Institutskolloquium des IPP 2018

End of 1981 the Electron Cyclotron Heating (ECH) Division in the Institute for Plasma Research (IPF) of the University of Stuttgart was founded with members of the former Belt-Pinch, Theta-Pinch and “Plasmaus” (linear 2.45 GHz RF plasma device) Groups in order to develop a 28 GHz, 0.2 MW, 40 ms ECH system for plasma start-up, heating and confinement experiments on the Wendelstein W7-A Stellarator at IPP Garching. Since these experiments were very successful, later 70 GHz and 140 GHz ECH and non-inductive current drive CD systems were installed. This colloquium talk will review the first 10 years of ECH&CD experiments on Wendelstein Stellarators. [mehr]

The future of the energy - a systems view

Based on scenario and energy systems modeling, the talk provides a perspecitve on the requirements and challenges for the future energy system and the role of energy sources such as nuclear, bio, solar and wind and the growth and development of energy carriers such as electricity, hydrogen and hydro-carbons. The talk provides a first principles perspective on resource costs, sustainability and reliability and highlights the trade-offs that different energy resources provide. The central role of energy technology research and innovation is highlighted. [mehr]

The multi-purpose electron accelerator ELBE and its applications, with a focus on THz spectroscopy

I will introduce the superconducting electron accelerator ELBE (Electron Linear accelerator with high Brilliance and low Emittance) as a source for different types of secondary radiation, including low- (meV) and high- (MeV) energy photons, positrons and neutrons. Being a solid-state spectroscopist, I will then focus on research we have performed using the terahertz free-electron laser FELBE. Here the high peak power can be employed for nonlinear optical experiments in the THz range, whereas the picosecond pulse structure enables time-resolved studies of relaxation processes. [mehr]

Physics Highlights from the LHCb Experiment

The LHCb detector at the Large Hadron Collider has been optimised for the study of rare decays of heavy flavour particles and the breaking of the symmetry between matter and antimatter. After a general introduction to the experiment, the talk will focus on the most important results obtained so far and their relevance for the search of physics beyond the Standard Model. [mehr]

Dense matter in neutron stars

Neutron stars are born as the endpoint of stellar evolution in core-collapse supernovae. The densities in a neutron star are extremely high, so high that nuclei are squeezed into their constituents, neutrons and protons. Exotic matter can appear in the core, either in the form of hyperons or as a new phase in the form of strange quark matter. The properties of neutron stars are determined by the nuclear equation of state of dense matter, so that the observation of neutron stars and neutron star merger can give a telltale signature of the properties of dense matter under extreme conditions. The present astrophysical data on neutron stars and pulsars, rotation-powered neutron stars, is reviewed. The equation of state of dense matter will be discussed and possible implications for future observations of core-collapse supernovae and neutron star mergers will be outlined. [mehr]

Homo Oeconomicus or Homo Reciprocans?

The last Nobel Prize in economics was awarded to Richard Thaler for his contributions to Behavioral Economics. Behavioral Economics challenges the standard, neoclassical assumption that economic decision makers are fully rational and only concerned about their own material well-being. Over the last decades new models of human behavior have been developed, often inspired and tested by fascinating economic experiments. In this lecture I will focus on "social preferences", a subfield of behavioral economics to which Thaler and many other economists and psychologists contributed. It seems obvious that many people are not only motivated by their own material well-being but often care about other people. Altruism, spite, inequality aversion, concerns for fairness and reciprocal behavior have an important impact on economic decision making. However, it is often difficult to disentangle the exact motivations that are driving economic behavior. In this lecture I will give a brief introduction into this field and discuss some recent experiments on the dark side of social preferences. [mehr]

