861 research outputs found
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
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
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.
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 -acyclic queries in time linear in the certificate
plus the output size, while for any -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.
Time-resolved characterization of a pulsed discharge in a stationary bubble
In recent years, plasma generation in water has been proposed for the application of water treatment. The process efficiency is believed to be improved by the introduction of bubbles in the plasma active region. For further optimization, the initiating and developmental mechanisms of plasma inside bubbles need to be understood to a greater extent. In order to meet this necessity, we investigated pulsed electrical discharge inside a stationary bubble in water. This paper deals with the evolution of the discharge and of the bubble shape during discharge, investigated by electrical characterization and fast imaging. Only several microseconds after the application of the voltage pulse, plasma light is observed. Different phases are observed during plasma formation. The plasma is strongest at the bubble surface, causing the surrounding water to evaporate. This leads to both the formation of propagating streamers into the water and the expansion and collapse of the bubble. These observations show that plasma inside a bubble has the strongest activity at the bubble surface, making it attractive for water treatment
Answering Conjunctive Queries under Updates
We consider the task of enumerating and counting answers to -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 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.
By sublinear we mean for some
, where is the size of the active domain of the current
database
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.
The world's drylands: a classification
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