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Shadwick, B A

Publications and source records attributed to Shadwick, B A.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Modeling asymmetric beams using higher-order phase-space moments

Shadwick et al. developed [Commun. Nonlinear. Sci. Numer. Simulat. 17, 2153-2160 (2012)] a self-consistent model of beam transport using a phase-space moment description (up to 2nd order) based on the Vlasov-Maxwell Hamiltonian structure. Tzoufras et al. have shown [Physics of Plasmas 16, 056705 (2009)] that for a laser-wakefield-accelerated electron beam, the spatial profile that minimizes the energy spread is a trapezoidal distribution. This profile is spatially asymmetric and thus cannot be described by only using moments less than order three, and so the moment method developed by Shadwick et al. was expanded upon to account for asymmetry by taking the profile to be a sum of two gaussian 'blobs.' It was found that, as long as the electric field across the beam was close to linear, treating the two blobs as separate species was a good approximation.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Generation of tunable, 100-800 MeV quasi-monoenergetic electron beams from a laser-wakefield accelerator in the blowout regime

In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30-80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100-800 MeV. These beams have angular divergence of 1-4 mrad, and 5%-25% energy spread, with a resulting brightness 10{sup 11} electrons mm{sup -2} MeV{sup -1} mrad{sup -2}. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Electron self-injection into an evolving plasma bubble: Quasi-monoenergetic laser-plasma acceleration in the blowout regime

An electron density bubble driven in a rarefied uniform plasma by a slowly evolving laser pulse goes through periods of adiabatically slow expansions and contractions. Bubble expansion causes robust self-injection of initially quiescent plasma electrons, whereas stabilization and contraction terminate self-injection thus limiting injected charge; concomitant phase space rotation reduces the bunch energy spread. In regimes relevant to experiments with hundred terawatt- to petawatt-class lasers, bubble dynamics and, hence, the self-injection process are governed primarily by the driver evolution. Collective transverse fields of the trapped electron bunch reduce the accelerating gradient and slow down phase space rotation. Bubble expansion followed by stabilization and contraction suppresses the low-energy background and creates a collimated quasi-monoenergetic electron bunch long before dephasing. Nonlinear evolution of the laser pulse (spot size oscillations, self-compression, and front steepening) can also cause continuous self-injection, resulting in a large dark current, degrading the electron beam quality.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Electron Self-Injection into an Evolving Plasma Bubble: The Way to a Dark Current Free GeV-Scale Laser Accelerator

A time-varying electron density bubble created by the radiation pressure of a tightly focused petawatt laser pulse traps electrons of ambient rarefied plasma and accelerates them to a GeV energy over a few-cm distance. Expansion of the bubble caused by the shape variation of the self-guided pulse is the primary cause of electron self-injection in strongly rarefied plasmas (n{sub e{approx}}10{sup 17} cm{sup -3}). Stabilization and contraction of the bubble extinguishes the injection. After the bubble stabilization, longitudinal non-uniformity of the accelerating gradient results in a rapid phase space rotation that produces a quasi-monoenergetic bunch well before the de-phasing limit. Combination of reduced and fully self-consistent (first-principle) 3-D PIC simulations complemented with the Hamiltonian diagnostics of electron phase space shows that the bubble dynamics and the self-injection process are governed primarily by the driver evolution; collective transverse fields of the trapped electron bunch reduce the accelerating gradient, slow down phase space rotation, and result in a formation of monoenergetic electron beam with higher energy than test-particle modeling predicts.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Numerical Methods for the Wave Equation

We examine a variety of numerical methods, both explicit and implicit, for the wave equation. Numerical dispersion relations are used to determine stability and to analyse the linear group velocity. We find that although implicit methods can allow for large time step, the group velocity becomes increasingly inaccurate with increasing step-size.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

All-Optical Control of Nonlinear Self-Focusing in Plasmas Using Non-Resonantly Driven Plasma Wave

Excitation of plasma density perturbations by an initially bi-color laser pulse helps to control nonlinear refraction in the plasma and enables various types of laser self-guiding. In this report we consider a setup that not only makes possible the transport of laser energy over cm-long relatively dense plasmas (n{sub 0} = 10{sup 18} cm{sup -3}) but also transforms the pulse into the unique format inaccessible to the conventional amplification techniques (relativistically intense periodic trains of few-cycle spikes). This well focusable pulse train is a novel light source interesting for ultra-fast high-field science applications. The opposite case of suppression of nonlinear self-focusing and dynamical self-guiding of an over-critical multi-frequency pulse is proposed for the proof-of-principle experimental study.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