Plasma cleaning of ITER first mirrors in magnetic field

To avoid reflectivity losses in ITER optical diagnostic systems, plasma sputtering of metallic First Mirrors is foreseen in order to remove deposits coming from the main wall (mainly beryllium and tungsten). Therefore plasma cleaning has to work on large mirrors (up to a size of 200*300 mm) and under the influence of strong magnetic fields (several Tesla). This work presents the results of plasma cleaning of aluminium and aluminium oxide (used as beryllium proxy) deposited on molybdenum mirrors. Using radio frequency (13.56 MHz) argon plasma, the removal of a 260 nm mixed aluminium/aluminium oxide film deposited by magnetron sputtering on a mirror (98 mm diameter) was demonstrated. 50 nm of pure aluminium oxide were removed from test mirrors (25 mm diameter) in a magnetic field of 0.35 T for various angles between the field lines and the mirrors surfaces. The cleaning efficiency was evaluated by performing reflectivity measurements, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy.Comment: 5 pages, 6 figures and 1 table. Results presented on the 21st Plasma Surface Interaction conference held in Kanazawa Japan, May 201

An Open Guide to Data Structures and Algorithms

This textbook serves as a gentle introduction for undergraduates to theoretical concepts in data structures and algorithms in computer science while providing coverage of practical implementation (coding) issues. The field of computer science (CS) supports a multitude of essential technologies in science, engineering, and communication as a social medium. The varied and interconnected nature of computer technology permeates countless career paths making CS a popular and growing major program. Mastery of the science behind computer science relies on an understanding of the theory of algorithms and data structures. These concepts underlie the fundamental tradeoffs that dictate performance in terms of speed, memory usage, and programming complexity that separate novice programmers from professional practitioners

A Scoping Review of Occupation-based Interventions for Women with Breast Cancer

Background: Globally, breast cancer is the most commonly diagnosed cancer. Women diagnosed with breast cancer often experience physical and psychosocial changes influencing occupational participation and quality of life. Limited evidence exists that reports outcomes of occupation-based occupational therapy interventions for women with breast cancer. Method: In this scoping review, we map occupation-based occupational therapy interventions conducted by occupational therapy professionals for women undergoing rehabilitation for breast cancer. The Joanna Briggs Institute Guidelines provided a structure for the review. Results: Six articles met the inclusion criteria. Interventions included health management, leisure, and social participation occupations. Conclusion: Occupational therapy interventions are important in promoting occupational engagement and quality of life for women experiencing breast cancer. Few studies exist that report these findings, thus necessitating more research

