Successful summer school online in 2020

Due to the current situation, this year's summer school did not take place in physics center in Bad Honnef, but online. The regular program consisting of basic plasma physics lectures and a master class on special topics could not take place as usual. Nevertheless, all teachers have agreed to present the basic lectures via video conferencing. The summer school was extended to two weeks with two lectures per day. This year more people could participate in the summer school due to its online format.
The lectures were technically flawless and the feedback from students and teachers was very positive. Many discussions and interactions could be made possible due to the high commitment of the teachers. Two practical workshops were also held by L. L. Alves on a Boltzmann solver and N. Braithwaite on the Paschen curve.
We hope for another summer school in 2021, then again on site at the physics center in bad Honnef. The latest information on planning will be published on the summer school homepage in March 2021.


The universe is more homogeneous than expected

Recent results of the Kilo-Degree Survey have shown that matter in the universe is about ten percent more evenly distributed than predicted by the standard model of cosmology. An international team, led by astronomers from the Netherlands, Scotland, Great Britain, and Germany - with the participation of the Ruhr-Universität Bochum - describes the results in five articles, three of which were published on a preprint server on 31 July 2020. They are also submitted for publication in the journal Astronomy and Astrophysics.

The data were recorded with the European Space Organisation's Very Large Telescope Survey Telescope (VST) on Cerro Paranal in northern Chile. The resulting map covers five percent of the extragalactic sky and includes 31 million galaxies, all of which were included in the analysis. They are up to ten billion light years away, and their light was emitted when the universe was only about a quarter of the age it is today.

Using the galaxies, the research consortium produced a map of the distribution of matter in the universe. To do this, the scientists used the so-called weak gravitational lensing effect: light from distant galaxies is deflected and distorted on its way to Earth by the gravitational effect of large accumulations of matter such as galaxy clusters. Based on this effect, the clumping tendency of matter can be determined - namely visible matter, gases and invisible dark matter, which makes up about 85 percent of the total matter in the universe.

The researchers make the distribution of matter visible in such maps. The grey square shows the size of a single map taken, with the moon in it for size comparison. The total map consists of more than 1,000 images. Areas of high density matter are shown in yellow, areas of low density in pink.

Over time, the gravitation of matter causes the universe to become less and less homogeneous. Areas with slightly more mass than average attract matter from their surroundings. Thus the differences in distribution become greater and greater. At the same time, the expansion of the universe counteracts this effect. Both processes are driven by gravity and are therefore suitable for putting the standard model of cosmology to the test. The equations predict precisely how much the density of matter will change over time.

However, the data from the Kilo-Degree Survey reveal a discrepancy: the universe is ten percent more homogeneous than it would appear to be according to the Standard Model. "The Standard Model of Cosmology has described all the cosmological observations we have made for the past 20 years. However, it is somewhat unsatisfactory that we have to accept mysterious substances such as dark matter and dark energy. That's why we're trying to test this model as best we can," says Prof. Dr. Hendrik Hildebrandt, head of the Observational Cosmology group at the Ruhr University Bochum.

The current analysis could indicate that the Standard Model is cracking. It is not the first discrepancy, even the so-called Hubble constant, which represents the expansion rate of the universe, does not match the model's predictions. "These discrepancies could of course be caused by systematic measurement errors," admits Prof. Dr. Catherine Heymans (University of Edinburgh), who together with Hendrik Hildebrandt heads the German Centre for Cosmological Lensing at the RUB, where she also holds a guest professorship. "But the measurements are becoming more and more precise, so that this is becoming increasingly unlikely," Heymans continues.

The researchers are not yet able to assess whether the Standard Model will eventually have to be replaced by a completely new theory, for example by replacing Einstein's General Theory of Relativity. "There are many theories that try to explain the measurements with new physics," says Hendrik Hildebrandt. "As an observing cosmologist, you try to remain impartial and make the measurements as accurate as possible without theoretical prejudice. One thing is clear, we live in exciting times!"
In one or two years the final map of the Kilo-Degree Survey will be available, with all the observations made in the project. It will be another 30 percent larger than the current map.

