arXiv
(422.153 recursos)
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Mostrando recursos 1 - 20 de 38
1.
Non-Gaussian statistics in space plasma turbulence, fractal properties
and pitfalls - de Wit, T. Dudok; Krasnosel'skikh, V. V.
Magnetic field fluctuations in the vicinity of the Earth's bow shock have
been investigated with the aim to characterize the intermittent behaviour of
strong plasma turbulence. The observed small-scale intermittency may be the
signature of a multifractal process but a deeper inspection reveals caveats in
such an interpretation. Several effects, including the anisotropy of the
wavefield, the violation of the Taylor hypothesis and the occasional occurrence
of coherent wave packets, strongly affect the higher order statistical
properties. After correcting these effects, a more Gaussian and scale-invariant
wavefield is recovered.
2.
Adaptive Mesh Refinement for Singular Current Sheets in Incompressible
Magnetohydrodynamic Flows - Friedel, Holger; Grauer, Rainer; Marliani, Christiane
The formation of current sheets in ideal incompressible magnetohydrodynamic
flows in two dimensions is studied numerically using the technique of adaptive
mesh refinement. The growth of current density is in agreement with simple
scaling assumptions. As expected, adaptive mesh refinement shows to be very
efficient for studying singular structures compared to non-adaptive treatments.
3.
Beat-wave generation of plasmons in semiconductor plasmas - Berezhiani, V. I.; Mahajan, S. M.
It is shown that in semiconductor plasmas, it is possible to generate large
amplitude plasma waves by the beating of two laser beams with frequency
difference close to the plasma frequency. For narrow gap semiconductors (for
example n-type InSb), the system can simulate the physics underlying beat wave
generation in relativistic gaseous plasmas.
4.
Duality Invariant Magnetohydrodynamics And Dyons - Coceal, Omduth; Sabra, Wafic A.; Thomas, Steven
The theory of magnetohydrodynamics is extended to the cases of a plasma of
separate magnetic and electric charges, as well as to a plasma of dyons
respectively. In both these cases the system possesses electric-magnetic
duality symmetry. In the former case we find that because of the existence of
two independent generalized Ohm's law equations, the limit of infinite electric
and magnetic conductivity results in the vanishing of both electric and
magnetic fields, as well as the corresponding currents. In the dyonic case, we
find that the resulting duality-invariant system of equations are equivalent to
those of ordinary MHD, after suitable field redefinitions.
5.
Nonlinear saturation of electrostatic waves: mobile ions modify trapping
scaling - Crawford, John David; Jayaraman, Anandhan
The amplitude equation for an unstable electrostatic wave in a multi-species
Vlasov plasma has been derived. The dynamics of the mode amplitude $\rho(t)$ is
studied using an expansion in $\rho$; in particular, in the limit
$\gamma\rightarrow0^+$, the singularities in the expansion coefficients are
analyzed to predict the asymptotic dependence of the electric field on the
linear growth rate $\gamma$. Generically $|E_k|\sim \gamma^{5/2}$, as
$\gamma\rightarrow0^+$, but in the limit of infinite ion mass or for
instabilities in reflection-symmetric systems due to real eigenvalues the more
familiar trapping scaling $|E_k|\sim \gamma^{2}$ is predicted.
6.
Overcoming the Back Reaction on Turbulent Motions in the Presence of
Magnetic Fields - Blackman, Eric G.
Standard magnetohydrodynamic theories, such as the mean field dynamo theory,
have been criticized when the back reaction of the magnetic field on turbulent
motions is neglected. For the dynamo, this back reaction has been argued to
suppress the turbulent motions required for optimal mean field production. Here
it is suggested that if the magnetic field is spatially intermittent, for
example residing in flux tubes, the back reaction on turbulent flows may be
significantly reduced.
7.
Conductivity of a Relativistic Plasma - Braams, Bastiaan J.; Karney, Charles F. F.
The collision operator for a relativistic plasma is reformulated in terms of
an expansion in spherical harmonics. In this formulation the collision operator
is expressed in terms of five scalar potentials which are given by
one-dimensional integrals over the distribution function. This formulation is
used to calculate the electrical conductivity of a uniform electron-ion plasma
with infinitely massive ions.
