65 research outputs found
Potential to measure quantum effects in recent all-optical radiation reaction experiments
The construction of 10 PW class laser facilities with unprecedented intensities has emphasized the need for a thorough understanding of the radiation reaction process. We describe simulations for a recent all-optical colliding pulse experiment, where a GeV scale electron bunch produced by a laser wakefield accelerator interacted with a counter-propagating laser pulse. In the rest frame of the electron bunch, the electric field of the laser pulse is increased by several orders of magnitude, approaching the Schwinger field and leading to substantial variation from the classical Landau-Lifshitz model. Our simulations show how the final electron and photon spectra may allow us to differentiate between stochastic and semi-classical models of radiation reaction, even when there is significant shot-to-shot variation in the experimental parameters. In particular, constraints are placed on the maximum energy spread and shot-to-shot variation permissible if a stochastic model is to be proven with confidence
Wavelength-selected Neutron Pulses Formed by a Spatial Magnetic Neutron Spin Resonator
AbstractWe present a novel type of spatial magnetic neutron spin resonator whose time and wavelength resolution can be de- coupled from each other by means of a travelling wave mode of operation. Combined with a pair of highly efficient polarisers such a device could act simultaneously as monochromator and chopper, able to produce short neutron pulses, whose wavelength, spectral width and duration could be varied almost instantaneously by purely electronic means with- out any mechanical modification of the experimental setup. To demonstrate the practical feasibility of this technique we have designed and built a first prototype resonator consisting of ten individually switchable modules which allows to produce neutron pulses in the microsecond regime. It was installed at a polarised 2.6Å neutron beamline at the 250kW TRIGA research reactor of the Vienna University of Technology where it could deliver pulses of 55μs duration, which is about three times less than the passage time of the neutrons through the resonator itself. In order to further improve the achievable wavelength resolution to about 3% a second prototype resonator, consisting of 48 individual modules with optimised field homogeneity and enlarged beam cross-section of 6 × 6cm2 was developed. We present the results of first measurements which demonstrate the successful operation of this device
A new locking-free formulation for planar, shear deformable, linear and quadratic beam finite elements based on the absolute nodal coordinate formulation
A 3D absolute nodal coordinate finite element model to compute the initial configuration of a railway catenary
In this paper we propose a method of finding the initial equilibrium configuration of cable structures discretized by finite elements applied to the shape-finding of the railway overhead system. Absolute
nodal coordinate formulation finite elements, which allow for axial and bending deformation, are used
for the contact and messenger wires. The other parts of the overhead system are discretized with
non-linear bars or equivalent springs. The proposed method considers the constraints introduced during
the assembly of the catenary, such as the position of droppers, cable tension, and height of the contact
wire. The formulation is general and can be applied to different catenary configurations or transitions
both in 2D and 3D with straight or curved track paths. A comparison of the results obtained for reference
catenaries in the bibliography is also included.The authors wish to thank Generatitat Valenciana for the financial support received in the framework of the Programme PROME-TEO 2012/023.Tur Valiente, M.; García, E.; Baeza González, LM.; Fuenmayor Fernández, FJ. (2014). A 3D absolute nodal coordinate finite element model to compute the initial configuration of a railway catenary. Engineering Structures. 71:234-243. https://doi.org/10.1016/j.engstruct.2014.04.015S2342437
Optimal parameters for radiation reaction experiments
As new laser facilities are developed with intensities on the scale of 10^22
- 10^24 W cm^-2 , it becomes ever more important to understand the effect of
strong field quantum electrodynamics processes, such as quantum radiation
reaction, which will play a dominant role in laser-plasma interactions at these
intensities. Recent all-optical experiments, where GeV electrons from a laser
wakefield accelerator encountered a counter-propagating laser pulse with a_0 >
10, have produced evidence of radiation reaction, but have not conclusively
identified quantum effects nor their most suitable theoretical description.
