45 research outputs found
DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters. Part 2: Model validation with experiments
Here, we present validation tests of the fluid dynamic solver presented in von Boetticher et al. (2016), simulating both laboratory-scale and large-scale debris-flow experiments. The new solver combines a Coulomb viscoplastic rheological model with a Herschel-Bulkley model based on material properties and rheological characteristics of the analyzed debris flow. For the selected experiments in this study, all necessary material properties were known - the content of sand, clay (including its mineral composition) and gravel as well as the water content and the angle of repose of the gravel. Given these properties, two model parameters are sufficient for calibration, and a range of experiments with different material compositions can be reproduced by the model without recalibration. One calibration parameter, the Herschel-Bulkley exponent, was kept constant for all simulations. The model validation focuses on different case studies illustrating the sensitivity of debris flows to water and clay content, channel curvature, channel roughness and the angle of repose. We characterize the accuracy of the model using experimental observations of flow head positions, front velocities, run-out patterns and basal pressures.Peer ReviewedPostprint (published version
DebrisInterMixing-2.3: a Finite Volume solver for three dimensional debris flow simulations based on a single calibration parameter – Part 2: Model validation
Here we present the validation of the fluid dynamic solver presented in part one of this work (von Boetticher et al., 2015), simulating laboratory-scale and large-scale debris-flow experiments. The material properties of the experiments, including water content, sand content, clay content and its mineral composition, and gravel content and its friction angle, were known. We show that given these measured properties, a single free model parameter is sufficient for calibration, and a range of experiments with different material compositions can be reproduced by the model without recalibration. The model validation focuses on different case studies illustrating the sensitivity of debris flows to water and clay content, channel curvature, channel roughness and the angle of repose of the gravel. We characterize the accuracy of the model using experimental observations of flow head positions, front velocities, run-out patterns and basal pressures.Peer ReviewedPostprint (published version
Measurement of the decay of laser-driven linear plasma wakefields.
We present measurements of the temporal decay rate of one-dimensional (1D), linear Langmuir waves excited by an ultrashort laser pulse. Langmuir waves with relative amplitudes of approximately 6% were driven by 1.7J, 50fs laser pulses in hydrogen and deuterium plasmas of density n_{e0}=8.4×10^{17}cm^{-3}. The wakefield lifetimes were measured to be τ_{wf}^{H_{2}}=(9±2) ps and τ_{wf}^{D_{2}}=(16±8) ps, respectively, for hydrogen and deuterium. The experimental results were found to be in good agreement with 2D particle-in-cell simulations. In addition to being of fundamental interest, these results are particularly relevant to the development of laser wakefield accelerators and wakefield acceleration schemes using multiple pulses, such as multipulse laser wakefield accelerators
Low-density hydrodynamic optical-field-ionized plasma channels generated with an axicon lens
We demonstrate optical guiding of high-intensity laser pulses in long, low
density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon
lens is used to generate HOFI plasma channels with on-axis electron densities
as low as and matched spot
sizes in the range . Control of these channel parameters via adjustment of the initial
cell pressure and the delay after the arrival of the channel-forming pulse is
demonstrated. For laser pulses with a peak axial intensity of , highly reproducible, high-quality guiding over
more than 14 Rayleigh ranges is achieved at a pulse repetition rate of 5 Hz,
limited by the available channel-forming laser and vacuum pumping system.
Plasma channels of this type would seem to be well suited to multi-GeV laser
wakefield accelerators operating in the quasi-linear regime
Meter-Scale, Conditioned Hydrodynamic Optical-Field-Ionized Plasma Channels
We demonstrate through experiments and numerical simulations that low-density, low-loss, meter-scale plasma channels can be generated by employing a conditioning laser pulse to ionize the neutral gas collar surrounding a hydrodynamic optical-field-ionized (HOFI) plasma channel. We use particle-in-cell simulations to show that the leading edge of the conditioning pulse ionizes the neutral gas collar to generate a deep, low-loss plasma channel which guides the bulk of the conditioning pulse itself as well as any subsequently injected pulses. In proof-of-principle experiments we generate conditioned HOFI (CHOFI) waveguides with axial electron densities of , and a matched spot size of . The power attenuation length of these CHOFI channels is , more than two orders of magnitude longer than achieved by HOFI channels. Hydrodynamic and particle-in-cell simulations demonstrate that meter-scale CHOFI waveguides with attenuation lengths exceeding 1 m could be generated with a total laser pulse energy of only J per meter of channel. The properties of CHOFI channels are ideally suited to many applications in high-intensity light-matter interactions, including multi-GeV plasma accelerator stages operating at high pulse repetition rates
Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements
Topics in laboratory plasma instability and turbulence
The first part of this thesis investigates the growth of ion density perturbations in large-amplitude laser-driven wakefields via two-dimensional particle-in-cell simulations. Growth rates and wave numbers of the perturbations are found to be consistent with a longitudinal strong-field modulational instability. We examine the transverse dependence of the instability for a Gaussian wakefield envelope and show that growth rates and wave numbers can be maximised off- axis. On-axis growth rates are found to decrease with increasing ion mass or electron temperature. These results are in close agreement with theoretical predictions obtained by solving the dispersion relation for a long-wavelength electrostatic oscillation in resonance with the plasma frequency and energy density that is large compared to the plasma thermal energy density. The implications for wakefield accelerators – in particular multi-pulse schemes – and methods to mitigate the effects of ion dynamics are discussed.
