2,870 research outputs found
Pegasus: A New Hybrid-Kinetic Particle-in-Cell Code for Astrophysical Plasma Dynamics
We describe Pegasus, a new hybrid-kinetic particle-in-cell code tailored for
the study of astrophysical plasma dynamics. The code incorporates an
energy-conserving particle integrator into a stable, second-order--accurate,
three-stage predictor-predictor-corrector integration algorithm. The
constrained transport method is used to enforce the divergence-free constraint
on the magnetic field. A delta-f scheme is included to facilitate a
reduced-noise study of systems in which only small departures from an initial
distribution function are anticipated. The effects of rotation and shear are
implemented through the shearing-sheet formalism with orbital advection. These
algorithms are embedded within an architecture similar to that used in the
popular astrophysical magnetohydrodynamics code Athena, one that is modular,
well-documented, easy to use, and efficiently parallelized for use on thousands
of processors. We present a series of tests in one, two, and three spatial
dimensions that demonstrate the fidelity and versatility of the code.Comment: 27 pages, 12 figures, accepted for publication in Journal of
Computational Physic
Beam Dynamics in High Intensity Cyclotrons Including Neighboring Bunch Effects: Model, Implementation and Application
Space charge effects, being one of the most significant collective effects,
play an important role in high intensity cyclotrons. However, for cyclotrons
with small turn separation, other existing effects are of equal importance.
Interactions of radially neighboring bunches are also present, but their
combined effects has not yet been investigated in any great detail. In this
paper, a new particle in cell based self-consistent numerical simulation model
is presented for the first time. The model covers neighboring bunch effects and
is implemented in the three-dimensional object-oriented parallel code
OPAL-cycl, a flavor of the OPAL framework. We discuss this model together with
its implementation and validation. Simulation results are presented from the
PSI 590 MeV Ring Cyclotron in the context of the ongoing high intensity upgrade
program, which aims to provide a beam power of 1.8 MW (CW) at the target
destination
Detecting shock waves in cosmological smoothed particle hydrodynamics simulations
We develop a formalism for the identification and accurate estimation of the
strength of structure formation shocks during cosmological smoothed particle
hydrodynamics simulations. Shocks not only play a decisive role for the
thermalization of gas in virialising structures but also for the acceleration
of relativistic cosmic rays (CRs) through diffusive shock acceleration. Our
formalism is applicable both to ordinary non-relativistic thermal gas, and to
plasmas composed of CRs and thermal gas. To this end, we derive an analytical
solution to the one-dimensional Riemann shock tube problem for a composite
plasma of CRs and thermal gas. We apply our methods to study the properties of
structure formation shocks in high-resolution hydrodynamic simulations of the
LCDM model. We find that most of the energy is dissipated in weak internal
shocks with Mach numbers M~2 which are predominantly central flow shocks or
merger shock waves traversing halo centres. Collapsed cosmological structures
are surrounded by external shocks with much higher Mach numbers up to M~1000,
but they play only a minor role in the energy balance of thermalization. We
show that after the epoch of cosmic reionisation the Mach number distribution
is significantly modified by an efficient suppression of strong external shock
waves due to the associated increase of the sound speed of the diffuse gas.
Invoking a model for CR acceleration in shock waves, we find that the average
strength of shock waves responsible for CR energy injection is higher than that
for shocks that dominate the thermalization of the gas. When combined with
radiative dissipation and star formation, our formalism can also be used to
study CR injection by supernova shocks, or to construct models for
shock-induced star formation in the interstellar medium. (abridged)Comment: 20 pages, 7 figures, just appeared in MNRAS, full resolution version
available at
http://www.cita.utoronto.ca/~pfrommer/Publications/MNRAS.367.113.pd
Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory
Multidimensional magneto-hydrodynamical (MHD) simulations coupled with
stochastic differential equations (SDEs) adapted to test particle acceleration
and transport in complex astrophysical flows are presented. The numerical
scheme allows the investigation of shock acceleration, adiabatic and radiative
losses as well as diffusive spatial transport in various diffusion regimes. The
applicability of SDEs to astrophysics is first discussed in regards to the
different regimes and the MHD code spatial resolution. The procedure is then
applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets.
The ability of SDE to reproduce analytical solutions of the
diffusion-convection equation for electrons is tested through the incorporation
of an increasing number of effects: shock acceleration, spatially dependent
diffusion coefficients and synchrotron losses. The SDEs prove to be efficient
in various shock configuration occurring in the inner jet during the
development of the Kelvin-Helmholtz instability. The particle acceleration in
snapshots of strong single and multiple shock acceleration including realistic
spatial transport is treated. In chaotic magnetic diffusion regime, turbulence
levels around are found to
be the most efficient to enable particles to reach the highest energies. The
spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows),
does not exhibit a power-law shape due to transverse momentum dependent
escapes. Out of this range, the confinement is not so efficient and the
spectrum cut-off above few hundreds of GeV, questioning the Chandra
observations of X-ray knots as being synchrotron radiation. The extension to
full time dependent simulations to X-ray extragalactic jets is discussed.Comment: Astronomy & Astrophysics (in press), 18 page
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