Gas, glass & light: 25+ years of photonic crystal fibres

The idea for a new kind of optical glass fibre—photonic crystal fibre (PCF)—first emerged in 1991. The aim was to realise a fibre with a two-dimensional periodic array of microscopic features (typically hollow channels) running along its entire length. These would be able to corral light within a central hollow or solid core, permitting light and matter waves to be tightly confined over long distances while precisely controlling the dispersion. More than a quarter of a century later, PCF has led to a whole series of new developments, some of which are already are moving into real-world applications. Solid-core PCFs are routinely used to transform invisible infrared laser pulses into white light 10 million times brighter than an arc lamp, and form the basis of commercial supercontinuum sources. Twisted PCF creates optical vortices that carry orbital angular momentum, as well as providing an elegant means of providing circular birefringence and dichroism. Intense interactions between light and sound in solid-core PCF enable stable all-optical mode-locking of fibre lasers at a high harmonic (a few GHz) of their round-trip frequency. Single-ring hollow-core PCF, comprising a ring of thin-walled capillaries surrounding a central hollow core, guides over an extremely wide frequency range and, through pressure-adjustable dispersion, provides a simple means of compressing pulses down to single-cycle durations, as well as underpinning a range of unique and extremely bright sources of tunable deep and vacuum ultraviolet light. [mehr]

New approaches to stable models for computational plasma physics

Due to the presence of multiple physical scales and complex nonlinear interactions, the numerical simulation of fusion plasmas often leads to computational problems of huge complexity. A long-standing challenge is then to design numerical methods that are computationally efficient, high order accurate and stable on very long time scales. Fortunately, steady progresses in the theory of structure-preserving discretizations have provided a solid mathematical ground for the development of stable high order numerical schemes. In this lecture I will give a brief review of the compatible Finite Element methods that have been developed in this direction, and I will explain how these tools are now being extended to design stable numerical models for the Vlasov-Maxwell equations. Recent ideas that allow to further improve the computational efficiency of such methods will be presented, along with a novel approach to low-noise particle approximations. [mehr]

From the beginnings to the (preliminary?) end of the North Korean missile program

Comparing North Korea's achievements in the field of missiles with the programs of other countries, one gets the impression that North Korea is a nation of rocket scientists. The recent glorious successes appear like a deja vu of the early days of North Korea's missile program in the 1980s and 1990s, when the country managed to present a full missile program out of the blue. But a close look from an engineer's perspective reveals some discrepancies in the common narrative, thus allowing for some surprising insights into the current situation of North Korea's missile threat. [mehr]

The European Spallation Source: New Opportunities for Science

The European Spallation Source (ESS), which is currently under construction in Lund, Sweden, is designed to push the limits of research with neutrons to new horizons. ESS will open up new scientific opportunities which are complementary to those at X-ray sources. These will include unprecedented in-situ and in-operando experiments which are only possible with neutrons due to their special properties. After a short summary of the design and the specifications of the European Spallation Source an overview of the current status and schedule of the ESS construction project will be given with a strong focus on the instruments and the surrounding scientific infrastructure. The overall goal of ESS is to begin user operation in 2023 and ramp up to 15 instruments by 2026. Selected examples of new scientific opportunities in the field of materials and life science will be discussed. [mehr]
Max Planck wurde in einem Akt der Verzweiflung zum Revolutionär seines Fachs und zum Begründer der modernen Physik. Mit seiner Quantentheorie zerbrach das bis dahin stabile wissenschaftliche Bild der Welt. Der Vortrag porträtiert den Physiker und Menschen Max Planck, in dessen Werk und Leben sich Triumph und Tragik vereinen. Dabei entsteht das Bild einer ganzen Epoche, die historisch, wissenschaftlich und philosophisch den Aufbruch in die Moderne darstellt. [mehr]

Quantum Logic Spectroscopy of Trapped Ions

Precision spectroscopy is a driving force for the development of our physical understanding. However, only few atomic and molecular systems of interest have been accessible for precision spectroscopy in the past, since they miss a suitable transition for laser cooling and internal state detection. This restriction can be overcome in trapped ions through quantum logic spectroscopy. Coherent laser manipulation originally developed in the context of quantum information processing with trapped ions allows us to combine the special spectroscopic properties of one ion species (spectroscopy ion) with the excellent control over another species (logic or cooling ion). The logic ion provides sympathetic cooling and is used to control and read out the internal state of the spectroscopy ion. In my presentation I will provide an overview of different implementations of quantum logic spectroscopy suitable for narrow (long-lived) and broad (dipole-allowed) transitions. Applications range from highly accurate optical clocks based on aluminium ions, over precision spectroscopy of broad and non-closed transitions in calcium isotopes, to non-destructive internal state detection and spectroscopy of molecular ions. Prospects to extend quantum logic spectroscopy to highly charged ions and first steps towards this goal will be discussed.Spectroscopy of these species enables a multitude of tests for physics beyond the Standard Model, such as probing for new force carriers and scalar fields that are dark matter candidates and could induce a variation of fundamental constants. Measurements of isotope shifts of narrow transitions in calcium isotopes probes nuclear structure and may allow to constrain new forces coupling electrons and neutrons. Precision spectroscopy of e.g. vibrational transitions in molecular ions will allow to put bounds on a possible variation of the electron-to-proton mass ratio, while highly charged ions are among the most sensitive systems to probe for a variation of the fine-structure constant.Picture: Sympathetically cooled highly-charged ions. Left: Ar13+ in a cloud of laser-cooled Be+ ions. Right: Two Be+ ions separated by a single Ar13+ ion. [mehr]

