3,755 research outputs found
How fast can the wall move? A study of the electroweak phase transition dynamics
We consider the dynamics of bubble growth in the Minimal Standard Model at
the electroweak phase transition and determine the shape and the velocity of
the phase boundary, or bubble wall. We show that in the semi-classical
approximation the friction on the wall arises from the deviation of massive
particle populations from thermal equilibrium. We treat these with Boltzmann
equations in a fluid approximation. This approximation is reasonable for the
top quarks and the light species while it underestimates the friction from the
infrared bosons and Higgs particles. We use the two-loop finite temperature
effective potential and find a subsonic bubble wall for the whole range of
Higgs masses GeV. The result is weakly dependent on : the wall
velocity falls in the range , while the wall thickness is
in the range . The wall is thicker than the phase equilibrium
value because out of equilibrium particles exert more friction on the back than
on the base of a moving wall. We also consider the effect of an infrared gauge
condensate which may exist in the symmetric phase; modelling it simplemindedly,
we find that the wall may become supersonic, but not ultrarelativistic.Comment: 42 pages, plain latex, with three figures. Minor editing August 1 (we
figured out how to do analytically some integrals we previously did
numerically, made corresponding (slight) changes to numerical results, and
corrected some typos.
Kinetic Solvers with Adaptive Mesh in Phase Space
An Adaptive Mesh in Phase Space (AMPS) methodology has been developed for
solving multi-dimensional kinetic equations by the discrete velocity method. A
Cartesian mesh for both configuration (r) and velocity (v) spaces is produced
using a tree of trees data structure. The mesh in r-space is automatically
generated around embedded boundaries and dynamically adapted to local solution
properties. The mesh in v-space is created on-the-fly for each cell in r-space.
Mappings between neighboring v-space trees implemented for the advection
operator in configuration space. We have developed new algorithms for solving
the full Boltzmann and linear Boltzmann equations with AMPS. Several recent
innovations were used to calculate the discrete Boltzmann collision integral
with dynamically adaptive mesh in velocity space: importance sampling,
multi-point projection method, and the variance reduction method. We have
developed an efficient algorithm for calculating the linear Boltzmann collision
integral for elastic and inelastic collisions in a Lorentz gas. New AMPS
technique has been demonstrated for simulations of hypersonic rarefied gas
flows, ion and electron kinetics in weakly ionized plasma, radiation and light
particle transport through thin films, and electron streaming in
semiconductors. We have shown that AMPS allows minimizing the number of cells
in phase space to reduce computational cost and memory usage for solving
challenging kinetic problems
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
Driven waves in a two-fluid plasma
We study the physics of wave propagation in a weakly ionised plasma, as it
applies to the formation of multifluid, MHD shock waves. We model the plasma as
separate charged and neutral fluids which are coupled by ion-neutral friction.
At times much less than the ion-neutral drag time, the fluids are decoupled and
so evolve independently. At later times, the evolution is determined by the
large inertial mismatch between the charged and neutral particles. The neutral
flow continues to evolve independently; the charged flow is driven by and
slaved to the neutral flow by friction. We calculate this driven flow
analytically by considering the special but realistic case where the charged
fluid obeys linearized equations of motion. We carry out an extensive analysis
of linear, driven, MHD waves. The physics of driven MHD waves is embodied in
certain Green functions which describe wave propagation on short time scales,
ambipolar diffusion on long time scales, and transitional behavior at
intermediate times. By way of illustration, we give an approximate solution for
the formation of a multifluid shock during the collision of two identical
interstellar clouds. The collision produces forward- and reverse J shocks in
the neutral fluid and a transient in the charged fluid. The latter rapidly
evolves into a pair of magnetic precursors on the J shocks, wherein the ions
undergo force free motion and the magnetic field grows monotonically with time.
The flow appears to be self similar at the time when linear analysis ceases to
be valid.Comment: 18 pages including 24 figures, accepted by MNRA
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