8.
Stellarator News for January, 1995 - Rome, James A.; Editor
The Stellarator News is a bi-monthly newsletter covering the international
stellarator community.
9.
Excitation of Large-ktheta Ion-Berstein Waves in Tokamaks - Valeo, E. J.; Fisch, N. J.
The mode-converted ion-Bernstein wave excited in tokamaks is shown to exhibit
certain very interesting behavior, including the attainment of very small
poloidal phase velocities, the reversal of poloidal direction, and up-down
asymmetries in propagation and damping. Because of these effects, this wave
holds promise for channeling {$\alpha$-particle}\ power to ions, something that
would make a tokamak fusion reactor far more attractive than presently
envisioned.
10.
Stellarator News, Issue 38, March 1995 - Rome, James A.; Editor
Stellarator News, an international journal of the stellarator community, is
Published by Fusion Energy Division, Oak Ridge National Laboratory, James A.
Rome, Editor
In the March 1995 issue . . .
**** Exerpts from the U.S. Congress Office of Technology Assment report on
TPX and Alternate Concepts.
**** Edge transport and turbulence studies on U-3M
The turbulent-driven particle flow is shown to be comparable with the
equilibrium flow at the boundary of the configuration under Alfven-heating
conditions in the U-3M torsatron.
**** Topological aspects of island divertor studies on W7-AS
The structure of the edge plasma in W7-AS, observed with probes, television
camera, and H-alpha light...
11.
Narrowing of Bennett Hole in Collisional Plasma - Podivilov, E. V.; Shapiro, D. A.; Stepanov, M. G.
The profile of a Bennett hole induced by laser field in ionic distribution in
collisional plasma is calculated. Influence of Chandrasekhar's dependence of
coefficients of velocity space transport on the profile is included into the
calculation for the first time. It is found that the hole narrows down as the
field detuning frequency increases. Physical cause of the effect is the falling
dependence of Coulomb collision frequency on the ionic velocity. Estimations
show that the effect is quite observable under conditions of high-current
gas-discharge plasma.
12.
Stellarator News, Issue 39, May 1995 - Rome, James A.; Editor
Special Issue: New Stellarators
New stellarators are being constructed throughout the world as part of a
well-coordinated international program. This issue concentrates on the
engineering and construction details of
Large Helical Device (LHD), Toki, Japan
Wendelstein 7-X (W7-X), Greifswald, Germany
Helias Stellarator Experiment (HSX), Madison Wisconsin, USA
TJ-II, Madrid, Spain
13.
Radiation Rates for Low Z Impurities in Edge Plasmas - Clark, R.; Abdallah, J.; Post, D.
The role of impurity radiation in the reduction of heat loads on divertor
plates in present experiments such as DIII-D, JET, JT-60, ASDEX, and Alcator
C-Mod, and in planned experiments such as ITER and TPX places a new degree of
importance on the accuracy of impurity radiation emission rates for electron
temperatures below 250 eV for ITER and below 150 eV for present experiments. We
have calculated the radiated power loss using a collisional radiative model for
Be, B, C, Ne and Ar using a multiple configuration interaction model which
includes density dependent effects, as well as a very detailed treatment of the
energy levels and meta-stable levels....
14.
Analytic Criteria for Power Exhaust in Divertors due to Impurity
Radiation - Post, D.; Putvinskaya, N.; Perkins, F. W.; Nevins, W.
Present divertor concepts for next step experiments such ITER and TPX rely
upon impurity and hydrogen radiation to transfer the energy from the edge
plasma to the main chamber and divertor chamber walls. The efficiency of these
processes depends strongly on the heat flux, the impurity species, and the
connection length. Using a database for impurity radiation rates constructed
from the ADPAK code package, we have developed criteria for the required
impurity fraction, impurity species, connection length and electron temperature
and density at the mid-plane. Consistent with previous work, we find that the
impurity radiation from coronal equilibrium rates is, in general, not adequate
to exhaust the highest expected...
15.
A Review of Recent Developments in Atomic Processes for Divertors and
Edge Plasmas - Post, D. E.
