Showing papers by "Phillip Sprangle published in 2002"
TL;DR: The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically and an equilibrium for a single optical-plasma filament is obtained, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing.
Abstract: The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically. A set of three-dimensional (3D), nonlinear propagation equations is derived, which includes the effects of dispersion, nonlinear self-focusing, stimulated molecular Raman scattering, multiphoton and tunneling ionization, energy depletion due to ionization, relativistic focusing, and ponderomotively excited plasma wakefields. The instantaneous frequency spread along a laser pulse in air, which develops due to various nonlinear effects, is analyzed and discussed. Coupled equations for the power, spot size, and electron density are derived for an intense ionizing laser pulse. From these equations we obtain an equilibrium for a single optical-plasma filament, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing. The equilibrium is shown to require a specific distribution of power along the filament. It is found that in the presence of ionization a self-guided optical filament is not realizable. A method for generating a remote spark in the atmosphere is proposed, which utilizes the dispersive and nonlinear properties of air to cause a low-intensity chirped laser pulse to compress both longitudinally and transversely. For optimally chosen parameters, we find that the transverse and longitudinal focal lengths can be made to coincide, resulting in rapid intensity increase, ionization, and white light generation in a localized region far from the source. Coupled equations for the laser spot size and pulse duration are derived, which can describe the focusing and compression process in the low-intensity regime. More general examples involving beam focusing, compression, ionization, and white light generation near the focal region are studied by numerically solving the full set of 3D, nonlinear propagation equations.
258 citations
TL;DR: In this article, a three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities.
Abstract: To achieve multi GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma without disruption A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities In the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel Long pulses (several plasma wavelengths) experience substantial modification due to these instabilities The short pulse LWFA, although having smaller accelerating fields, can provide acceleration for longer distances in a plasma channel By allowing the plasma density to increase along the propagation path electron dephasing can be deferred, increasing the energy gain A simulation example of a GeV channel guided LWFA accelerator is presented Simulations also show that multi-GeV energies can be achieved by optimally tapering the plasma channel
41 citations
TL;DR: In this article, the authors investigated the possibility of using a short plasma channel to provide focusing or control of the spot size of a laser pulse at intensities far above the usual damage limits of conventional optical elements.
Abstract: Plasma channels have been used to guide intense laser pulses over distances of many Rayleigh lengths. This paper investigates the possibility of using a short plasma channel to provide focusing or control of the spot size of a laser pulse at intensities far above the usual damage limits of conventional optical elements. Analytical models for the focal length and focused spot size of a single plasma channel lens and a nonconverging laser pulse are presented, and results are compared with the two-dimensional simulation code LEM [J. Krall et al., Phys. Rev. E 48, 2157 (1993)]. Several advanced thin lens configurations, including multiple lens transport systems, and both focusing and defocusing lenses for externally focused converging laser pulses are also analyzed. Experimental techniques for producing appropriate plasma profiles are reviewed, and evidence for plasma channel focusing in a capillary discharge guiding experiment is analyzed. Thick “overmoded” lenses offer a possible alternative if there are experimental difficulties in producing sufficiently thin plasma channels. A variety of potential applications exist for the various proposed lens configurations.
26 citations
TL;DR: In this article, the authors evaluated several reduced models of a plasma and showed that the quasistatic approximation causes a much more severe ultraviolet catastrophe than does the paraxial approximation alone.
Abstract: Certain reduced models of a plasma are subject to a phenomenon known as the ultraviolet catastrophe, whereby the Raman growth rate diverges with increasing transverse wave number. Numerical solutions that make use of such models cannot account for large angle scattering. Several of these models are evaluated in terms of their ability to reproduce the growth rates given by the two-dimensional Raman dispersion relation. The quasistatic approximation causes a much more severe ultraviolet catastrophe than does the paraxial approximation alone. Removal of the quasistatic approximation eliminates the ultraviolet catastrophe provided nonparaxial terms are retained. Fully explicit particle-in-cell simulations show that large angle scattering leads to a fine structure which perturbs the pump wave and heats the plasma.
18 citations
TL;DR: The propagation of intense laser pulses with durations longer than the plasma period through tapered plasma channels is investigated theoretically and numerically and full-scale numerical fluid simulations indicate parameters that favorably excite either the forward Raman or self-modulation instability.
Abstract: The propagation of intense laser pulses with durations longer than the plasma period through tapered plasma channels is investigated theoretically and numerically. General propagation equations are presented and reduced partial differential equations that separately describe the forward Raman (FR) and self-modulation (SM) instabilities in a nonuniform plasma are derived. Local dispersion relations for FR and SM instabilities are used to analyze the detuning process arising from a longitudinal density gradient. Full-scale numerical fluid simulations indicate parameters that favorably excite either the FR or SM instability. The suppression of the FR instability and the enhancement of the SM instability in a tapered channel in which the density increases longitudinally is demonstrated. For a pulse undergoing a self-modulation instability, calculations show that the phase velocity of the wakefield in an untapered channel can be significantly slower than the pulse group velocity. Simulations indicate that this wake slippage can be forestalled through the use of a tapered channel.
16 citations
26 Nov 2002
TL;DR: In this paper, the index of refraction through variation of plasma density using a segmented capillary discharge was used to control the laser group velocity in long, multistage plasma channels.
Abstract: To achieve multi‐GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma channels while maintaining a proper phase with the accelerated electrons. Using capillary discharge we have demonstrated a method that allows controlling the laser group velocity in long, multistage plasma channels. The control is achieved by modifying the index of refraction through variation of plasma density using a segmented capillary discharge.
2 citations
01 May 2002-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, it was shown that for a continuous beam, increasing the beam current can improve the beam quality and that when both effects are treated self-consistently, an equilibrium exists wherein the space charge forces exactly compensate for the betatron motion.
Abstract: In order to obtain optimal performance from a free-electron laser (FEL), the axial velocity spread on the electron beam must be small as it propagates through the wiggler field. Treated separately, both the wiggler-induced betatron motions and the self-induced space-charge forces tend to increase the axial velocity spread and degrade the performance of the FEL. However, it has been shown analytically by Hafizi and Roberson (Phys. Plasmas 3 (1996) 2156) that when both effects are treated self-consistently an equilibrium exists wherein the space charge forces exactly compensate for the betatron motion. This leads to the surprising result that for a continuous beam, increasing the beam current can improve the beam quality.