4,062 research outputs found
ASCOT: solving the kinetic equation of minority particle species in tokamak plasmas
A comprehensive description of methods, suitable for solving the kinetic
equation for fast ions and impurity species in tokamak plasmas using Monte
Carlo approach, is presented. The described methods include Hamiltonian
orbit-following in particle and guiding center phase space, test particle or
guiding center solution of the kinetic equation applying stochastic
differential equations in the presence of Coulomb collisions, neoclassical
tearing modes and Alfv\'en eigenmodes as electromagnetic perturbations relevant
to fast ions, together with plasma flow and atomic reactions relevant to
impurity studies. Applying the methods, a complete reimplementation of the
well-established minority species code ASCOT is carried out as a response both
to the increase in computing power during the last twenty years and to the
weakly structured growth of the code, which has made implementation of
additional models impractical. Also, a benchmark between the previous code and
the reimplementation is accomplished, showing good agreement between the codes.Comment: 13 pages, 9 figures, submitted to Computer Physics Communication
Additional Acceleration of Protons and Energetic Neutrino Production in a Filamentary Jet of the Blazar Markarian 501
Blazars have been regarded as one of the most powerful sources of the highest
energy cosmic rays and also their byproducts, neutrinos. Provided that a
magnetized filamentary system is established in a blazar jet as well, we could
apply the mechanism of multi-stage diffusive shock acceleration to a feasible
TeV emitter, Mrk 501 to evaluate the achievable maximum energy of protons.
Taking conceivable energy restriction into account systematically, it seems
adequate to say that EeV-protons are produced at this site by our present
model. We also estimate neutrino fluxes generated by these accelerated protons
and discuss the detectability based on an updated kilometre-scale telescope
such as IceCube.Comment: 17 pages, 3 Postscript figure
Stochastic reacceleration of relativistic electrons by turbulent reconnection: a mechanism for cluster-scale radio emission ?
In this paper we investigate a situation where relativistic particles are
reaccelerated diffusing across regions of reconnection and magnetic dynamo in
super-Alfvenic, incompressible large-scale turbulence. We present an
exploratory study of this mechanism in the intra-cluster-medium (ICM). In view
of large-scale turbulence in the ICM we adopt a reconnection scheme that is
based on turbulent reconnection and MHD turbulence. In this case particles are
accelerated and decelerated in a systematic way in reconnecting and
magnetic-dynamo regions, respectively, and on longer time-scales undergo a
stochastic process diffusing across these sites (similar to second-order
Fermi). Our study extends on larger scales numerical studies that focused on
the acceleration in and around turbulent reconnecting regions. We suggest that
this mechanism may play a role in the reacceleration of relativistic electrons
in galaxy clusters providing a new physical scenario to explain the origin of
cluster-scale diffuse radio emission. Indeed differently from current turbulent
reacceleration models proposed for example for radio halos this mechanism is
based on the effect of large-scale incompressible and super-Alfvenic
turbulence. In this new model turbulence governs the interaction between
relativistic particles and magnetic field lines that diffuse, reconnect and are
stretched in the turbulent ICM.Comment: 13 pages, 2 figures, MNRAS in pres
Hierarchical bounding structures for efficient virial computations: Towards a realistic molecular description of cholesterics
We detail the application of bounding volume hierarchies to accelerate
second-virial evaluations for arbitrary complex particles interacting through
hard and soft finite-range potentials. This procedure, based on the
construction of neighbour lists through the combined use of recursive
atom-decomposition techniques and binary overlap search schemes, is shown to
scale sub-logarithmically with particle resolution in the case of molecular
systems with high aspect ratios. Its implementation within an efficient
numerical and theoretical framework based on classical density functional
theory enables us to investigate the cholesteric self-assembly of a wide range
of experimentally-relevant particle models. We illustrate the method through
the determination of the cholesteric behaviour of hard, structurally-resolved
twisted cuboids, and report quantitative evidence of the long-predicted phase
handedness inversion with increasing particle thread angles near the
phenomenological threshold value of . Our results further highlight
the complex relationship between microscopic structure and helical twisting
power in such model systems, which may be attributed to subtle geometric
variations of their chiral excluded-volume manifold