Plasma cleaning of diagnostic first mirrors for the nuclear fusion machine ITER

First Mirrors (FMs) will play a crucial role in optical diagnostic systems of future fusion devices like the International Thermonuclear Experimental Reactor (ITER). Unlike today's tokamaks, forthcoming reactors are expected to produce a high level of radiations and neutrons, preventing the use of common optical components (windows, fibers). Instead, an alternative solution based on an optical labyrinth embedded in the neutron shielding and employing metallic mirrors was proposed. Being the first element of the optical path which allows light to cross the neutron shielding, FMs will be placed close to the thermonuclear plasma and will, therefore, be subject to intense thermal and radiations loads, bombardment by plasma particles (mainly charge-exchange neutrals, CXNs) and deposition of material eroded from the plasma-facing components. Especially net deposition of particles eroded from the First Wall (FW), i.e. mainly beryllium (Be) and tungsten (W), can degrade the reflectivity of FMs severely compromising the reliability of the optical diagnostics. Although passive mitigation techniques are predicted to reduce the amount of Be and W on FMs, optical degradation cannot be fully suppressed; in situ mirror cleaning techniques are indispensable. Discharge plasma cleaning is currently considered as the most promising method to tackle this issue. The main goal of this thesis is to provide an exhaustive study of plasma cleaning techniques of FMs in conditions as close as possible to future fusion reactors. To obtain suitable results for the fusion community and in particular for ITER, experiments were not only conducted in the University of Basel but in numerous international facility as the Swiss Plasma Center in Lausanne, the Paul Scherrer Insitute in Villingen, the National Institute for Laser, Plasma and Radiation Physics (INFLPR) in Bucharest or in two tokamaks, the Joint European Torus (JET) and the Experimental Advanced Superconducting Tokamak (EAST), respectively located in England and China. Significant outcomes have been obtained from the experimental investigations. Several types of plasma generation processes were considered and one of them exhibited very promising results, namely capacitively coupled radio-frequency (RF) discharge. Different types of deposits, trying to mimic the one that could be found in ITER were obtained by magnetron sputtering technique and intensively studied. Cleaning tests done on typical ITER contaminant material (Be or W) as well as on a Be proxy (Be is toxic), aluminium (Al), either in pure or mixed thin films, obtained from existing tokamak or deposited in laboratory showed very promising results. After that, cleaning experiments using larger mirrors up to 200x300 mm2, approaching the final design of ITER's edge Thomson scattering (ETS) FM, were performed in Basel and provided encouraging results. Despite the type of dielectric properties of the mirror's surface (conducting, insulating or half-insulating/half-conducting) the cleaning process systematically exhibited homogeneous etching over the surface. Furthermore, as a strong magnetic field (up to 3.5 T on the mirror locations) will be present in ITER, a thorough experimental investigation on the effect of external B-field on the etching proprieties was conducted. The strength (up to 3.5 T) and orientation of the B-field to the normal of the mirror surface (from 0 to 90°) were shown to be parameters strongly affecting etching rate and homogeneity of the cleaning process. With the B-field being always more tangential to the surface, the etching was getting more and more inhomogeneous. When the B-field was at 90°, e.g. parallel to the surface, the sputtering was impossible on half the mirror. This observation led to a major review of some ITER diagnostics designs with all first mirrors being now repositioned to have a maximum angle of 85° between their normal and the B-field. The first ever cleaning trials in an existing tokamak (EAST) performed on the ETS mock-up will be introduced. Finally, in the optic of in situ RF plasma cleaning integration in ITER, extensive work was started on the so-called driven grounded electrode technique, where the idea is to ground (e.g. short-circuit) the FMs to ease the implementation in ITER

Why Delannoy numbers?

This article is not a research paper, but a little note on the history of combinatorics: We present here a tentative short biography of Henri Delannoy, and a survey of his most notable works. This answers to the question raised in the title, as these works are related to lattice paths enumeration, to the so-called Delannoy numbers, and were the first general way to solve Ballot-like problems. These numbers appear in probabilistic game theory, alignments of DNA sequences, tiling problems, temporal representation models, analysis of algorithms and combinatorial structures.Comment: Presented to the conference "Lattice Paths Combinatorics and Discrete Distributions" (Athens, June 5-7, 2002) and to appear in the Journal of Statistical Planning and Inference

Entwicklung einer low-cost Messstation für urbane Hitzeinseln inkl. automatischer Datenübertragung