Meanwhile, two other projects, one in the US and one in Japan, are working on similar analyses based on observational data. From 2022, even better measuring technology will be available: the Ruby telescope, which is 60 times more powerful than the VST, and the Euclid satellite, which will take much sharper images outside the atmosphere.

The Kilo-Degree Survey is an international project led by astronomers in the Netherlands, Scotland, England and Germany. The project coordinator is Prof. Dr. Koen Kuijken from the Leiden Observatory in the Netherlands. Other partners come from Italy, Australia, Poland, the USA, and China.

adapted from Julia Weiler, RUB

New cosmic magnetic field structures discovered in galaxy NGC 4217

The spiral galaxy NGC 4217 has a huge magnetic field that is shown here as green lines. The data for this visualisation were recorded with the radio telescope Karl G. Jansky Very Large Array (VLA) of the National Science Foundation. The image of the galaxy shown from the side is taken from data from the Sloan Digital Sky Survey and Kitt Peak National Observatory.

Spiral galaxies such as our Milky Way can have sprawling magnetic fields. There are various theories about their formation, but so far the process is not well understood. An international research team has now analysed the magnetic field of the Milky Way-like galaxy NGC 4217 in detail on the basis of radio astronomical observations and has discovered as yet unknown magnetic field structures. The data suggest that star formation and star explosions, so-called supernovae, are responsible for the visible structures.

The team led by Dr. Yelena Stein from Ruhr-Universität Bochum, the Centre de Données astronomiques de Strasbourg and the Max Planck Institute for Radio Astronomy in Bonn together with US-American and Canadian colleagues, published their report in the journal Astronomy and Astrophysics, released online on 21 July 2020.

The analysed data had been compiled in the project “Continuum Halos in Nearby Galaxies”, where radio waves were utilised to measure 35 galaxies. “Galaxy NGC 4217 is of particular interest to us,” explains Yelena Stein, who began the study at the Chair of Astronomy at Ruhr-Universität Bochum under Professor Ralf-Jürgen Dettmar and who currently works at the Centre de Données astronomiques de Strasbourg. NGC 4217 is similar to the Milky Way and is only about 67 million light years away, which means relatively close to it, in the Ursa Major constellation. The researchers therefore hope to successfully transfer some of their findings to our home galaxy.

Magnetic fields and origins of star formation
When evaluating the data from NGC 4217, the researchers found several remarkable structures. The galaxy has an X-shaped magnetic field structure, which has also been observed in other galaxies, extending far outwards from the galaxy disk, namely over 20,000 light years.

In addition to the X-shape, the team found a helix structure and two large bubble structures, also called superbubbles. The latter originate from places where many massive stars explode as supernovae, but also where stars are formed that emit stellar winds in the process. Researchers therefore suspect a connection between these phenomena.

“It is fascinating that we discover unexpected phenomena in every galaxy whenever we use radio polarisation measurements,” points out Dr. Rainer Beck from the MPI for Radio Astronomy in Bonn, one of the authors of the study. “Here in NGC 4217, it is huge magnetic gas bubbles and a helix magnetic field that spirals upwards into the galaxy’s halo.”

The analysis moreover revealed large loop structures in the magnetic fields along the entire galaxy. “This has never been observed before,” explains Yelena Stein. “We suspect that the structures are caused by star formation, because at these points matter is ejected outward.”

Image shows magnetic field structures
For their analysis, the researchers combined different methods that enabled them to visualise the ordered and chaotic magnetic fields of the galaxy both along the line of sight and perpendicular to it. The result was a comprehensive image of the structures.

