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

Simulation-Based Parallel Training

Numerical simulations are ubiquitous in science and engineering. Machine learning for science investigates how artificial neural architectures can learn from these simulations to speed up scientific discovery and engineering processes. Most of these architectures are trained in a supervised manner. They require tremendous amounts of data from simulations that are slow to generate and memory greedy. In this article, we present our ongoing work to design a training framework that alleviates those bottlenecks. It generates data in parallel with the training process. Such simultaneity induces a bias in the data available during the training. We present a strategy to mitigate this bias with a memory buffer. We test our framework on the multi-parametric Lorenz's attractor. We show the benefit of our framework compared to offline training and the success of our data bias mitigation strategy to capture the complex chaotic dynamics of the system

Comparative chemistry of diffuse clouds III: sulfur-bearing molecules

Using data from IRAM's Plateau de Bure Interferometer and 30 m Telescope, we discuss the mm-wave absorption lines of CS, SO, H2S and HCS+ which arise in diffuse clouds occulting several extragalactic continuum sources. Typical relative abundances are X(CS)/X(HCO+) ~ 2, X(CS)/X(SO) ~ 2, X(CS)/X(H2S) ~ 6 and X(CS)/X(HCS+) ~ 13.Comment: Accepted by A&A 2002-Jan-1

Connectivity Optimization in Robotic Networks

La collaboration entre multiple appareils électroniques (e.g. smartphones, ordinateurs, robots, senseurs et routeurs) est une tendance qui suscite un vif intérêt tant ses applications semblent prometteuses. Les maisons autonomes ou villes intelligentes figurent parmi la prodigieuse variété d’exemples. La communication entre appareils est une des clés du succès de leur coopération. Sans un bon système de communication, les appareils se retrouvent vite incapables d’échanger l’information nécessaire à la prise de décision. Pour garantir une bonne communication, il faut un réseau solide sur lequel elle puisse reposer. Nous pourrions envisager une organisation centralisée, puisqu’elles sont si répandues. Nos téléphones portables communiquent grâce à des antennes-relais ; et nous naviguons sur l’internet grâce à des routeurs. Dans un réseau centralisé, si un noeud principal, tel qu’une antenne ou un routeur, est défaillant, la capacité à communiquer en est dramatiquement diminuée. Or, certaines collaborations entre appareils interviennent, parfois, dans des situations où les infrastructures classiques ne sont pas accessibles. C’est le cas pour les opérations de sauvetages, où les moyens de communications classiques ont pu être endommagés à la suite d’un sinistre. D’autres organisations sont alors plus judicieuses. Dans les réseaux ad hoc, par exemple, il n’existe pas de noeud central, car chaque appareil peut servir au transit de l’information. Cette dissertation s’intéresse à la mise en place de réseaux ad hoc et mobiles entre smartphones et drones. Elle s’inscrit dans le cadre d’un partenariat, entre Humanitas Solutions et l’École Polytechnique de Montréal, qui vise à établir un moyen de communication basé sur ces appareils, pour connecter victimes et premiers secours lors d’opérations de sauvetages. Pour mener à bien ce projet, nous devons permettre aux appareils électroniques de communiquer sans recourir à quelconque infrastructure. Pour relayer l’information, nous devons également maintenir les drones connectés au-dessus de la zone sinistrée.----------ABSTRACT: Because of their promising applications, the interest for machine-to-machine interaction has soared (e.g. between smartphones, laptops, robots, sensors, or routers). Autonomous homes and smart cities are just two examples among the many. Without a good communication system, devices are unable to share relevant information and take effective decisions. Thus, inter-device communication is key for successful cooperations. To guarantee suitable communication, devices need to rely on a robust network. One might think of classical centralized network architecture since it is so common – antennae relay our smartphone communications, and routers provide us with an Internet connection at home. However, this architecture is not adequate for every application. When a central node (e.g. an antenna or a router) fails, it can cripple all the network. Moreover, fixed infrastructure is not always available, which is detrimental for applications like search and rescue operations. Hence, other network designs can be more suitable, like ad hoc networks, where there is no central node and every device can route information. This work aims at establishing mobile ad hoc networks between multiple devices for search and rescue operations. This thesis is framed by a partnership between Humanitas Solutions and École Polytechnique de Montréal, whose goal is to relay information between victims and first responders by the use of smartphones and flying robots (i.e. drones). For this purpose, we have to enable infrastructureless communications between devices and maintain drones connected over the disaster area

Reconfiguration of plane trees in convex geometric graphs

A non-crossing spanning tree of a set of points in the plane is a spanning tree whose edges pairwise do not cross. Avis and Fukuda in 1996 proved that there always exists a flip sequence of length at most $2n-4$ between any pair of non-crossing spanning trees (where $n$ denotes the number of points). Hernando et al. proved that the length of a minimal flip sequence can be of length at least $\frac 32 n$. Two recent results of Aichholzer et al. and Bousquet et al. improved the Avis and Fukuda upper bound by proving that there always exists a flip sequence of length respectively at most $2n - \log n$ and $2n - \sqrt{n}$. We improve the upper bound by a linear factor for the first time in 25 years by proving that there always exists a flip sequence between any pair of non-crossing spanning trees $T_1,T_2$ of length at most $c n$ where $c \approx 1.95$. Our result is actually stronger since we prove that, for any two trees $T_1,T_2$, there exists a flip sequence from $T_1$ to $T_2$ of length at most $c |T_1 \setminus T_2|$. We also improve the best lower bound in terms of the symmetric difference by proving that there exists a pair of trees $T_1,T_2$ such that a minimal flip sequence has length $\frac 53 |T_1 \setminus T_2|$, improving the lower bound of Hernando et al. by considering the symmetric difference instead of the number of vertices. We generalize this lower bound construction to non-crossing flips (where we close the gap between upper and lower bounds) and rotations