Urbane Hitzeinseln sind städtische Gebiete, die sich stärker erwärmen als ihr Umland. Die höheren Temperaturen bringen in den Städten eine Reihe von schwerwiegenden gesundheitlichen und wirtschaftlichen Problemen mit sich, von welchen immer mehr Menschen betroffen sein werden. Forschende auf der ganzen Welt versuchen, die genauen Ursachen und den exakten Nutzen von Gegenmassnahmen zu quantifizieren. Eine wichtige Grundlage dieser Untersuchungen ist die Messung der Temperaturverteilung in Städten. Am exaktesten kann diese mittels einer Vielzahl von Messstationen, die auf dem ganzen Stadtgebiet verteilt sind, erfasst werden. Da Messstationen teuer sind, ist es schwierig, ein dichtes Messnetz zu etablieren, weshalb oft nur über kurze Zeit oder mit zu wenigen Messpunkten gemessen werden kann. Zudem weisen gängige Stationen meist einen zu hohen Strahlungsfehler auf oder benötigen zum dauerhaften Betrieb eines Ventilators einen Netzanschluss. Das Ziel dieser Bachelorarbeit war die Entwicklung einer Messstation, welche die Temperatur und relative Luftfeuchtigkeit exakt erfasst, die Messwerte automatisch und drahtlos übermittelt und weniger als Fr. 100.- kostet. Die Messstation soll mindestens ein Jahr ohne externe Energieversorgung betrieben werden können. Dies erforderte die Entwicklung eines Strahlenschutzgehäuses, von Mess- und Funkelektronik und von Steuerungssoftware. Zudem wurde eine Kostenschätzung für eine Serienproduktion der Messstation unternommen. Basierend auf Recherchen zu meteorologischen Messmethoden und Strahlenschutzgehäusen wurden Bauteile, Baumaterialien, elektronische Komponenten und Fertigungsverfahren evaluiert und getestet. Die ausgewählte Sensorik und Elektronik wurde miniaturisiert und in den Gehäuseprototypen, der aus 3D-Druck- und Blechteilen besteht, eingebaut. LoRaWAN wurde als Funkstandard ausgewählt und seine Funktionalität getestet. Zudem wurde die Software für die Messstation und einen Server, welcher die empfangenen Messdaten auswertet, entwickelt. Mittels einer einfachen physikalischen Modellbildung, verschiedenen Versuchen und nichtlinearen Ausgleichsrechnungen wurde zudem ein Extrapolationsalgorithmus etabliert, welcher exakte Messungen bei geringem Energieverbrauch ermöglicht. Aus der Arbeit resultieren zwei Prototypen, welche automatisch Messwerte sammeln und übermitteln. Die Messstation weist auch bei starker Sonneneinstrahlung eine Messgenauigkeit von 1K auf. In der Testphase wiesen die Prototypen im Vergleich mit einer Referenzstation in 93.5% der Fälle eine Abweichung von weniger als ±0.5K auf. Die Messgenauigkeit der relativen Luftfeuchtigkeit betrug ±3%RH. Zudem wurden Kostenschätzungen für die Produktion von 100, 500 und 1000 Messstationen erstellt. Der Materialpreis bei 1000 Stück liegt bei Fr. 131.- pro Messstation. Obwohl das geforderte Kostendach nicht eingehalten werden konnte, resultieren eine Vielzahl von wichtigen Erkenntnissen aus dieser Arbeit. Im Ausblick werden zudem mögliche Untersuchungen und Anpassungen, welche die Messstation bezüglich Kosten, Energieeffizienz und Messgenauigkeit weiter optimieren könnten, diskutiert. Mit dieser Arbeit konnte ein wichtiger Beitrag zur besseren Untersuchung des urbanen Hitzeinseleffekts geleistet werden

Ion flux-energy distributions across grounded grids in an RF plasma source with DC-grounded electrodes

We present an experimental investigation of the ion flux–energy distribution functions (IFEDFs) obtained across grounded grids in an asymmetric capacitively coupled RF source using a helium discharge. The powered electrode in the RF source is DC-grounded via a λ/4 filter, which lifts its DC potential to zero. Grids of different dimensions (hole width, thickness, and geometric transparency) were used to confine the plasma, while the IFEDF of the ion beam departing the grid and reaching the reactor walls was studied using a retarding field energy analyser. The IFEDF obtained was double-peaked, indicating the presence of fast ions arriving from the plasma source, and cold ions generated upon charge exchange collisions between the fast ions and neutrals. The flux, as well as the peak energies of the two ion groups, depended significantly on the process parameters: RF power, He pressure, the distance between grids and walls, and the dimensions of the grids. The results indicate that confining plasma with grids can reduce the ion flux at the walls by over 60%, significantly lowering the wall sputtering rate. This was confirmed with a dedicated long-exposure plasma discharge with a gridded plasma reactor, wherein less than 1 nm of Cu deposition was found on the DC-grounded powered electrode, and the surface reflectivity was preserved to pristine values. In contrast, a similar experiment in a gridless reactor resulted in Cu deposition of 35 nm with a drastic drop in surface reflectivity. These studies are of great importance for the application of similar RF plasma sources with in-situ cleaning of diagnostic mirrors in fusion devices, as well as in a variety of plasma processing applications

Demography of sea lamprey (Petromyzon marinus) ammocoete populations in relation to potential spawning-migration obstructions

