Streamers in air splitting into three branches

We investigate the branching of positive streamers in air and present the first systematic investigation of splitting into more than two branches. We study discharges in 100 mbar artificial air that is exposed to voltage pulses of 10 kV applied to a needle electrode 160 mm above a grounded plate. By imaging the discharge with two cameras from three angles, we establish that about every 200th branching event is a branching into three. Branching into three occurs more frequently for the relatively thicker streamers. In fact, we find that the surface of the total streamer cross-sections before and after a branching event is roughly the same.Comment: 6 pages, 7 figure

Power laws and self-similar behavior in negative ionization fronts

We study anode-directed ionization fronts in curved geometries. When the magnetic effects can be neglected, an electric shielding factor determines the behavior of the electric field and the charged particle densities. From a minimal streamer model, a Burgers type equation which governs the dynamics of the electric shielding factor is obtained. A Lagrangian formulation is then derived to analyze the ionization fronts. Power laws for the velocity and the amplitude of streamer fronts are observed numerically and calculated analytically by using the shielding factor formulation. The phenomenon of geometrical diffusion is explained and clarified, and a universal self-similar asymptotic behavior is derived.Comment: 25 pages, 9 figure

Experimental evaluation of a solid oxide fuel cell system exposed to inclinations and accelerations by ship motions

Solid Oxide Fuel Cell (SOFC) systems have the potential to reduce emissions from seagoing vessels. However, it is unknown whether ship motions influence the system's operation. In this research, a 1.5 kW SOFC module is operated on an inclination platform that emulates ship motions, to evaluate the influence of static and dynamic inclinations on the system's safety, operation, and lifetime. The test campaign consists of a static inclination test, a dynamic test, a degradation test, and a high acceleration test. There were no interruptions in the power supply during the different tests, and no detectable gas leakages or safety hazards. Although the SOFC does not fail in any test condition, dynamic inclinations result in forced oscillations in the fuel regulation, which propagate through the system by different feedback loops in the control architecture, leading to significant deviations in the operational parameters of the system. Additionally, for motion periods from 16 to 26 s, reoccurring exceedance of the fuel utilisation results in a gradual reduction of the power supply. Several enhancements are recommended to improve the design of SOFCs and marine fuel cell regulations to ensure their safe operation on ships.</p

Deviations from the local field approximation in negative streamer heads

Negative streamer ionization fronts in nitrogen under normal conditions are investigated both in a particle model and in a fluid model in local field approximation. The parameter functions for the fluid model are derived from swarm experiments in the particle model. The front structure on the inner scale is investigated in a 1D setting, allowing reasonable run-time and memory consumption and high numerical accuracy without introducing super-particles. If the reduced electric field immediately before the front is >= 50kV/(cm bar), solutions of fluid and particle model agree very well. If the field increases up to 200kV/(cm bar), the solutions of particle and fluid model deviate, in particular, the ionization level behind the front becomes up to 60% higher in the particle model while the velocity is rather insensitive. Particle and fluid model deviate because electrons with high energies do not yet fully run away from the front, but are somewhat ahead. This leads to increasing ionization rates in the particle model at the very tip of the front. The energy overshoot of electrons in the leading edge of the front actually agrees quantitatively with the energy overshoot in the leading edge of an electron swarm or avalanche in the same electric field.Comment: The paper has 17 pages, including 15 figures and 3 table

