Positive streamers in ambient air and a N2:O2-mixture (99.8 : 0.2)

Photographs show distinct differences between positive streamers in air or in a nitrogen-oxygen mixture (0.2% O2). The streamers in the mixture branch more frequently, but the branches also extinguish more easily. Probably related to that, the streamers in the mixture propagate more in a zigzag manner while they are straighter in air. Furthermore, streamers in the mixture can become longer; they are thinner and more intense.Comment: 2 pages, 4 figures, paper is accepted for IEEE Trans. Plasma Sci. and scheduled to appear in June 200

Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy

Positive and negative streamers are studied in ambient air at 1 bar; they emerge from a needle electrode placed 40 mm above a planar electrode. The amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses have rise times of 30 ns or shorter. Diameters, velocities and energies of the streamers are measured. Two regimes are identified; a low voltage regime where only positive streamers appear and a high voltage regime where both positive and negative streamers exist. Below 5 kV, no streamers emerge. In the range from 5 to 40 kV, positive streamers form, while the negative discharges only form a glowing cloud at the electrode tip, but no streamers. For 5 to 20 kV, diameters and velocities of the positive streamers have the minimal values of d=0.2 mm and v \approx 10^5 m/s. For 20 to 40 kV, their diameters increase by a factor 6 while the voltage increases only by a factor 2. Above the transition value of 40 kV, streamers of both polarities form; they strongly resemble each other, though the positive ones propagate further; their diameters continue to increase with applied voltage. For 96 kV, positive streamers attain diameters of 3 mm and velocities of 4*10^6 m/s, negative streamers are about 20 % slower and thinner. An empirical fit formula for the relation between velocity v and diameter d is v=0.5 d^2/(mm ns) for both polarities. Streamers of both polarities dissipate energies of the order of several mJ per streamer while crossing the gap.Comment: 20 pages, 9 figures, accepted for J. Phys.

Answering Conjunctive Queries under Updates

We consider the task of enumerating and counting answers to $k$-ary conjunctive queries against relational databases that may be updated by inserting or deleting tuples. We exhibit a new notion of q-hierarchical conjunctive queries and show that these can be maintained efficiently in the following sense. During a linear time preprocessing phase, we can build a data structure that enables constant delay enumeration of the query results; and when the database is updated, we can update the data structure and restart the enumeration phase within constant time. For the special case of self-join free conjunctive queries we obtain a dichotomy: if a query is not q-hierarchical, then query enumeration with sublinear$^\ast$ delay and sublinear update time (and arbitrary preprocessing time) is impossible. For answering Boolean conjunctive queries and for the more general problem of counting the number of solutions of k-ary queries we obtain complete dichotomies: if the query's homomorphic core is q-hierarchical, then size of the the query result can be computed in linear time and maintained with constant update time. Otherwise, the size of the query result cannot be maintained with sublinear update time. All our lower bounds rely on the OMv-conjecture, a conjecture on the hardness of online matrix-vector multiplication that has recently emerged in the field of fine-grained complexity to characterise the hardness of dynamic problems. The lower bound for the counting problem additionally relies on the orthogonal vectors conjecture, which in turn is implied by the strong exponential time hypothesis. $^\ast)$ By sublinear we mean $O(n^{1-\varepsilon})$ for some $\varepsilon>0$, where $n$ is the size of the active domain of the current database

Nanosecond pulsed discharges in N2 and N2/H2O mixtures

Nanosecond pulsed discharges in N2 and N2/H2O at atmospheric pressure between two pin-shaped electrodes are studied. The evolution of the discharge is investigated with time-resolved imaging and optical emission spectroscopy. The discharge consists of three phases, the ignition (mainly molecular emission), spark (mainly atomic and ionic emission) and recombination phase (mainly atomic emission). The electron density is obtained by broadening of the N 746 nm and Ha line and reaches very high values up to 4 10^24 m^-3. The gas temperature is obtained by OES and Rayleigh scattering

Beyond Worst-Case Analysis for Joins with Minesweeper

We describe a new algorithm, Minesweeper, that is able to satisfy stronger runtime guarantees than previous join algorithms (colloquially, `beyond worst-case guarantees') for data in indexed search trees. Our first contribution is developing a framework to measure this stronger notion of complexity, which we call {\it certificate complexity}, that extends notions of Barbay et al. and Demaine et al.; a certificate is a set of propositional formulae that certifies that the output is correct. This notion captures a natural class of join algorithms. In addition, the certificate allows us to define a strictly stronger notion of runtime complexity than traditional worst-case guarantees. Our second contribution is to develop a dichotomy theorem for the certificate-based notion of complexity. Roughly, we show that Minesweeper evaluates $\beta$-acyclic queries in time linear in the certificate plus the output size, while for any $\beta$-cyclic query there is some instance that takes superlinear time in the certificate (and for which the output is no larger than the certificate size). We also extend our certificate-complexity analysis to queries with bounded treewidth and the triangle query.Comment: [This is the full version of our PODS'2014 paper.

Positive streamers in air and nitrogen of varying density: experiments on similarity laws

Positive streamers in ambient air at pressures from 0.013 to 1 bar are investigated experimentally. The voltage applied to the anode needle ranges from 5 to 45 kV, the discharge gap from 1 to 16 cm. Using a "slow" voltage rise time of 100 to 180 ns, the streamers are intentionally kept thin. For each pressure p, we find a minimal diameter d_{min}. To test whether streamers at different pressures are similar, the minimal streamer diameter d_{min} is multiplied by its pressure p; we find this product to be well approximated by p*d_{min}=0.20 \pm 0.02 mm*bar over two decades of air pressure at room temperature. The value also fits diameters of sprite discharges above thunderclouds at an altitude of 80 km when extrapolated to room temperature (as air density rather than pressure determines the physical behavior). The minimal velocity of streamers in our measurements is approximately 0.1 mm/ns = 10^5 m/s. The same minimal velocity has been reported for tendrils in sprites. We also investigate the size of the initial ionization cloud at the electrode tip from which the streamers emerge, and the streamer length between branching events. The same quantities are also measured in nitrogen with a purity of approximately 99.9 %. We characterize the essential differences with streamers in air and find a minimal diameter of p*d_{min}=0.12 \pm 0.02 mm*bar in our nitrogen.Comment: 24 pages, 11 figures, accepted for J. Phys.