Revisión de literatura de jerarquÃa volúmenes acotantes enfocados en detección de colisiones
(Eng) A bounding volume is a common method to simplify object representation by using the composition of geometrical shapes that enclose the object; it encapsulates complex objects by means of simple volumes and it is widely useful in collision detection applications and ray tracing for rendering algorithms. They are popular in computer graphics and computational geometry. Most popular bounding volumes are spheres, Oriented-Bounding Boxe s (OBB’ s), Axis-Align ed Bound ing Boxes (AABB’ s); moreover , the literature review includes ellipsoids, cylinders, sphere packing, sphere shells , k-DOP’ s, convex hulls, cloud of points, and minimal bounding boxe s, among others. A Bounding Volume Hierarchy is ussualy a tree in which the complete object is represented thigter fitting every level of the hierarchy. Additionally, each bounding volume has a cost associated to construction, update, and interference te ts. For instance, spheres are invariant to rotation and translations, then they do not require being updated ; their constructions and interference tests are more straightforward then OBB’ s; however, their tightness is lower than other bounding volumes. Finally , three comparisons between two polyhedra; seven different algorithms were used, of which five are public libraries for collision detection.(Spa) Un volumen acotante es un método común para simplificar la representación de los objetos por medio de composición
de formas geométricas que encierran el objeto; estos encapsulan objetos complejos por medio de volúmenes simples y
son ampliamente usados en aplicaciones de detección de colisiones y trazador de rayos para algoritmos de renderización.
Los volúmenes acotantes son populares en computación gráfica y en geometrÃa computacional; los más populares son las
esferas, las cajas acotantes orientadas (OBB’s) y las cajas acotantes alineadas a los ejes (AABB’s); no obstante, la literatura
incluye elipses, cilindros empaquetamiento de esferas, conchas de esferas, k-DOP’s, convex hulls, nubes de puntos y cajas
acotantes mÃnimas, entre otras. Una jerarquÃa de volúmenes acotantes es usualmente un árbol, en el cual la representación
de los objetos es más ajustada en cada uno de los niveles de la jerarquÃa. Adicionalmente, cada volumen acotante tiene
asociado costos de construcción, actualización, pruebas de interferencia. Por ejemplo, las esferas so invariantes a rotación
y translación, por lo tanto no requieren ser actualizadas en comparación con los AABB no son invariantes a la rotación.
Por otro lado la construcción y las pruebas de solapamiento de las esferas son más simples que los OBB’s; sin embargo, el
ajuste de las esferas es menor que otros volúmenes acotantes. Finalmente, se comparan dos poliedros con siete algoritmos
diferentes de los cuales cinco son librerÃas públicas para detección de colisiones
Isospin effects and symmetry energy studies with INDRA
The equation of state of asymmetric nuclear matter is still controversial, as
predictions at subsaturation as well as above normal density widely diverge. We
discuss several experimental results measured in heavy-ion collisions with the
INDRA array in the incident energy range 5-80 MeV/nucleon. In particular an
estimate of the density dependence of the symmetry energy is derived from
isospin diffusion results compared with a transport code: the potential part of
the symmetry energy linearly increases with the density. We demonstrate that
isospin equilibrium is reached in mid-central collisions for the two reactions
Ni+Au at 52 MeV/nucleon and Xe+Sn at 32 MeV/nucleon. New possible variables and
an improved modelization to investigate symmetry energy are discussed.Comment: Review for the special issue of EPJA on symmetry energy. 20 pages, 16
figure
Distance estimation and collision prediction for on-line robotic motion planning
An efficient method for computing the minimum distance and predicting collisions between moving objects is presented. This problem has been incorporated in the framework of an in-line motion planning algorithm to satisfy collision avoidance between a robot and moving objects modeled as convex polyhedra. In the beginning the deterministic problem, where the information about the objects is assumed to be certain is examined. If instead of the Euclidean norm, L(sub 1) or L(sub infinity) norms are used to represent distance, the problem becomes a linear programming problem. The stochastic problem is formulated, where the uncertainty is induced by sensing and the unknown dynamics of the moving obstacles. Two problems are considered: (1) filtering of the minimum distance between the robot and the moving object, at the present time; and (2) prediction of the minimum distance in the future, in order to predict possible collisions with the moving obstacles and estimate the collision time
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