Here we show the number of collisions and the conditions required to accomplish
this, based on a simulation campaign of radiation reaction experiments under
realistic conditions. We conclude that while the critical energy of the photon
spectrum distinguishes classical and quantum-corrected models, a better means
of distinguishing the stochastic and deterministic quantum models is the change
in the electron energy spread. This is robust against shot-to-shot fluctuations
and the necessary laser intensity and electron beam energies are already
available. For example, we show that so long as the electron energy spread is
below 25%, collisions at a_0 = 10 with electron energies of 500 MeV could
differentiate between different quantum models in under 30 shots, even with
shot to shot variations at the 50% level.Comment: 12 pages, 7 figure
A comparison of Finite Elements for Nonlinear Beams: The absolute nodal coordinate and geometrically exact formulations
Two of the most popular finite element formulations for solving nonlinear beams are the absolute nodal coordinate and the geometrically exact approaches. Both can be applied to problems with very large deformations and strains, but they differ substantially at the continuous and the discrete levels. In addition, implementation and run-time computational costs also vary significantly. In the current work, we summarize the main features of the two formulations, highlighting their differences and similarities, and perform numerical benchmarks to assess their accuracy and robustness. The article concludes with recommendations for the choice of one formulation over the other
Generation of meter-scale hydrogen plasmas and efficient, pump-depletion-limited wakefield excitation using 10 GeV electron bunches
High repetition rates and efficient energy transfer to the accelerating beam
are important for a future linear collider based on the beam-driven plasma
wakefield acceleration scheme (PWFA-LC). This paper reports the first results
from the Plasma Wakefield Acceleration Collaboration (E300) that are beginning
to address both of these issues using the recently commissioned FACET-II
facility at SLAC. We have generated meter-scale hydrogen plasmas using
time-structured 10 GeV electron bunches from FACET-II, which hold the promise
of dramatically increasing the repetition rate of PWFA by rapidly replenishing
the gas between each shot compared to the hitherto used lithium plasmas that
operate at 1-10 Hz. Furthermore, we have excited wakes in such plasmas that are
suitable for high gradient particle acceleration with high drive-bunch to wake
energy transfer efficiency -- a first step in achieving a high overall energy
transfer efficiency. We have done this by using time-structured electron drive
bunches that typically have one or more ultra-high current (>30 kA) femtosecond
spike(s) superimposed on a longer (~0.4 ps) lower current (<10 kA) bunch
structure. The first spike effectively field-ionizes the gas and produces a
meter-scale (30-160 cm) plasma, whereas the subsequent beam charge creates a
wake. The length and amplitude of the wake depends on the longitudinal current
profile of the bunch and plasma density. We find that the onset of pump
depletion, when some of the drive beam electrons are nearly fully depleted of
their energy, occurs for hydrogen pressure >1.5 Torr. We also show that some
electrons in the rear of the bunch can gain several GeV energies from the wake.
These results are reproduced by particle-in-cell simulations using the QPAD
code. At a pressure of ~2 Torr, simulations results and experimental data show
that the beam transfers about 60% of its energy to the wake
Wakefield Generation in Hydrogen and Lithium Plasmas at FACET-II: Diagnostics and First Beam-Plasma Interaction Results
Plasma Wakefield Acceleration (PWFA) provides ultrahigh acceleration
gradients of 10s of GeV/m, providing a novel path towards efficient, compact,
TeV-scale linear colliders and high brightness free electron lasers. Critical
to the success of these applications is demonstrating simultaneously high
gradient acceleration, high energy transfer efficiency, and preservation of
emittance, charge, and energy spread. Experiments at the FACET-II National User
Facility at SLAC National Accelerator Laboratory aim to achieve all of these
milestones in a single stage plasma wakefield accelerator, providing a 10 GeV
energy gain in a <1 m plasma with high energy transfer efficiency. Such a
demonstration depends critically on diagnostics able to measure emittance with
mm-mrad accuracy, energy spectra to determine both %-level energy spread and
broadband energy gain and loss, incoming longitudinal phase space, and matching
dynamics. This paper discusses the experimental setup at FACET-II, including
the incoming beam parameters from the FACET-II linac, plasma sources, and
diagnostics developed to meet this challenge. Initial progress on the
generation of beam ionized wakes in meter-scale hydrogen gas is discussed, as
well as commissioning of the plasma sources and diagnostics
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