1,440 research outputs found
Nonlinear Breit-Wheeler pair creation with bremsstrahlung rays
Electron-positron pairs are produced through the Breit-Wheeler process when
energetic photons traverse electromagnetic fields of sufficient strength. Here
we consider a possible experimental geometry for observation of pair creation
in the highly nonlinear regime, in which bremsstrahlung of an ultrarelativistic
electron beam in a high- target is used to produce rays that
collide with a counterpropagating laser pulse. We show how the target thickness
may be chosen to optimize the yield of Breit-Wheeler positrons, and verify our
analytical predictions with simulations of the cascade in the material and in
the laser pulse. The electron beam energy and laser intensity required are well
within the capability of today's high-intensity laser facilities.Comment: 12 pages, 5 figure
On the contribution of exchange interactions to the Vlasov equation
Exchange effects play an important role in determining the equilibrium
properties of dense matter systems, as well as for magnetic phenomena. There
exists an extensive literature concerning, e.g., the effects of exchange
interactions on the equation of state of dense matter. Here, a generalization
of the Vlasov equation to include exchange effects is presented allowing for
electromagnetic mean fields, thus incorporating some of the dynamic effects due
to the exchange interactions. Treating the exchange term perturbatively, the
correction to classical Langmuir waves in plasmas is found, and the results are
compared with previous work. It is noted that the relative importance of
exchange effects scales similarly with density and temperature as particle
dispersive effects, but that the overall magnitude is sensitive to the details
of the specific problem. The implications of our results are discussed.Comment: 9 page
Photon acceleration in vacuum
A new process associated with the nonlinear optical properties of the
electromagnetic vacuum, as predicted by quantum electrodynamics, is described.
This can be called photon acceleration in vacuum, and corresponds to the
frequency shift that takes place when a given test photon interacts with an
intense beam of background radiation.Comment: 10 pages, 2 figures, version to appear in Phys. Lett.
Particle-in-Cell simulations of electron spin effects in plasmas
We have here developed a particle-in-cell code accounting for the magnetic
dipole force and for the magnetization currents associated with the electron
spin. The electrons is divided into spin-up and spin-down populations relative
to the magnetic field, where the magnetic dipole force acts in opposite
directions for the two species. To validate the code, we have studied the
wakefield generation by an electromagnetic pulse propagating parallel to an
external magnetic field. The properties of the generated wakefield is shown to
be in good quantitative agreement with previous theoretical results.
Generalizations of the code to account for more quantum effects is discussedComment: 5 pages, 6 figure
Circularly polarized modes in magnetized spin plasmas
The influence of the intrinsic spin of electrons on the propagation of
circularly polarized waves in a magnetized plasma is considered. New eigenmodes
are identified, one of which propagates below the electron cyclotron frequency,
one above the spin-precession frequency, and another close to the
spin-precession frequency.\ The latter corresponds to the spin modes in
ferromagnets under certain conditions. In the nonrelativistic motion of
electrons, the spin effects become noticeable even when the external magnetic
field is below the quantum critical\ magnetic field strength, i.e.,
and the electron density
satisfies m. The importance of electron
spin (paramagnetic) resonance (ESR) for plasma diagnostics is discussed.Comment: 10 page
Ferromagnetic behavior in magnetized plasmas
We consider a low-temperature plasma within a newly developed MHD Fluid
model. In addition to the standard terms, the electron spin, quantum particle
dispersion and degeneracy effects are included. It turns out that the electron
spin properties can give rise to Ferromagnetic behavior in certain regimes. If
additional conditions are fulfilled, a homogenous magnetized plasma can even be
unstable. This happen in the low-temperature high-density regime, when the
magnetic properties associated with the spin can overcome the stabilizing
effects of the thermal and Fermi pressure, to cause a Jeans like instability.Comment: 4 pages, 1 figur
On the ionospheric coupling of auroral electric fields
The quasi-static coupling of high-altitude potential structures and electric fields to the ionosphere is discussed with particular focus on the downward field-aligned current (FAC) region. Results are presented from a preliminary analysis of a selection of electric field events observed by Cluster above the acceleration region. The degree of coupling is here estimated as the ratio between the magnetic field-aligned potential drop, &Delta;&Phi;<sub>II</sub>, as inferred from the characteristic energy of upward ion (electron) beams for the upward (downward) current region and the high-altitude perpendicular (to <b>B</b>) potential, &Delta;&Phi;<sub>bot</sub>, as calculated by integrating the perpendicular electric field across the structure. For upward currents, the coupling can be expressed analytically, using the linear current-voltage relation, as outlined by Weimer et al. (1985). This gives a scale size dependent coupling where structures are coupled (decoupled) above (below) a critical scale size. For downward currents, the current-voltage relation is highly non-linear which complicates the understanding of how the coupling works. Results from this experimental study indicate that small-scale structures are decoupled, similar to small-scale structures in the upward current region. There are, however, exceptions to this rule as illustrated by Cluster results of small-scale intense electric fields, correlated with downward currents, indicating a perfect coupling between the ionosphere and Cluster altitude
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