85 research outputs found
BPS states in string theory
No full textIn this thesis we discuss a number of interesting and important properties of BPS states in string theory. We study wall-crossing behavior of BPS states at large volume limit and implications of it for the OSV conjecture. We find that the weak topological coupling OSV conjecture can be true at most in a special chamber of the K"ahler cone. We also clarify an interesting puzzle arising in the description of BPS states on the Higgs branch of supersymmetic quantum mechanics. Using methods of toric geometry we compute Hilbert spaces of BPS states on the compactified Higgs branch and arrive at completely consistent picture of spatial structure of those spaces. We introduce new kinds of walls, called Bound State Transformation(BST) walls, in the moduli space across which the nature of BPS bound states changes but the index remains continuous. These walls are necessary to explain the continuity of BPS index. BPS states can undergo recombination, conjugation or hybrids of the two when crossing a BST wall. Conjugation phenomenon happens near singularities in the moduli space and we relate massless spectra of BPS states at such singularities to monodromies around them. In cases when massless vector BPS particles are present we find new constraints on the spectrum and in particular predict the existence of magnetic monopoles becoming massless at such singularities. We give a simple physical derivation of the Kontsevich-Soibelman wall-crossing formula. Considering galaxy-like configurations of BPS particles with a central supermassive black hole with a number of stellar BPS systems around it we derive a consistency requirement on the partition function of such BPS galaxies. This requirement turns out to be nothing but Kontsevich-Soibelman wall-crossing formula. Our approach gives a generalization of the formula for the case when massless BPS particles are present.Ph.D.Includes bibliographical referencesIncludes vitaby Evgeny Andriyas
Laser-plasma interactions with a Fourier-Bessel particle-in-cell method
No full textA new spectral particle-in-cell (PIC) method for plasma modeling is presented and discussed. In the proposed scheme, the Fourier-Bessel transform is used to translate the Maxwell equations to the quasi-cylindrical spectral domain. In this domain, the equations are solved analytically in time, and the spatial derivatives are approximated with high accuracy. In contrast to the finite-difference time domain (FDTD) methods, that are used commonly in PIC, the developed method does not produce numerical dispersion and does not involve grid staggering for the electric and magnetic fields. These features are especially valuable in modeling the wakefield acceleration of particles in plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the test simulations of laser plasma interactions are compared to the ones done with the quasi-cylindrical FDTD PIC code CALDER-CIRC
All-optical Compton scattering at shallow interaction angles
No full textInternational audienceAll-optical Compton sources combine laser-wakefield accelerators and intense scattering pulses to generate ultrashort bursts of backscattered radiation. The scattering pulse plays the role of a small-period undulator (∼1 µm) in which relativistic electrons oscillate and emit X-ray radiation. To date, most of the working laser-plasma accelerators operate preferably at energies of a few hundreds of megaelectronvolts and the Compton sources developed so far produce radiation in the range from hundreds of kiloelectronvolts to a few megaelectronvolts. However, for such applications as medical imaging and tomography the relevant energy range is 10-100 keV. In this article, we discuss different scattering geometries for the generation of X-rays in this range. Through numerical simulations, we study the influence of electron beam parameters on the backscattered photons. We find that the spectral bandwidth remains constant for beams of the same emittance regardless of the scattering geometry. A shallow interaction angle of 30 • or less seems particularly promising for imaging applications given parameters of existing laser-plasma accelerators. Finally, we discuss the influence of the radiation properties for potential applications in medical imaging and non-destructive testing
Axiparabola: a new tool for high-intensity optics
No full textInternational audienceAbstract An axiparabola is a reflective aspherical optics that focuses a light beam into an extended focal line. The light intensity and group velocity profiles along the focus are adjustable through the proper design. The on-axis light velocity can be controlled, for instance, by adding spatio-temporal couplings via chromatic optics on the incoming beam. Therefore the energy deposition along the axis can be either subluminal or superluminal as required in various applications. This article first explores how the axiparabola design defines its properties in the geometric optics approximation. Then the obtained description is considered in numerical simulations for two cases of interest for laser-plasma acceleration. We show that the axiparabola can be used either to generate a plasma waveguide to overcome diffraction or for driving a dephasingless wakefield accelerator
PIConGPU 0.4.3: System Updates and Bug Fixes
