496 research outputs found
Viriato: a Fourier-Hermite spectral code for strongly magnetised fluid-kinetic plasma dynamics
We report on the algorithms and numerical methods used in Viriato, a novel
fluid-kinetic code that solves two distinct sets of equations: (i) the Kinetic
Reduced Electron Heating Model (KREHM) equations [Zocco & Schekochihin, Phys.
Plasmas 18, 102309 (2011)] (which reduce to the standard Reduced-MHD equations
in the appropriate limit) and (ii) the kinetic reduced MHD (KRMHD) equations
[Schekochihin et al., Astrophys. J. Suppl. 182:310 (2009)]. Two main
applications of these equations are magnetised (Alfvenic) plasma turbulence and
magnetic reconnection. Viriato uses operator splitting (Strang or Godunov) to
separate the dynamics parallel and perpendicular to the ambient magnetic field
(assumed strong). Along the magnetic field, Viriato allows for either a
second-order accurate MacCormack method or, for higher accuracy, a
spectral-like scheme composed of the combination of a total variation
diminishing (TVD) third order Runge-Kutta method for the time derivative with a
7th order upwind scheme for the fluxes. Perpendicular to the field Viriato is
pseudo-spectral, and the time integration is performed by means of an iterative
predictor-corrector scheme. In addition, a distinctive feature of Viriato is
its spectral representation of the parallel velocity-space dependence, achieved
by means of a Hermite representation of the perturbed distribution function. A
series of linear and nonlinear benchmarks and tests are presented, including a
detailed analysis of 2D and 3D Orszag-Tang-type decaying turbulence, both in
fluid and kinetic regimes.Comment: 42 pages, 15 figures, submitted to J. Comp. Phy
Towards a unified linear kinetic transport model with the trace ion module for EIRENE
Linear kinetic Monte Carlo particle transport models are frequently employed
in fusion plasma simulations to quantify atomic and surface effects on the main
plasma flow dynamics. Separate codes are used for transport of neutral
particles (incl. radiation) and charged particles (trace impurity ions).
Integration of both modules into main plasma fluid solvers provides then self
consistent solutions, in principle. The required interfaces are far from
trivial, because rapid atomic processes in particular in the edge region of
fusion plasmas require either smoothing and resampling, or frequent transfer of
particles from one into the other Monte Carlo code. We propose a different
scheme here, in which despite the inherently different mathematical form of
kinetic equations for ions and neutrals (e.g. Fokker-Planck vs. Boltzmann
collision integrals) both types of particle orbits can be integrated into one
single code. We show that the approximations and shortcomings of this "single
sourcing" concept (e.g., restriction to explicit ion drift orbit integration)
can be fully tolerable in a wide range of typical fusion edge plasma
conditions, and be overcompensated by the code-system simplicity, as well as by
inherently ensured consistency in geometry (one single numerical grid only) and
(the common) atomic and surface process modulesComment: 15 pages, 7 figure
Trinity: A Unified Treatment of Turbulence, Transport, and Heating in Magnetized Plasmas
To faithfully simulate ITER and other modern fusion devices, one must resolve
electron and ion fluctuation scales in a five-dimensional phase space and time.
Simultaneously, one must account for the interaction of this turbulence with
the slow evolution of the large-scale plasma profiles. Because of the enormous
range of scales involved and the high dimensionality of the problem, resolved
first-principles global simulations are very challenging using conventional
(brute force) techniques. In this thesis, the problem of resolving turbulence
is addressed by developing velocity space resolution diagnostics and an
adaptive collisionality that allow for the confident simulation of velocity
space dynamics using the approximate minimal necessary dissipation. With regard
to the wide range of scales, a new approach has been developed in which
turbulence calculations from multiple gyrokinetic flux tube simulations are
coupled together using transport equations to obtain self-consistent,
steady-state background profiles and corresponding turbulent fluxes and
heating. This approach is embodied in a new code, Trinity, which is capable of
evolving equilibrium profiles for multiple species, including electromagnetic
effects and realistic magnetic geometry, at a fraction of the cost of
conventional global simulations. Furthermore, an advanced model physical
collision operator for gyrokinetics has been derived and implemented, allowing
for the study of collisional turbulent heating, which has not been extensively
studied. To demonstrate the utility of the coupled flux tube approach,
preliminary results from Trinity simulations of the core of an ITER plasma are
presented.Comment: 187 pages, 53 figures, Ph.D. thesis in physics at University of
Maryland, single-space versio
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
A Drift-Kinetic Analytical Model for SOL Plasma Dynamics at Arbitrary Collisionality
A drift-kinetic model to describe the plasma dynamics in the scrape-off layer
region of tokamak devices at arbitrary collisionality is derived. Our
formulation is based on a gyroaveraged Lagrangian description of the charged
particle motion, and the corresponding drift-kinetic Boltzmann equation that
includes a full Coulomb collision operator. Using a Hermite-Laguerre velocity
space decomposition of the gyroaveraged distribution function, a set of
equations to evolve the coefficients of the expansion is presented. By
evaluating explicitly the moments of the Coulomb collision operator,
distribution functions arbitrarily far from equilibrium can be studied at
arbitrary collisionalities. A fluid closure in the high-collisionality limit is
presented, and the corresponding fluid equations are compared with
previously-derived fluid models
Plasma propulsion simulation using particles
This perspective paper deals with an overview of particle-in-cell / Monte
Carlo collision models applied to different plasma-propulsion configurations
and scenarios, from electrostatic (E x B and pulsed arc) devices to
electromagnetic (RF inductive, helicon, electron cyclotron resonance)
thrusters, with an emphasis on plasma plumes and their interaction with the
satellite. The most important elements related to the modeling of plasma-wall
interaction are also presented. Finally, the paper reports new progress in the
particle-in-cell computational methodology, in particular regarding
accelerating computational techniques for multi-dimensional simulations and
plasma chemistry Monte Carlo modules for molecular and alternative propellan
- …