46,129 research outputs found
A Lloyd-model generalization: Conductance fluctuations in one-dimensional disordered systems
We perform a detailed numerical study of the conductance through
one-dimensional (1D) tight-binding wires with on-site disorder. The random
configurations of the on-site energies of the tight-binding
Hamiltonian are characterized by long-tailed distributions: For large
, with . Our
model serves as a generalization of 1D Lloyd's model, which corresponds to
. First, we verify that the ensemble average is proportional to the length of the wire for all values of
, providing the localization length from . Then, we show that the probability distribution
function is fully determined by the exponent and
. In contrast to 1D wires with standard
white-noise disorder, our wire model exhibits bimodal distributions of the
conductance with peaks at and . In addition, we show that
is proportional to , for , with , in
agreement to previous studies.Comment: 5 pages, 5 figure
Wakefield-Induced Ionization injection in beam-driven plasma accelerators
We present a detailed analysis of the features and capabilities of
Wakefield-Induced Ionization (WII) injection in the blowout regime of beam
driven plasma accelerators. This mechanism exploits the electric wakefields to
ionize electrons from a dopant gas and trap them in a well-defined region of
the accelerating and focusing wake phase, leading to the formation of
high-quality witness-bunches [Martinez de la Ossa et al., Phys. Rev. Lett. 111,
245003 (2013)]. The electron-beam drivers must feature high-peak currents
() and a duration comparable to the plasma
wavelength to excite plasma waves in the blowout regime and enable WII
injection. In this regime, the disparity of the magnitude of the electric field
in the driver region and the electric field in the rear of the ion cavity
allows for the selective ionization and subsequent trapping from a narrow phase
interval. The witness bunches generated in this manner feature a short duration
and small values of the normalized transverse emittance (). In addition, we show that the amount of injected
charge can be adjusted by tuning the concentration of the dopant gas species,
which allows for controlled beam loading and leads to a reduction of the total
energy spread of the witness beams. Electron bunches, produced in this way,
fulfil the requirements to drive blowout regime plasma wakes at a higher
density and to trigger WII injection in a second stage. This suggests a
promising new concept of self-similar staging of WII injection in steps with
increasing plasma density, giving rise to the potential of producing electron
beams with unprecedented energy and brilliance from plasma-wakefield
accelerators
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
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