No full text<p>This release adds updates and new HPC system templates. Important bug fixes include I/O work-arounds for issues in OpenMPI 2.0-4.0 (mainly with HDF5), guards for particle creation with user-defined profiles, a fixed binomial current smoothing, checks for the number of devices in grid distributions and container (Docker & Singularity) modernizations.</p>
<p>Please refer to our <a href="https://github.com/ComputationalRadiationPhysics/picongpu/blob/0.4.3/CHANGELOG.md#043">ChangeLog</a> for a full list of features, fixes and user interface changes before getting started.</p>
<p>Thanks to Axel Huebl, Alexander Debus, Igor Andriyash, Marco Garten, Sergei Bastrakov, Adam Simpson, Richard Pausch, Juncheng E, Klaus Steiniger, and René Widera for contributions to this release!</p>
A spectral, quasi-cylindrical and dispersion-free Particle-In-Cell algorithm
No full textWe propose a spectral Particle-In-Cell (PIC) algorithm that is based on the combination of a Hankel transform and a Fourier transform. For physical problems that have close-to-cylindrical symmetry, this algorithm can be much faster than full 3D PIC algorithms. In addition, unlike standard finite-difference PIC codes, the proposed algorithm is free of spurious numerical dispersion, in vacuum. This algorithm is benchmarked in several situations that are of interest for laser–plasma interactions. These benchmarks show that it avoids a number of numerical artifacts, that would otherwise affect the physics in a standard PIC algorithm — including the zero-order numerical Cherenkov effect
All-optical Compton scattering at shallow interaction angles
No full textAll-optical Compton sources combine laser wakefield accelerators and intense
scattering pulses to generate ultrashort bursts of backscattered radiation. The
scattering pulse plays the role of a short-period undulator in which
relativistic electrons oscillate and emit x-ray radiation. To date, most of the
working laser-plasma accelerators operate preferably at energies of a few
hundreds of MeV and the Compton sources developed so far produce radiation in
the range from hundreds of keV to a few MeV. However, for such applications as
medical imaging and tomography the relevant energy range is 10-100 keV. In this
article, we discuss different scattering geometries for the generation of
X-rays in this range. Through numerical simulations, we study the influence of
electron beam parameters on the backscattered photons. We find that the
spectral bandwidth remains constant for beams of the same emittance regardless
of the scattering geometry. A shallow interaction angle of 30 degrees or less
seems particularly promising for imaging applications given parameters of
existing laser-plasma accelerators. Finally, we discuss the influence of the
radiation properties for potential applications in medical imaging and
non-destructive testing
Quasi-monoenergetic multi-GeV electron acceleration in a plasma waveguide
No full textInternational audienceLaser-plasma accelerators present a promising alternative to conventional accelerators. To fully exploit the extreme amplitudes of the plasma fields and produce high-quality beams, precise control over electron injection into the accelerating structure is required, along with effective laser pulse guiding to extend the acceleration length. Recent studies have demonstrated efficient guiding and acceleration using hydrodynamic optically field-ionized (OFI) plasma channels. This guiding technique has also been combined with controlled electron injection to produce high-quality electron beams at the GeV level using a 50 TW laser. The present work extends these results to higher laser energies, demonstrating the generation of quasi-monoenergetic electron beams with peak energies exceeding 2 GeV, for a PW-class laser
X-ray amplification from a Raman Free Electron Laser
No full textaccepted for publication in Phys. Rev. Lett. 03/11/2012We demonstrate that a mm-scale free electron laser can operate in the X-ray range, in the interaction between a moderately relativistic electron bunch, and a transverse high intensity optical lattice. The corrugated light-induced ponderomotive potential acts simultaneously as a guide and as a low-frequency wiggler, triggering stimulated Raman scattering. The gain law in the small signal regime is derived in a fluid approach, and confirmed from Particle-In-Cell simulations. We describe the nature of bunching, and discuss the saturation properties. The resulting all-optical Raman X-ray laser opens perspectives for ultra-compact coherent light sources up to the hard X-ray range
Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate
No full textProton beams with up to 100 pC bunch charge, 0.48 MeV cut-off energy and
divergence as low as a were generated from solid targets at kHz
repetition rate by a few-mJ femtosecond laser under controlled plasma
conditions. The beam spatial profile was measured using a small aperture
scanning time-of-flight detector. Detailed parametric studies were performed by
varying the surface plasma scale length from 8 to 80 nm and the laser pulse
duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with
observations and, together with an in-depth theoretical analysis of the
acceleration mechanism, indicate that high repetition rate femtosecond laser
technology could be used to produce few-MeV protons beams for applications.Comment: 6 pages, 4 figures (main text). 7 pages, 6 figures (supplemental
material
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