Exploring Turbulence in Fusion Plasmas through Experiment and Simulation

A profound understanding of turbulence in fusion plasmas is paramount because of its strong impact on both core and edge plasma transport. Turbulence is a key player in determining particle, energy, and momentum fluxes and thus dictates the shape of the density, temperature, and rotation profiles, which set the the efficiency of a fusion reactor. Gyrokinetic theory is considered nowadays to be the state of the art when it comes to a compromise between realism and efficiency, but must be validated to improve the reliability of predicting profiles for future fusion devices. The fundamentals of turbulence, its generation and its characterization with both measurements and simulations will be presented. For the core plasma, particular attention is paid to the comparison between measurements and gyrokinetic simulations, which has led to significant gains in understanding. The edge turbulence behavior in different confinement regimes is contrasted. Strong turbulence is usually observed in the L-mode edge while the H-mode edge plasma exhibits a reduced turbulence level. In yet another confinement regime, the improved energy confinement mode (I-mode), edge turbulence is reduced to a large degree. However, strongly intermittent high amplitude events are observed. An analytic candidate generation mechanisms is presented and first results from accompanying gyrokinetic simulations of the I-mode edge – consistent with the observations – are reported. [mehr]

Solar geoengineering - taking the edge off climate change?

Despite the progress made at the Paris climate talks with respect to reducing emissions of greenhouse gases it is possible that the rate of environmental change may result in extensive negative impacts for humans and natural ecosystems. Clearly, emissions cuts are essential to managing climate risk and reducing climate change. However, given the potential consequences of a high rate of environmental change, it is important to investigate complementary approaches. For example, geoengineering methods aimed at altering earth's radiation budget may offer a fast-acting way of moderating the rate of climate change. However, such geoengineering approaches entail a number of new risks, and cannot replace reducing CO2 emissions/levels. I will discuss different geoengineering methods, focusing on new approaches to stratospheric solar radiation management, highlighting technical capabilities and risk. I will also briefly discuss questions surrounding the potential implementation of such approaches. [mehr]

Learning across space and time in spiking neural networks

The brain routinely discovers sensory clues that predict opportunities or dangers. However, it is unclear how neural learning processes can bridge the typically long delays between sensory clues and behavioral outcomes. Here, I introduce a learning concept, aggregate-label learning, that enables biologically plausible model neurons to solve this temporal credit assignment problem. Aggregate-label learning matches a neuron’s number of output spikes to a feedback signal that is proportional to the number of clues but carries no information about their timing. Aggregate-label learning outperforms stochastic reinforcement learning at identifying predictive clues and is able to solve unsegmented speech-recognition tasks. Furthermore, it allows unsupervised neural networks to discover reoccurring constellations of sensory features even when they are widely dispersed across space and time. [mehr]
Large-scale plasma instabilities with the potential of damaging wall structures or reducing their lifetime are a significant concern for magnetic confinement fusion. Among the most critical are disruptions and edge localized modes. Predicting the behavior of such instabilities and their control for ITER and beyond is a challenging task for which input from experiments, theory, and simulations is needed. This presentation describes non-linear MHD simulations of edge localized modes, disruptions and control strategies. The talk will explain why simulations or large-scale instabilities are needed and show that our simulations have already revealed a lot of aspects about the physics of large-scale instabilities. It will also give some insights into the actual work involved “behind the scenes” and challenges we are facing for the future. [mehr]
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