15,608 research outputs found
The interaction between the Moon and the solar wind
We study the interaction between the Moon and the solar wind using a
three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical
fields are presented for typical solar wind conditions with different magnetic
field directions. We find two different wake structures for an interplanetary
magnetic field that is perpendicular to the solar wind flow, and for one that
is parallell to the flow. The wake for intermediate magnetic field directions
will be a mix of these two extreme conditions. Several features are consistent
with a fluid interaction, e.g., the presence of a rarefaction cone, and an
increased magnetic field in the wake. There are however several kinetic
features of the interaction. We find kinks in the magnetic field at the wake
boundary. There are also density and magnetic field variations in the far wake,
maybe from an ion beam instability related to the wake refill. The results are
compared to observations by the WIND spacecraft during a wake crossing. The
model magnetic field and ion velocities are in agreement with the measurements.
The density and the electron temperature in the central wake are not as well
captured by the model, probably from the lack of electron physics in the hybrid
model.Comment: Accepted for publication in Earth, Planets and Spac
Identifying cross country skiing techniques using power meters in ski poles
Power meters are becoming a widely used tool for measuring training and
racing effort in cycling, and are now spreading also to other sports. This
means that increasing volumes of data can be collected from athletes, with the
aim of helping coaches and athletes analyse and understanding training load,
racing efforts, technique etc. In this project, we have collaborated with
Skisens AB, a company producing handles for cross country ski poles equipped
with power meters. We have conducted a pilot study in the use of machine
learning techniques on data from Skisens poles to identify which "gear" a skier
is using (double poling or gears 2-4 in skating), based only on the sensor data
from the ski poles. The dataset for this pilot study contained labelled
time-series data from three individual skiers using four different gears
recorded in varied locations and varied terrain. We systematically evaluated a
number of machine learning techniques based on neural networks with best
results obtained by a LSTM network (accuracy of 95% correctly classified
strokes), when a subset of data from all three skiers was used for training. As
expected, accuracy dropped to 78% when the model was trained on data from only
two skiers and tested on the third. To achieve better generalisation to
individuals not appearing in the training set more data is required, which is
ongoing work.Comment: Presented at the Norwegian Artificial Intelligence Symposium 201
Some Environmental Policy Implications of Recycling Paper Products in Western Europe
We live in a wasteful society, and are becoming increasingly aware of this fact. Our concern for conservation of our natural resources and about the deleterious effects on the environment of disposal of waste products is increasingly reflected in proposed legislation aimed at reducing waste. The preferred technique is recycling of waste products.
While laudable in its objectives, a narrow focus on recycling is also limited, and can result in unexpected effects that can at least partially offset the expected benefits. This is particularly true of paper for at least three basic reasons. First, paper is a major component, about 35%, of household waste volume. Second, unlike most waste, paper has a very high energy content. And third, unlike coal or oil, paper is a renewable resource, and in Europe is produced mostly from forests managed on sustainable principles.
This report summarizes a forthcoming feasibility study of large-scale paper recycling in Europe which investigated the entire production and disposal process using a "life-cycle" methodology and data base developed at IIASA
Kinetic modelling of runaway electron avalanches in tokamak plasmas
Runaway electrons (REs) can be generated in tokamak plasmas if the
accelerating force from the toroidal electric field exceeds the collisional
drag force due to Coulomb collisions with the background plasma. In ITER,
disruptions are expected to generate REs mainly through knock-on collisions,
where enough momentum can be transferred from existing runaways to slow
electrons to transport the latter beyond a critical momentum, setting off an
avalanche of REs. Since knock-on runaways are usually scattered off with a
significant perpendicular component of the momentum with respect to the local
magnetic field direction, these particles are highly magnetized. Consequently,
the momentum dynamics require a full 3-D kinetic description, since these
electrons are highly sensitive to the magnetic non-uniformity of a toroidal
configuration. A bounce-averaged knock-on source term is derived. The
generation of REs from the combined effect of Dreicer mechanism and knock-on
collision process is studied with the code LUKE, a solver of the 3-D linearized
bounce-averaged relativistic electron Fokker-Planck equation, through the
calculation of the response of the electron distribution function to a constant
parallel electric field. This work shows that the avalanche effect can be
important even in non-disruptive scenarios. RE formation through knock-on
collisions is found to be strongly reduced when taking place off the magnetic
axis, since trapped electrons cannot contribute to the RE population. The
