1,373 research outputs found
Performance of Agronomic Crop Varieties in Alaska 1978 –2002
There is no such thing as the perfect variety for Alaska. Some varieties are adapted to a wide range of climatic and geographic locations, while others are more specific in their adaptation. The change in elevation of a few hundred feet or a move of a few miles can have a considerable effect on the performance of any variety. Also, cultural practices such as tillage, fertilizer rates, planting date, seeding rate, pest control, and a multitude of other factors can also influence crop yields. This is especially noticeable in northern environments such as Alaska. For example, date-of-planting studies done by F.J. Wooding (1973) and C.W. Knight (1989) found that any date after the middle of May for planting an agronomic crop can result in delayed maturity, low yields, and low quality grain, even for the best adapted varieties for Alaska
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
On the numerical approximation of the rotation number
AbstractThe starting point of this paper is a polygonal approximation of an invariant curve of a map. Using this polygonal approximation an approximation for the circle map (the restriction of the map to the invariant curve) is obtained. The rotation number of the circle map is then approximated by the rotation number of the approximated circle map. The error in the obtained approximate rotation number is discussed, and related to the error in the polygonal approximation of the invariant curve. Simple algorithms for the approximation of the rotation number are described. A numerical example illustrates the theory
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.
Multi-agent collaborative search : an agent-based memetic multi-objective optimization algorithm applied to space trajectory design
This article presents an algorithm for multi-objective optimization that blends together a number of heuristics. A population of agents combines heuristics that aim at exploring the search space both globally and in a neighbourhood of each agent. These heuristics are complemented with a combination of a local and global archive. The novel agent-based algorithm is tested at first on a set of standard problems and then on three specific problems in space trajectory design. Its performance is compared against a number of state-of-the-art multi-objective optimization algorithms that use the Pareto dominance as selection criterion: non-dominated sorting genetic algorithm (NSGA-II), Pareto archived evolution strategy (PAES), multiple objective particle swarm optimization (MOPSO), and multiple trajectory search (MTS). The results demonstrate that the agent-based search can identify parts of the Pareto set that the other algorithms were not able to capture. Furthermore, convergence is statistically better although the variance of the results is in some cases higher
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.
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
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