17 research outputs found
A spectral, quasi-cylindrical and dispersion-free Particle-In-Cell algorithm
We 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 numerical
dispersion. 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.Comment: 23 pages, 8 figure
All-optical Compton scattering at shallow interaction angles
All-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 (∼1μm) 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
Laser-plasma interactions with a Fourier-Bessel Particle-in-Cell method
A 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 commonly used 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.Comment: submitted to Phys. Plasma
Revealing Josephson vortex dynamics in proximity junctions below critical current
Made of a thin non-superconducting metal (N) sandwiched by two
superconductors (S), SNS Josephson junctions enable novel quantum
functionalities by mixing up the intrinsic electronic properties of N with the
superconducting correlations induced from S by proximity. Electronic properties
of these devices are governed by Andreev quasiparticles [1] which are absent in
conventional SIS junctions whose insulating barrier (I) between the two S
electrodes owns no electronic states. Here we focus on the Josephson vortex
(JV) motion inside Nb-Cu-Nb proximity junctions subject to electric currents
and magnetic fields. The results of local (Magnetic Force Microscopy) and
global (transport) experiments provided simultaneously are compared with our
numerical model, revealing the existence of several distinct dynamic regimes of
the JV motion. One of them, identified as a fast hysteretic entry/escape below
the critical value of Josephson current, is analyzed and suggested for
low-dissipative logic and memory elements.Comment: 11 pages, 3 figures, 1 table, 43 reference
All-optical Compton scattering at shallow interaction angles
International 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
Identifying observable carrier-envelope phase effects in laser wakefield acceleration with near-single-cycle pulses
International audienceDriving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realization of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum, and the direction of emitted betatron radiation become CEP dependent. For injection in a density gradient, the effect on beam pointing is reduced and the electron energy spectrum is CEP independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam
Direct observation of relativistic broken plasma waves
International audiencePlasma waves contribute to many fundamental phenomena, including astrophysics, thermonuclear fusion and particle acceleration. Such waves can develop in numerous ways, from classic Langmuir oscillations carried by electron thermal motion, to the waves excited by an external force and travelling with a driver. In plasma-based particle accelerators, a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit, which is the maximum wave amplitude that a plasma can sustain. In this limit, plasma electrons gain sufficient energy from the wave to outrun it and to get trapped inside the wave bucket. Theory and numerical simulations predict multi-dimensional wavebreaking, which is crucial in the electron self-injection process that determines the accelerator performances. Here we present a real-time experimental visualization of the laser-driven nonlinear relativistic plasma waves by probing them with a femtosecond high-energy electron bunch from another laser-plasma accelerator coupled to the same laser system. This single-shot electron deflectometry allows us to characterize nonlinear plasma wakefield with femtosecond temporal and micrometre spatial resolutions revealing features of the plasma waves at the breaking point
Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate
Proton 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