In the second part of the thesis, we develop a linearised Fokker-Planck collision model for gyrokinetic simulations that satisfies conservation laws and is accurate at arbitrary collisionality. The differential test-particle component of the operator is exact; the implementation of the integrodifferential field-particle component uses a spherical harmonic expansion and a modified Laguerre polynomial expansion introduced by Hirshman and Sigmar [S. P. Hirshman, D. J. Sigmar, Phys. Fluids 19, 1532 (1976)]. The numerical methods of the implementation in the δf-gyrokinetic code stella [M. Barnes, F. I. Parra, M. Landreman, Journal of Comp. Phys. 391, 365-380 (2019)] are discussed, and conservation properties of the operator are verified. The accuracy of the collision model is demonstrated by computing transport coefficients for the classical Spitzer problem and by performing benchmarks against the collision model of the gyrokinetic solver GS2.</p
Modulational instability in large-amplitude linear laser wakefields
We investigate the growth of ion density perturbations in large-amplitude linear laser wakefields
via two-dimensional particle-in-cell simulations. Growth rates and wave numbers are found to
be consistent with a longitudinal strong-field modulational instability (SFMI). We examine the
transverse dependence of the instability for a Gaussian wakefield envelope and show that growth
rates and wavenumbers can be maximised off-axis. On-axis growth rates are found to decrease with
increasing ion mass or electron temperature. These results are in close agreement with the dispersion
relation of a Langmuir wave with energy density that is large compared to the plasma thermal energy
density. The implications for wakefield accelerators, in particular multi-pulse schemes, are discussed
DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters – Part 2: Model validation with experiments
Here, we present validation tests of the fluid dynamic solver presented in
von Boetticher et al. (2016), simulating both laboratory-scale and large-scale
debris-flow experiments. The new solver combines a Coulomb viscoplastic
rheological model with a Herschel–Bulkley model based on material properties
and rheological characteristics of the analyzed debris flow. For the selected
experiments in this study, all necessary material properties were known –
the content of sand, clay (including its mineral composition) and gravel as
well as the water content and the angle of repose of the gravel. Given these
properties, two model parameters are sufficient for calibration, and a range
of experiments with different material compositions can be reproduced by the
model without recalibration. One calibration parameter, the Herschel–Bulkley
exponent, was kept constant for all simulations. The model validation focuses
on different case studies illustrating the sensitivity of debris flows to
water and clay content, channel curvature, channel roughness and the angle of
repose. We characterize the accuracy of the model using experimental
observations of flow head positions, front velocities, run-out patterns and
basal pressures
DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters – Part 1: Model description
Here, we present a three-dimensional fluid dynamic solver that simulates
debris flows as a mixture of two fluids (a Coulomb viscoplastic model of the
gravel mixed with a Herschel–Bulkley representation of the fine material
suspension) in combination with an additional unmixed phase representing the
air and the free surface. We link all rheological parameters to the material
composition, i.e., to water content, clay content, and mineral composition,
content of sand and gravel, and the gravel's friction angle; the user must
specify only two free model parameters. The volume-of-fluid (VoF) approach is
used to combine the mixed phase and the air phase into a single cell-averaged
Navier–Stokes equation for incompressible flow, based on code adapted
from standard solvers of the open-source CFD software OpenFOAM. This
effectively single-phase mixture VoF method saves computational costs
compared to the more sophisticated drag-force-based multiphase models. Thus,
complex three-dimensional flow structures can be simulated while accounting
for the pressure- and shear-rate-dependent rheology