4,937 research outputs found
Quasimonoenergetic electron beams produced by colliding cross-polarized laser pulses in underdense plasmas
The interaction of two laser pulses in an underdense plasma has proven to be
able to inject electrons in plasma waves, thus providing a stable and tunable
source of electrons. Whereas previous works focused on the "beatwave" injection
scheme in which two lasers with the same polarization collide in a plasma, this
present letter studies the effect of polarization and more specifically the
interaction of two colliding cross-polarized laser pulses. It is shown both
theoretically and experimentally that electrons can also be pre-accelerated and
injected by the stochastic heating occurring at the collision of two
cross-polarized lasers and thus, a new regime of optical injection is
demonstrated. It is found that injection with cross-polarized lasers occurs at
higher laser intensities.Comment: 4 pages, 4 figure
Computationally efficient methods for modelling laser wakefield acceleration in the blowout regime
Electron self-injection and acceleration until dephasing in the blowout
regime is studied for a set of initial conditions typical of recent experiments
with 100 terawatt-class lasers. Two different approaches to computationally
efficient, fully explicit, three-dimensional particle-in-cell modelling are
examined. First, the Cartesian code VORPAL using a perfect-dispersion
electromagnetic solver precisely describes the laser pulse and bubble dynamics,
taking advantage of coarser resolution in the propagation direction, with a
proportionally larger time step. Using third-order splines for macroparticles
helps suppress the sampling noise while keeping the usage of computational
resources modest. The second way to reduce the simulation load is using
reduced-geometry codes. In our case, the quasi-cylindrical code CALDER-CIRC
uses decomposition of fields and currents into a set of poloidal modes, while
the macroparticles move in the Cartesian 3D space. Cylindrical symmetry of the
interaction allows using just two modes, reducing the computational load to
roughly that of a planar Cartesian simulation while preserving the 3D nature of
the interaction. This significant economy of resources allows using fine
resolution in the direction of propagation and a small time step, making
numerical dispersion vanishingly small, together with a large number of
particles per cell, enabling good particle statistics. Quantitative agreement
of the two simulations indicates that they are free of numerical artefacts.
Both approaches thus retrieve physically correct evolution of the plasma
bubble, recovering the intrinsic connection of electron self-injection to the
nonlinear optical evolution of the driver
Residual amplitude modulation in interferometric gravitational wave detectors
The effects of residual amplitude modulation (RAM) in laser interferometers using heterodyne sensing can be substantial and difficult to mitigate. In this work, we analyze the effects of RAM on a complex laser interferometer used for gravitational wave detection. The RAM introduces unwanted offsets in the cavity length signals and thereby shifts the operating point of the optical cavities from the nominal point via feedback control. This shift causes variations in the sensing matrix, and leads to degradation in the performance of the precision noise subtraction scheme of the multiple-degree-of-freedom control system. In addition, such detuned optical cavities produce an optomechanical spring, which also perturbs the sensing matrix. We use our simulations to derive requirements on RAM for the Advanced LIGO (aLIGO) detectors, and show that the RAM expected in aLIGO will not limit its sensitivity
Observation of beam loading in a laser-plasma accelerator
Beam loading is the phenomenon which limits the charge and the beam quality
in plasma based accelerators. An experimental study conducted with a
laser-plasma accelerator is presented. Beam loading manifests itself through
the decrease of the beam energy, the reduction of dark current and the increase
of the energy spread for large beam charge. 3D PIC simulations are compared to
the experimental results and confirm the effects of beam loading. It is found
that, in our experimental conditions, the trapped electron beams generate
decelerating fields on the order of 1 GV/m/pC and that beam loading effects are
optimized for trapped charges of about 20 pC.Comment: 10 pages,4 figure
Complementary use of TEM and APT for the investigation of steels nanostructured by severe plastic deformation
The properties of bulk nanostructured materials are often controlled by
atomic scale features like segregation along defects or composition gradients.