To optimise the results, Yelena Stein combined the data evaluated by means of radio astronomy with an image of NGC 4217 that was taken in the visible light range. The image is available for download on the website “Visualising the data was important to me,” stresses Stein. “Because when you think about galaxies, magnetic fields is not the first thing that comes to mind, although they can be gigantic and display unique structures. The image is supposed to shift the magnetic fields more into focus.”

adapted from Julia Weiler, RUB, translated by Donata Zuber

Lukas Mai and colleagues on new chemistry for ultra-thin gas sensors 

© RUB, Marquard

A Bochum team has developed a new process for zinc oxide coatings that can be used in nitrogen oxide sensors and as protective coatings on plastics.
The application of zinc oxide coatings in industry is manifold and ranges from the protection of perishable goods from air to the detection of toxic nitrogen oxides. Such layers can be produced by means of atomic layer deposition (ALD), which normally uses precursor chemicals, so-called precursors, which ignite immediately in air. An interdisciplinary research team at the Ruhr-Universität Bochum (RUB) has now established a new production process based on non-self-igniting precursors that takes place at such low temperatures that plastics can also be coated. The team reported in the magazine "Small", which selected the article for its title in the issue of 4 June 2020.

Applying ultra-thin coatings

To produce a sensor for nitrogen dioxide (NO2), a thin layer of nanostructured zinc oxide (ZnO) must be applied to a sensor substrate and then integrated into an electrical component. Prof. Dr. Anjana Devi's team used ALD to apply ultra-thin ZnO layers to such sensor substrates.

In general, ALD processes are used in industry to miniaturize electrical components by means of ultra-thin layers, some of which are only a few atomic layers thick, while at the same time increasing the efficiency. This requires precursors that react on a surface in the ALD process to form a thin layer. "The chemistry behind ALD processes is therefore essential and has a great influence on the resulting layers," emphasizes Anjana Devi.

Safe handling and highest quality

In industry, ZnO coatings have so far been produced with an extremely reactive zinc precursor that ignites immediately in air, experts call it pyrophoric. "The key to developing a safe ALD process was to research a new, non-pyrophoric precursor that can be handled safely and is capable of producing ZnO coatings of the highest quality," said Lukas Mai, lead author of the study. "The challenge was to find an alternative chemistry capable of replacing pyrophoric, industrially used compounds".
The special feature of the new process is that it is even possible at low temperatures, which makes it possible to coat plastics. Thus, the new process is not only suitable for the production of gas sensors, but also for gas barrier layers. These are applied to plastic in industry and are used to protect sensitive goods such as food and medicines from air.

This was made possible by the interdisciplinary cooperation of natural scientists and engineers. The team included the working groups Chemistry of Inorganic Materials headed by Anjana Devi and General Electrical Engineering and Plasma Technology headed by Prof. Dr. Peter Awakowicz, researchers from Heinrich Heine University Düsseldorf and the company Paragon.

The work was funded by the European Fund for Regional Development (EFRE) in the Funald project and by the German Research Foundation in the framework of the Collaborative Research Centre/Transregional TR87. Lukas Mai was supported by the Stiftung der Deutschen Wirtschaft.

adapted from Meike Drießen, RUB
Eickhoff prize

Dr.-Ing. Schmidt is awarded for his outstanding dissertation

Technical plasmas are among the things that have a significant influence on the world around us, without many people knowing about it. "You can, for example, process surfaces with plasmas; but they are crucial in the production of modern computer chips, which are built into almost all modern technical devices - from cars to smart phones," explains Frederik Schmidt. "A better understanding of this technology leads to innovations that make our lives easier, network people and shape our future.

© RUB, Marquard

In his dissertation, he investigated how the energy gets into a plasma. The path from the power socket to nanometer-sized semiconductor tracks is being investigated by various specialists and is in part well understood. Frederik Schmidt has brought together two of these specialist areas: the electrical network between the power socket and the plasma on the one hand, and detailed plasma simulations on the other. This makes it possible to investigate the relationship between the two. "For example, I have looked at the paths along which energy flows and how much is lost on its way into the plasma. That is sometimes quite a lot," says the researcher. The results help to make systems and superstructures more efficient and thus more economical and ecological. In addition, he has developed his own electrical network that can be implemented for certain applications with considerably less effort and losses than before. "I was able to show theoretically that this works. Colleagues in France were then able to prove in experiments that it is also practically possible to build something like this," says Schmidt.

adapted from Meike Drießen, RUB