Copyright © 2017 John Wiley & Sons, Ltd. Recent advances in the understanding of lamprey migrations have led to concerns over the impacts of obstructions on the demography of many species. This study investigated sea lamprey (Petromyzon marinus) larvae (ammocoetes) in two adjacent but contrasting rivers, both designated Special Areas of Conservation under the EC Habitats Directive (92/43/EEC), one (the River Wye) with a small number of potential migration obstructions in its upper reaches and one (the River Usk) with obstacles along its course. The geographical distributions, densities and age structures of the ammocoete populations were examined in relation to the locations of potential obstructions to the spawning migrations of anadromous adults. A minimum of three age classes was recorded as far as 200 km upstream of the mouth of the River Wye (93% of the length of the mainstem), demonstrating that adults regularly migrate to the upper reaches of the catchment (downstream of a natural waterfall). By contrast, sea lamprey ammocoetes appeared to be absent (in suitable habitat) from 20 km (17%) of the River Usk, and there was a reduction in density, prevalence and the number of age classes upstream of two putative spawning-migration obstructions. This study highlights some of the potential impacts of habitat fragmentation by obstructions on the spawning migrations of anadromous species, as inferred from ammocoete demography. When used in combination to compare contiguous reaches, ammocoete densities, prevalence and age structure may be a useful indicator of which structures are likely to be important migration obstructions, and where further studies or mitigation efforts should be focused. It is likely that passage past some obstructions is enhanced if high river levels occur during the spawning migration, but there is a need to facilitate passage during all conditions, to improve access to under-exploited spawning and nursery areas

Study of Wall Re-Deposition on DC-grounded ITER-relevant Mirrors with RF Plasma in a First Mirror Unit

In ITER, several first mirrors (FMs) are expected to be DC-grounded with the water cooling lines being implemented as a quarter wavelength ($lambda$/4) RF-filter. DC-grounding of the FMs can significantly increase the plasma potential V p, which could trigger an increased wall sputtering and associated re-deposition on the FMs during plasma cleaning. To understand the scope of this impact, helium discharges were excited with DC-grounded FMs in an ITER-sized mock-up of a first mirror unit (FMU) using wall materials with different sputtering energy thresholds (E th). Additionally, a part of the FM was electrically isolated from the RF to study its impact on the erosion/re-deposition properties on the surface. The E th of the wall materials, as well as its native oxide layers, had a significant influence on the re-deposition observed on the FMs. With high E th where walls were unsputtered, both the DC-grounded and electrically isolated parts of the FM were free of deposits. However, with low E th where the walls were sputtered, there was a net wall re-deposition on the DC-grounded parts of the FM, while electrically isolated parts were still relatively clean. Further, to study the impact of floating wall components, Cu walls in the FMU were isolated from the ground. Here the walls developed a floating potential V f and the ion energy at the walls was lowered to e(V p - V f). The floating walls, in this case, were relatively unsputtered and the FMs experienced a net cleaning with total reflectivity of the mirror preserved at pristine mirror levels. This work shows that electrically isolating the FM as well as the wall surface minimizes wall re-deposition in presence of $lambda$/4 filter and therefore are promising techniques for effective FM cleaning in ITER

Plasma Cleaning of Steam Ingressed ITER First Mirrors

In ITER, the first mirrors (FMs) are vulnerable to an in-vessel coolant leak which could severely diminish their optical properties. To understand the scope of this potential impact, several FM samples were exposed to a steam and humidity test simulating the event in ITER. Both rhodium and molybdenum mirrors, observed a loss in specular reflectivity as a result (the loss being greater for the Mo mirror). Their surfaces were tarnished with the development a thin Rh oxide and a thick Mo oxide (120–170 nm). This study focusses on capacitively coupled radio frequency (CCRF) plasma cleaning of steam ingressed (SI) FM samples and follow their optical recovery. Plasma cleaning experiments were performed with 13.56 MHz CCRF plasma using argon and/or hydrogen as process gas (with 230 eV ion energy). Initial and final reflectivity measurements, chemical surface analysis using in vaccuo X-ray photoelectron spectroscopy, scanning electron microscopy, focused ion beam and roughness measurements, were carried out for each sample to evaluate the cleaning efficiency. Using the plasma cleaning technique, it was possible to remove the SI induced contamination from the mirror surfaces and recover their optical properties to the pristine levels. Several ‘voids/inclusions’ were seen to arise along the grain boundaries as a result of the SI procedure. The concentration of these ‘voids/inclusions’ was observed to increase till a certain point followed by a decrease with increasing cleaning time