arXiv subjects

A. Davidson

Publications and source records attributed to A. Davidson.

6 recordsLinked to original sources

arXiv2021

Diagnostic Techniques for Particle-in-Cell Simulations of Laser-produced Gamma-rays in the Strong-field QED Regime

The emission of multi-MeV ($\gamma$-ray) photons from the interaction of a high-powered laser pulse with a dense plasma target is studied using particle-in-cell simulations. A new set of diagnostic techniques is presented and applied to analyze the intense field and ultra-relativistic electrons. Such methods elucidate the dominant processes responsible for efficient laser-to-$\gamma$ energy conversion, which include nonlinear Compton scattering and magneto-bremsstrahlung radiation, and provide a clear picture of the interaction on a microscopic level. We identify regions in the plasma target of high photon energy-density and obtain an energy conversion efficiency as high as 30%. The essential characteristics of the interaction are validated with full-3D simulations.

physics.plasm-ph↗

arXiv2018

Optimizing Laser Wakefield Acceleration in the Nonlinear Self-Guided Regime for Fixed Laser Energy

The scaling laws for laser wakefield acceleration in the nonlinear, self-guided regime [Lu et al. Phys. Rev. Spec. Top. Accel. Beams 10, 061301 (2007)] are examined in detail using the quasi-3D version of the particle-in-cell code OSIRIS. We find that the scaling laws continue to work well as the plasma density is reduced while the normalized laser amplitude is kept fixed. For fixed laser energy, the energy gain of an isolated bunch of electrons can be improved with some loss in the bunch charge by shortening the normalized pulse length until self-guiding no longer occurs, and through the use of asymmetric longitudinal profiles with rapid rise times. For example, without any external guiding a 15 J,.8\mu m laser with a pulse length of 46 fs (39 fs) is found to generate a quasi-mono-energetic bunch of 355pC (227pC) with a max energy of 3.25 (4.04) GeV with an acceleration distance of 2.43 cm (3.08cm). Furthermore, a bunch with 39.4pC and a maximum energy 4.6 GeV is produced for an asymmetric laser with a rapid rise time. Studies for 30 J and 100 J lasers are also presented.

physics.plasm-ph↗

arXiv2015

Maximal Steiner Trees in the Stochastic Mean-Field Model of Distance

Consider the complete graph on $n$ vertices, with edge weights drawn independently from the exponential distribution with unit mean. Janson showed that the typical distance between two vertices scales as $\log{n}/n$, whereas the diameter (maximum distance between any two vertices) scales as $3\log{n}/n$. Bollob\'{a}s et al. showed that, for any fixed k, the weight of the Steiner tree connecting $k$ typical vertices scales as $(k-1)\log{n}/n$, which recovers Janson's result for $k=2$. We extend this result to show that the worst case $k$-Steiner tree, over all choices of $k$ vertices, has weight scaling as $(2k-1)\log{n}/n$ and finally, we generalise this result to Steiner trees with a mixture of typical and worst case vertices.

math.PR↗

arXiv2014

Implementation of a hybrid particle code with a PIC description in r-z and a gridless description in $\phi$ into OSIRIS

For many plasma physics problems, three-dimensional and kinetic effects are very important. However, such simulations are very computationally intensive. Fortunately, there is a class of problems for which there is nearly azimuthal symmetry and the dominant three-dimensional physics is captured by the inclusion of only a few azimuthal harmonics. Recently, it was proposed [A. Lifschitz et al., J. Comp. Phys. 228 (5) (2009) 1803-1814] to model one such problem, laser wakefield acceleration, by expanding the fields and currents in azimuthal harmonics and truncating the expansion after only the first harmonic. The complex amplitudes of the fundamental and first harmonic for the fields were solved on an r-z grid and a procedure for calculating the complex current amplitudes for each particle based on its motion in Cartesian geometry was presented using a Marder's correction to maintain the validity of Gauss's law. In this paper, we describe an implementation of this algorithm into OSIRIS using a rigorous charge conserving current deposition method to maintain the validity of Gauss's law. We show that this algorithm is a hybrid method which uses a particles-in-cell description in r-z and a gridless description in $\phi$. We include the ability to keep an arbitrary number of harmonics and higher order particle shapes. Examples, for laser wakefield acceleration, plasma wakefield acceleration, and beam loading are also presented and directions for future work are discussed.

physics.comp-ph↗

arXiv2002

Clash of symmetries on the brane

If our 3+1-dimensional universe is a brane or domain wall embedded in a higher dimensional space, then a phenomenon we term the ``clash of symmetries'' provides a new method of breaking some continuous symmetries. A global $G_{\text{cts}} \otimes G_{\text{discrete}}$ symmetry is spontaneously broken to $H_{\text{cts}} \otimes H_{\text{discrete}}$, where the continuous subgroup $H_{\text{cts}}$ can be embedded in several different ways in the parent group $G_{\text{cts}}$, and $H_{\text{discrete}} < G_{\text{discrete}}$. A certain class of topological domain wall solutions connect two vacua that are invariant under {\it differently embedded} $H_{\text{cts}}$ subgroups. There is then enhanced symmetry breakdown to the intersection of these two subgroups on the domain wall. This is the ``clash''. In the brane limit, we obtain a configuration with $H_{\text{cts}}$ symmetries in the bulk but the smaller intersection symmetry on the brane itself. We illustrate this idea using a permutation symmetric three-Higgs-triplet toy model exploiting the distinct $I-$, $U-$ and $V-$spin U(2) subgroups of U(3). The three disconnected portions of the vacuum manifold can be treated symmetrically through the construction of a three-fold planar domain wall junction configuration, with our universe at the nexus. A possible connection with $E_6$ is discussed.

hep-th↗

arXiv1995

Light-Heavy Symmetry: Geometric Mass Hierarchy for Three Families

The Universal Seesaw pattern coupled with a Light$\leftrightarrow$Heavy symmetry principle leads to the Diophantine equation $\displaystyle N = \sum_{i=1}^Nn_i$, where $n_i\geq 0$ and distinct. Its unique non-trivial solution $(3=0+1+2)$ gives rise to the geometric mass hierarchy $m_W$, $m_Wε$, $m_Wε^2$ for $N=3$ fermion families. This is realized in a model where the hybrid (yet Up$\leftrightarrow$Down symmetric) quark mass relations $m_d m_t \approx m_c^2\leftrightarrow m_u m_b \approx m_s^2 $ play a crucial role in expressing the CKM mixings in terms of simple mass ratios, notably $\sinθ_C \approx {m_c\over m_b}$.

hep-ph↗