research

Preface Selected publications Statistics (Publications and patents, Concluded scientific degrees; Appointments and honors; Invited conference contributions, colloquia, lectures and talks; Conferences, workshops, colloquia and seminars; Exchange of researchers; Projects) Doctoral training programme Experimental equipment User facilities and services Organization chart and personne