The most promising concepts for power and particle control in tokamaks and
other fusion experiments rely upon atomic processes to transfer the power and
momentum from the edge plasma to the plasma chamber walls. This places a new
emphasis on processes at low temperatures (1-200 eV) and high densities
(10^20-10^22 m^-3). The most important atomic processes are impurity and
hydrogen radiation, ionization, excitation, recombination, charge exchange,
radiation transport, molecular collisions, and elastic scattering of atoms,
molecules and ions. Important new developments have occurred in each of these
areas. The best available data for these processes and an assessment of their
role in plasma wall interactions are summarized, and the...
16.
Calculations of Energy Losses due to Atomic Processes in Tokamaks with
Applications to the ITER Divertor - Post, D.; Abdallah, J.; Clark, R. E. H.; Putvinskaya, N.
Reduction of the peak heat loads on the plasma facing components is essential
for the success of the next generation of high fusion power tokamaks such as
the International Thermonuclear Experimental Reactor (ITER) 1 . Many present
concepts for accomplishing this involve the use of atomic processes to transfer
the heat from the plasma to the main chamber and divertor chamber walls and
much of the experimental and theoretical physics research in the fusion program
is directed toward this issue. The results of these experiments and
calculations are the result of a complex interplay of many processes. In order
to identify the key features of these experiments and...
17.
Studies of Plasma Detachment Using a One Dimensional Model for Divertor
Operation - Vesey, R. A.; Post, D. E.; Bateman, G.
To characterize the conditions required to reach advanced divertor regimes, a
one-dimensional computational model has been developed based on a coordinate
transformation to incorporate two-dimensional effects. This model includes
transport of ions, two species each of atoms and molecules, momentum, and ion
and electron energy both within and across the flux surfaces. Impurity
radiation is calculated using a coronal equilibrium model which includes the
effects of charge-exchange recombination. Numerical results indicate that
impurity radiation acts to facilitate plasma detachment and enhances the power
lost from the divertor channel in escaping neutral atoms by cooling the
electrons and suppressing ionization. As divertor particle densities increase,
cold and thermal molecules become increasingly...
18.
Detailed Radiative Transport Modeling of a Radiative Divertor - Wan, A. S.; Dalhed, H. E.; Scott, H. A.; Post, D. E.; Rognlien, T. D.
An effective radiative divertor maximizes the utilization of atomic processes
to spread out the energy deposition to the divertor chamber walls and to reduce
the peak heat flux. Because the mixture of neutral atoms and ions in the
divertor can be optically thick to a portion of radiated power, it is necessary
to accurately model the magnitude and distribution of line radiation in this
complex region. To assess their importance we calculate the effects of
radiation transport using CRETIN, a multi-dimensional, non-local thermodynamic
equilibrium simulation code that includes the atomic kinetics and radiative
transport processes necessary to model the complex environment of a radiative
divertor. We also include neutral...
19.
A New Look at the Landau's Theory of Spreading and Damping of Waves in
Collisionless Plasmas - Soshnikov, V. N.
The theory of plasma waves and Landau damping in Maxwellian plasmas, Landau's
``rule of pass around poles'' include doubtful statements, particularly related
to an artificial ``constructing'' of the dispersion equation, what should allow
the possibility of its solution otherwise not existing at all, and the
possibility of analytical continuations of corresponding very specific ruptured
functions in the one-dimensional Laplace transformation, used by Landau, what
is the base of his theory.
We represent, as an accessible variant, a more general alternative theory
based on a two-dimensional Laplace transformation, leading to an asymptotical
in time and space solution as a complicated superposition of coupled damping
and {\em non-damping \/} plane waves...
20.
On the conical refraction of hydromagnetic waves in plasma with
anisotropic thermal pressure - Tsiklauri, David
A phenomenon analogous to the conical refraction widely known in the
crystalooptics and crystaloacoustics is discovered for the
magnetohydrodynamical waves in the collisionless plasma with anisotropic
thermal pressure. Angle of the conical refraction is calculated for the medium
under study which is predicted to be $18^{\circ}26^{\prime}$. Possible
experimental corroborating of the discovered phenomenon is discussed.
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