806 research outputs found
Wake excited in plasma by an ultrarelativistic pointlike bunch
We study propagation of a relativistic electron bunch through a cold plasma assuming that the transverse and longitudinal dimensions of the bunch are much smaller than the plasma collisionless skin depth. Treating the bunch as a point charge and assuming that its charge is small, we derive a simplified system of equations for the plasma electrons and show that, through a simple rescaling of variables, the bunch charge can be eliminated from the equations. The equations demonstrate an ion cavity formed behind the driver. They are solved numerically and the scaling of the cavity parameters with the driver charge is obtained. A numerical solution for the case of a positively charged driver is also found.Department of Energy DE-AC03-76SF00515U.S. Department of Energy DEFG02-04ER54742 DE-SC0007889 DE-SC0010622Air Force Office of Scientific Research (AFOSR) FA9550-14-1-0045Physic
Modeling of long range frequency sweeping for energetic particle modes
Long range frequency sweeping events are simulated numerically within a one-dimensional, electrostatic bump-on-tail model with fast particle sources and collisions. The numerical solution accounts for fast particle trapping and detrapping in an evolving wave field with a fixed wavelength, and it includes three distinct collisions operators: Drag (dynamical friction on the background electrons), Krook-type collisions, and velocity space diffusion. The effects of particle trapping and diffusion on the evolution of holes and clumps are investigated, and the occurrence of non-monotonic (hooked) frequency sweeping and asymptotically steady holes is discussed. The presented solution constitutes a step towards predictive modeling of frequency sweeping events in more realistic geometries
Resonant Excitation of Shear Alfv\'en Perturbations by Trapped Energetic Ions in a Tokamak
A new analytic expression is derived for the resonant drive of high n
Alfvenic modes by particles accelerated to high energy by Ion Cyclotron
Resonance Heating. This derivation includes finite orbit effects, and the
formalism is completely non-perturbative. The high-n limit is used to calculate
the complex particle response integrals along the orbits explicitly. This new
theory is applied to downward sweeping Alfven Cascade quasimodes completing the
theory of these modes, and making testable predictions. These predictions are
found to be consistent with experiments carried out on the Joint European Torus
[P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)].Comment: 31 pages, 6 figure
Deep connection between f(R) gravity and the interacting dark sector model
We examine the conformal equivalence between the gravity and the
interacting dark sector model. We review the well-known result that the
conformal transformation physically corresponds to the mass dilation which
marks the strength of interaction between dark sectors. Instead of modeling
f(R) gravity in the Jordan frame, we construct the gravity in terms of
mass dilation function in the Einstein frame. We find that the condition to
keep gravity consistent with CMB observations ensures the energy flow
from dark energy to dark matter in the corresponding interacting model, which
meets the requirement to alleviate the coincidence problem in the Einstein
framework.Comment: 9 pages, 2 figures, revised version, accepted for publication in
Phys. Rev.
Alfvén Eigenmodes in shear reversed plasmas
Experiments on JT-60U and JET have shown that plasma configurations with shear reversal are prone to the excitation of unusual Alfvén eigenmodes by energetic particles. These modes emerge outside the TAE frequency gap, where one might expect them to be strongly damped. The modes often appear in bunches and they exhibit a quasi-periodic pattern of predominantly upward frequency sweeping (Alfvén Cascades) as the safety factor q changes in time. This work presents a theory that explains the key features of the observed unusual modes including their connection to TAE’s as well as the modifications of TAE’s themselves near the shear reversal point. The developed theory has been incorporated into a reduced numerical model and verified with full geometry codes. JET experimental data on Alfvén spectroscopy have been simulated to infer the mode numbers and the evolution of qmin in the discharge. This analysis confirms the values of q that characterize the internal transport barrier triggering in reversed shear plasmas
On analytical solutions of f(R) modified gravity theories in FLRW cosmologies
A novel analytical method for f(R) modified theories without matter in
Friedmann-Lemaitre-Robertson-Walker spacetimes is introduced. The equation of
motion for the scale factor in terms of cosmic time is reduced to the equation
for the evolution of the Ricci scalar R with the Hubble parameter H. The
solution of equation of motion for actions of the form of power law in Ricci
scalar R, is presented with a detailed elaboration of the action quadratic in
R. The reverse use of the introduced method is exemplified in finding
functional forms f(R) which lead to specified scale factor functions. The
analytical solutions are corroborated by numerical calculations with excellent
agreement. Possible further applications to the phases of inflationary
expansion and late-time acceleration as well as f(R) theories with radiation
are outlined.Comment: 16 pages, 6 figures. v2: minor changes, references added. v3: minor
changes, more references added. v4: version to appear in IJMPD. v5: DOI and
journal reference adde
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Spontaneous hole-clump pair creation in weakly unstable plasmas
A numerical simulation of a kinetic instability near threshold shows how a hole and clump spontaneously appear in the particle distribution function. The hole and clump support a pair of Bernstein, Greene, Kruskal (BGK) nonlinear waves that last much longer than the inverse linear damping rate while they are upshifting and downshifting in frequency. The frequency shifting allows a balance between the power nonlinearly extracted from the resonant particles and the power dissipated into the background plasma. These waves eventually decay due to phase space gradient smoothing caused by collisionality
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