Here we discuss about the complimentary use of TEM and APT to obtain a full
description of nanostructures. The advantages and limitations of both
techniques are highlighted on the basis of experimental data collected in
severely deformed steels with a special emphasis on carbon spatial
distribution
The Frenchness of Marcel Lefebvre and the Society of St Pius X:a new reading
The case of Marcel Lefebvre and the Society of St Pius X (SSPX) deserves fresh perspectives. The current historiography is too franco-centric, focused on selective aspects of Lefebvre’s biography and the actions of isolated individuals, rather than with the life of the SSPX itself. After evaluating the current state of the historiography, this article proposes a new analysis of the SSPX’s political discourses in France and internationally and undertakes to reframe the relationship between Lefebvre’s life and his congregation by re-examining his African missionary experiences. Such new perspectives will be helpful as the SSPX moves towards regularisation under the pontificate of Pope Francis
Recent high-magnetic-field studies of unusual groundstates in quasi-two-dimensional crystalline organic metals and superconductors
After a brief introduction to crystalline organic superconductors and metals,
we shall describe two recently-observed exotic phases that occur only in high
magnetic fields. The first involves measurements of the non-linear electrical
resistance of single crystals of the charge-density-wave (CDW) system
(Per)Au(mnt) in static magnetic fields of up to 45 T and temperatures
as low as 25 mK. The presence of a fully gapped CDW state with typical CDW
electrodynamics at fields higher that the Pauli paramagnetic limit of 34 T
suggests the existence of a modulated CDW phase analogous to the
Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall
potential of single crystals of -(BEDT-TTF)KHg(SCN), made using
a variant of the Corbino geometry in quasistatic magnetic fields, show
persistent current effects that are similar to those observed in conventional
superconductors. The longevity of the currents, large Hall angle, flux
quantization and confinement of the reactive component of the Hall potential to
the edge of the sample are all consistent with the realization of a new state
of matter in CDW systems with significant orbital quantization effects in
strong magnetic fields.Comment: SNS 2004 Conference presentatio
Angular control of optical cavities in a radiation-pressure-dominated regime: the Enhanced LIGO case
We describe the angular sensing and control (ASC) of 4 km detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO). Enhanced LIGO, the culmination of the first generation LIGO detectors, operated between 2009 and 2010 with about 40 kW of laser power in the arm cavities. In this regime, radiation-pressure effects are significant and induce instabilities in the angular opto-mechanical transfer functions. Here we present and motivate the ASC design in this extreme case and present the results of its implementation in Enhanced LIGO. Highlights of the ASC performance are successful control of opto-mechanical torsional modes, relative mirror motions of ≤ 1×10^−7 rad rms, and limited impact on in-band strain sensitivity
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
Planetary detection limits taking into account stellar noise. I. Observational strategies to reduce stellar oscillation and granulation effects
The radial velocity signature of stellar noise is small, around the
meter-per-second, but already too much for the detection of Earth mass planets
in habitable zones. In this paper, we address the important role played by
observational strategies in averaging out the radial velocity signature of
stellar noise. We also derive the planetary mass detection limits expected in
presence of stellar noise. We start with HARPS asteroseismology measurements
for 4 stars (beta Hyi, alpha Cen A, mu Ara and tau Ceti) available in the ESO
archive plus very precise measurements of alpha Cen B. This sample covers
different spectral types, from G2 to K1 and different evolutionary stage, from
subgiant to dwarf stars. Since the span of our data ranges between 5 to 8 days,
we will have access to oscillation modes and granulation phenomena, without
important contribution of activity noise which is present at larger time
scales. For those 5 stars, we generate synthetic radial velocity measurements
after fitting corresponding models of stellar noise in Fourier space. These
measurements allows us to study the radial velocity variation due to stellar
noise for different observational strategies as well as the corresponding
planetary mass detection limits. Applying 3 measurements per night of 10
minutes exposure each, 2 hours apart, seems to average out most efficiently the
stellar noise considered. For quiet K1V stars as alpha Cen B, such a strategy
allows us to detect planets of ~3 times the mass of Earth with an orbital
period of 200 days, corresponding to the habitable zone of the star. Since
activity is not yet included in our simulation, these detection limits
correspond to a case, which exist, where the host star has few magnetic
features. In this case stellar noise is dominated by oscillation modes and
granulation phenomena.Comment: 12 pages, 6 figures, Accepted for publication in A&
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