Dutch higher education and Chinese students in the Netherlands

The number of Chinese students in the Dutch higher education sector has grown rapidly. In 2014 the number of Chinese BA and MA students reached 4638, or about 7 percent of the population of international students in the Netherlands. The number of formally employed PhD students in that year was 427. After the Germans, the Chinese have become the second largest group of international students. Dutch HBO schools (‘universities of applied sciences’) used to attract about half of all Chinese students in the Netherlands, but their share has been dropping perceptibly in recent years. The presence of Chinese students has become a structural aspect of the Dutch higher education sector. In the competition for student talent from China, the Netherlands lose out to top-ranked universities in the English-speaking world. Nevertheless, the Netherlands has proven to be quite successful as one of the most English of all non-English-speaking countries with a very wide range of study programmes in English. When looking at the relative costs of studying abroad the Netherlands occupies a middle position. As a result, Chinese students often come to the Netherlands as an excellent second-best choice. Self-development and exposure to foreign cultures are the most important reasons to study abroad. Another consideration is the hope of gaining a competitive advantage in the Chinese job market upon return. Students view the Netherlands as safe, egalitarian, and open to different cultures. This appreciation of certain aspects of Dutch society is also a reflection on satisfaction with some social changes inherent to China’s rapid modernization. Students appreciate the fact that Dutch education puts more emphasis on skills and the use of knowledge. Contacts with international students is an important aspect of positive experiences of Chinese students. Relations with Dutch students seems to be less common. Students encounter problems with a language, different customs, integration, and even prejudice in Dutch society. Many students also report being uncomfortable being exposed by questions and criticisms of Chinese politics. These tensions could potentially harm the image of the Netherlands in China and affect student inflow. Chinese bachelor’s students perform relatively well in comparison with Dutch and many other international students. A relatively high percentage successfully completes their education and mostly within the nominal study time. In comparison with students from other non-EEA countries the stay rate for employment reasons after graduation is relatively low and continues to drop. Most Chinese students are enrolled in programmes in the field of business, science, and engineering. As a result, most of the graduates who stay find work in financial services, trade, business, communication and hospitality. Only students in technology and science stay relatively less often for employment after graduation.Asian Studie

Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures

Photo-ionization is the accepted mechanism for the propagation of positive streamers in air though the parameters are not very well known; the efficiency of this mechanism largely depends on the presence of both nitrogen and oxygen. But experiments show that streamer propagation is amazingly robust against changes of the gas composition; even for pure nitrogen with impurity levels below 1 ppm streamers propagate essentially with the same velocity as in air, but their minimal diameter is smaller, and they branch more frequently. Additionally, they move more in a zigzag fashion and sometimes exhibit a feathery structure. In our simulations, we test the relative importance of photo-ionization and of the background ionization from pulsed repetitive discharges, in air as well as in nitrogen with 1 ppm O2 . We also test reasonable parameter changes of the photo-ionization model. We find that photo- ionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition frequency has to be included above 1 Hz. Finally, we explain the feather-like structures around streamer channels that are observed in experiments in nitrogen with high purity, but not in air.Comment: 12 figure

TURNING TEENS INTO FOSSILPHILES: CITIZEN SCIENCE AND ADVANCED VISUALIZATION OF PALEONTOLOGY COLLECTIONS

In 2016, the North Carolina Museum of Natural Sciences (NCMNS) received funding from NSF’s Collections in Support of Biological Research Program to launch a new citizen science initiative—FossilPhiles—aimed at improving publically accessible natural history specimen data. The FossilPhiles project supported NCMNS’ ongoing efforts to digitize paleontology collections and provide STEM opportunities for historically underrepresented student populations by engaging middle and high school students in authentic data collection. Five students were chosen from area schools with underserved populations to digitize highly significant or visually impactful vertebrate, invertebrate, and paleobotanical fossil specimens (e.g., type specimens, rare collections, specimens of high public interest). Students were trained in specimen handling, collections data, and archiving. They collected standard measurement data, photographed specimens in 2D, and constructed 3D photorealistic models using photogrammetry. Over a period of six months, students took over 13,000 photos, documenting 176 specimens in 2D and 137 in 3D. Of these, 124 photos have already been uploaded to the NCMNS’ open-access collections database, accessible through the NCMNS’ website, GBIF, VertNet, and iDigBio. Future project plans include creation of a publicly accessible, interactive portal of the 3D specimen models. Throughout their internships, FossilPhiles students were provided training and opportunities to communicate their experiences with the broader community. The entirety of the FossilPhiles project took place within the glass-walled Paleontology Research Lab (PRL) in the Nature Research Center of the NCMNS, on view to NCMNS’ ~1 million annual visitors. Additionally, students were regularly engaged with communicating about the project in real-time via social media outlets (e.g., Twitter, blogs), sharing photos of fossils they worked on, facts and skills that they learned, and challenges they overcame. FossilPhiles students also partnered with peers engaged in non-STEM museum internships to promote cross-learning. They collaborated with the NCMNS’ Teen Newsroom program to produce a video interview about their evolving impressions on what it means to be a scientist