297 research outputs found
Analytic slowing-down distributions as modified by turbulent transport
The effect of electrostatic microturbulence on fast particles rapidly
decreases at high energy, but can be significant at moderate energy. Previous
studies found that, in addition to changes in the energetic particle density,
this results in nontrivial changes to the equilibrium velocity distribution.
These effects have implications for plasma heating and the stability of
Alfv\'en eigenmodes, but make multiscale simulations much more difficult
without further approximations. Here, several related analytic model
distribution functions are derived from first principles with reasonable
approximations. A single dimensionless parameter characterizes the relative
strength of turbulence relative to collisions, and this parameter appears as an
exponent in the model distribution functions. Even the most simple of these
models reproduces key features of the numerical phase-space transport solution
and provides a useful a priori heuristic for determining how strong the effect
of turbulence is on the redistribution of energetic particles in toroidal
plasmas.Comment: 13 pages; 6 figure
Validating modelling assumptions of alpha particles in electrostatic turbulence
To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium
distribution must be used. In the work, the response of high-energy alpha
particles to electrostatic turbulence has been analyzed for several different
tokamak parameters. Our results are consistent with known scalings and
experimental evidence that alpha particles are generally well-confined: on the
order of several seconds. It is also confirmed that the effect of alphas on the
turbulence is negligible at realistically low concentrations, consistent with
linear theory. It is demonstrated that the usual practice of using a
high-temperature Maxwellian gives incorrect estimates for the radial alpha
particle flux, and a method of correcting it is provided. Furthermore, we see
that the timescales associated with collisions and transport compete at
moderate energies, calling into question the assumption that alpha particles
remain confined to a flux surface that is used in the derivation of the
slowing-down distribution.Comment: 23 pages, 13 figures, submitted to the Journal of Plasma Physic
First principles of modelling the stabilization of microturbulence by fast ions
The observation that fast ions stabilize ion-temperature-gradient-driven
microturbulence has profound implications for future fusion reactors. It is
also important in optimizing the performance of present-day devices. In this
work, we examine in detail the phenomenology of fast ion stabilization and
present a reduced model which describes this effect. This model is derived from
the high-energy limit of the gyrokinetic equation and extends the existing
"dilution" model to account for nontrivial fast ion kinetics. Our model
provides a physically-transparent explanation for the observed stabilization
and makes several key qualitative predictions. Firstly, that different classes
of fast ions, depending on their radial density or temperature variation, have
different stabilizing properties. Secondly, that zonal flows are an important
ingredient in this effect precisely because the fast ion zonal response is
negligible. Finally, that in the limit of highly-energetic fast ions, their
response approaches that of the "dilution" model; in particular, alpha
particles are expected to have little, if any, stabilizing effect on plasma
turbulence. We support these conclusions through detailed linear and nonlinear
gyrokinetic simulations.Comment: 29 pages, 10 figures, 3 table
Autonomic Sonar Sensor Fault Manager for Mobile Robots
NASA, ESA, and NSSC space agencies have plans to put planetary rovers on Mars in 2020. For these future planetary rovers to succeed, they will heavily depend on sensors to detect obstacles. This will also become of vital importance in the future, if rovers become less dependent on commands received from earth-based control and more dependent on self-configuration and self-decision making. These planetary rovers will face harsh environments and the possibility of hardware failure is high, as seen in missions from the past. In this paper, we focus on using Autonomic principles where self-healing, self-optimization, and self-adaption are explored using the MAPE-K model and expanding this model to encapsulate the attributes such as Awareness, Analysis, and Adjustment (AAA-3). In the experimentation, a Pioneer P3-DX research robot is used to simulate a planetary rover. The sonar sensors on the P3-DX robot are used to simulate the sensors on a planetary rover (even though in reality, sonar sensors cannot operate in a vacuum). Experiments using the P3-DX robot focus on how our software system can be adapted with the loss of sonar sensor functionality. The autonomic manager system is responsible for the decision making on how to make use of remaining 'enabled' sonars sensors to compensate for those sonar sensors that are 'disabled'. The key to this research is that the robot can still detect objects even with reduced sonar sensor capability
Mobile Robots and Autonomic Ambient Assisted Living
The use of Smart Environments in the delivery
of pervasive care is a research topic that has witnessed increasing
interest in recent years. These environments aim
to deliver pervasive care through ubiquitous sensing by
monitoring the occupants Activities of Daily Living. In order
for these environments to succeed in achieving their
goal, it is crucial that sensors deployed in the environment
perform faultlessly. In this research we investigate
addressing anomalous sensor behavior through the utilization
of a mobile robot. The robot’s role is twofold; it
must provide substitution in the presence of suspected
sensor faults and act as an observer of anomalous sensor
behavior in order to understand the changes that occur in
the behavior of sensors deployed within the environment
over time. The aim of this work is to explore a paradigm
shift to the use of Autonomic Ambient Assisted Living.We
have discovered that the use of a mobile robot is a viable
means of introducing this paradigm to a Smart Environment
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