Harvest and Nitrogen Management of Three Perennial Grasses as Biomass Feedstock in Subarctic Alaska

High energy costs in high-latitude regions have generated interest in the feasibility of bioenergy cropping. The goal of this study was to determine the N response and best harvest regime for biomass production of three perennial, cool-season grass species—tufted hairgrass (Deschampsia caespitosa (L.) P. Beauv.), slender wheatgrass (Elymus trachy­caulus (Link) Gould ex Shinners), and smooth bromegrass (Bromus inermis Leyss)—at two locations in central Alaska. Maximum dry matter yields were 11.3 Mg ha-1 for smooth bromegrass, 8.1 Mg ha-1 for tufted hairgrass, and 8.0 Mg ha-1 for slender wheatgrass, but yields varied greatly among years. We found a linear N response in most cases, with highest yields at the 100 kg N ha-1 application rate. Yields for the double-harvest regime usually did not vary significantly from those of the fall harvest, but spring harvest sometimes reduced yields dramatically. Biomass in the spring harvest was usually dry enough not to require additional drying for storage. Results of this study indicate it may be possible to produce grass biomass yields high enough for use as bioenergy feedstocks in central Alaska, but questions remain about the best management practices and the economics of growing bioenergy crops in Alaska.Les coûts élevés de l’énergie en haute latitude incitent les gens à se pencher sur la faisabilité d’entreprendre des cultures bioénergétiques. L’objectif de cette étude consistait à déterminer la réponse à l’azote et le meilleur régime d’exploitation pour la bioproduction de trois espèces de graminées vivaces en saison fraîche, soit la deschampsie cespiteuse (Deschampsia caespitosa (L.) P. Beauv.), l’élyme à chaumes rudes (Elymus trachycaulus (Link) Gould ex Shinners) et le brome inerme (Bromus inermis Leyss), à deux endroits du centre de l’Alaska. Le rendement maximum de matière sèche était de 11,3 tm ha-1 dans le cas du brome inerme, de 8,1 tm ha-1 dans le cas de la deschampsie cespiteuse et de 8,0 tm ha-1 dans le cas de l’élyme à chaumes rudes, bien que les rendements aient connu d’importantes variations d’une année à l’autre. Nous avons trouvé une réponse linéaire à l’azote dans la plupart des cas, les rendements les plus élevés étant ceux de la dose d’application de 100 kg N ha-1. Le rendement du régime à double récolte ne variait généralement pas beaucoup du régime à récolte d’automne, bien que les récoltes du printemps donnaient parfois un rendement considérablement réduit. De manière générale, la biomasse de la récolte du printemps était suffisamment sèche pour ne pas avoir besoin d’être asséchée davantage avant d’être stockée. Les résultats de cette étude indiquent qu’il peut être possible de produire des rendements en biomasse suffisamment élevés à partir de graminées pour être utilisés comme charge bioénergétique dans le centre de l’Alaska, mais cela dit, il y a toujours lieu de répondre aux questions portant sur les pratiques de gestion exemplaires et le caractère économique des productions bioéner­gétiques en Alaska

Conceptualisatie en parameterisatie van landgebruik, bodem, beregening en buisdrainage in het NHI

Bij de modellering van de hydrologie van de bodem en het landgebruik in het NHI (Nationaal Hydrologisch Instrumentarium) is voortgebouwd op de hydrologie voor STONE 2.3. Bij STONE is Nederland landsdekkend gemodelleerd met behulp van een beperkt aantal (6405) plots. Een plot is een unieke combinatie van gewastypen, en geclassificeerde hydrologische eigenschappen en bodemchemische eigenschappen. Bij het bouwen van het NHI is deze piotbenadering verlaten. Dit betekent dat voor elke gridcel van 250 bij 250 m een model moet worden gebouwd van het gewas-onverzadigde zone deelsysteem.Daarbij is gebruik gemaakt van de modelcode metaSWAP, de onverzadigde zone module van SIMGRO. In dit artikel wordt zowel de conceptualisatie als de bijbehorende parametrisatie en de daarvoor gebruikte bestanden in meer detail beschreve

Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise-rate

Controlling streamer morphology is important for numerous applications. Up to now, the effect of the voltage rise rate was only studied across a wide range. Here we show that even slight variations in the voltage rise can have significant effects. We have studied positive streamer discharges in a 16 cm point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a duration of 130 ns. The voltage rise varies by a rise rate from 1.9 kV/ns to 2.7 kV/ns and by the first peak voltage of 22 to 28 kV. A structural link is found between smaller discharges with a larger inception cloud caused by a faster rising voltage. This relation is explained by the greater stability of the inception cloud due to a faster voltage rise, causing a delay in the destabilisation. Time-resolved measurements show that the inception cloud propagates slower than an earlier destabilised, more filamentary discharge. This explains that the discharge with a faster rising voltage pulse ends up to be shorter. Furthermore, the effect of remaining background ionisation in a pulse sequence has been studied, showing that channel thickness and branching rate are locally affected, depending on the covered volume of the previous discharge.Comment: 16 pages, 9 figure