relative importance of the avalanche mechanism is investigated as a function of
the key parameters for RE formation; the plasma temperature and the electric
field strength. In agreement with theoretical predictions, the simulations show
that in low temperature and E-field knock-on collisions are the dominant source
of REs and can play a significant role for RE generation, including in
non-disruptive scenarios.Comment: 23 pages, 12 figure
Role of low- component in deformed wave functions near the continuum threshold
The structure of deformed single-particle wave functions in the vicinity of
zero energy limit is studied using a schematic model with a quadrupole deformed
finite square-well potential. For this purpose, we expand the single-particle
wave functions in multipoles and seek for the bound state and the Gamow
resonance solutions. We find that, for the states, where is
the -component of the orbital angular momentum, the probability of each
multipole components in the deformed wave function is connected between the
negative energy and the positive energy regions asymptotically, although it has
a discontinuity around the threshold. This implies that the
resonant level exists physically unless the component is inherently large
when extrapolated to the well bound region. The dependence of the multipole
components on deformation is also discussed
Depressed clad hollow optical fiber with fundamental LP01 mode cut-off
We propose a depressed clad hollow optical fiber with fundamental (LP01) mode cut-off suitable for high power short-wavelength, especially three-level, fiber laser operation by introducing highly wavelength dependent losses at longer wavelengths. The cut-off characteristic of such fiber structure was investigated. A Yb-doped depressed clad hollow optical fiber laser generating 59.1W of output power at 1046nm with 86% of slope efficiency with respect to the absorbed pump power was realised by placing the LP01 mode cut-off at ~1100nm
RGB generation by four-wave mixing in small-core holey fibers
We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled fiber laser by an efficient four-wave mixing process in submicron-sized cores of microstructured holey fibers. A master-oscillator power amplifier (MOPA) source based on Yb-doped fiber is employed to generate 80 ps pulses at 1060 nm wavelength with 32 MHz repetition rate, which are then frequency-doubled in an LBO crystal to generate up to 2 W average power of green light. The green pump is then carefully launched into secondary cores of the cladding of photonic bandgap fibers. These secondary cores with diameters of about 400 to 800 nm act as highly nonlinear waveguides. At the output, we observe strong red and blue sidebands which, together with the remaining green pump light, form a visible white light source of about 360 mW. The generating process is identified as four-wave mixing where phase matching is achieved by birefringence in the secondary cores which arises from non-symmetric deformation during the fiber fabrication. Numerical models of the fiber structure and of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of the white light source due to the damage threshold of silica fibers
Nuclear pairing reduction due to rotation and blocking
Nuclear pairing gaps of normally deformed and superdeformed nuclei are
investigated using the particle-number conserving (PNC) formalism for the
cranked shell model, in which the blocking effects are treated exactly. Both
rotational frequency -dependence and seniority (number of unpaired
particles) -dependence of the pairing gap are
investigated. For the ground-state bands of even-even nuclei, PNC calculations
show that in general decreases with increasing , but
the -dependence is much weaker than that calculated by the
number-projected Hartree-Fock-Bogolyubov approach. For the multiquasiparticle
bands (seniority ), the pairing gaps keep almost -independent.
As a function of the seniority , the bandhead pairing gaps
decrease slowly with increasing . Even for
the highest seniority bands identified so far,
remains greater than 70% of
.Comment: 15 pages, 5 figure
High fidelity readout scheme for rare-earth solid state quantum computing
We propose and analyze a high fidelity readout scheme for a single instance
approach to quantum computing in rare-earth-ion-doped crystals. The scheme is
based on using different species of qubit and readout ions, and it is shown
that by allowing the closest qubit ion to act as a readout buffer, the readout
error can be reduced by more than an order of magnitude. The scheme is shown to
be robust against certain experimental variations, such as varying detection
efficiencies, and we use the scheme to predict the expected quantum fidelity of
a CNOT gate in these solid state systems. In addition, we discuss the potential
scalability of the protocol to larger qubit systems. The results are based on
parameters which we believed are experimentally feasible with current
technology, and which can be simultaneously realized.Comment: 7 pages, 5 figure
On the Entropy of a Family of Random Substitutions
The generalised random Fibonacci chain is a stochastic extension of the
classical Fibonacci substitution and is defined as the rule mapping and with probability , where with
, and where the random rule is applied each time it acts on
a 1. We show that the topological entropy of this object is given by the growth
rate of the set of inflated generalised random Fibonacci words.Comment: A more appropriate tile and minor misprints corrected, compared to
the previous versio
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