Biomedical subjects

P Sprangle

Publications and source records attributed to P Sprangle.

At least 19 recordsLinked to original sources

PubMed2005

Transmission of intense femtosecond laser pulses into dielectrics.

The interaction of intense, femtosecond laser pulses with a dielectric medium is examined using a numerical simulation. The simulation uses the one-dimensional electromagnetic wave equation to model laser pulse propagation. In addition, it includes multiphoton ionization, electron attachment, Ohmic heating of free electrons, and temperature-dependent collisional ionization. Laser pulses considered in this study are characterized by peak intensities approximately 10(12) -10(14) W/cm(2) and pulse durations approximately 10-100 fsec . These laser pulses interacting with fused silica are shown to produce above-critical plasma densities and electron energy densities sufficient to attain experimentally measured damage thresholds. Significant transmission of laser energy is observed even in cases where the peak plasma density is above the critical density for reflection. A damage fluence based on absorbed laser energy is calculated for various pulse durations. The calculated damage fluence threshold is found to be consistent with recent experimental results.

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PubMed2005

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air.

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.

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PubMed2005

Quasimonoenergetic electrons from unphased injection into channel guided laser wakefield accelerators.

A high-quality electron beam can be extracted from a channel guided laser wakefield accelerator without confining the injected particles to a small region of phase. By careful choice of the injection energy, a regime can be found where uniformly phased particles are quickly bunched by the accelerator itself and subsequently accelerated to high energy. The process is particularly effective in a plasma channel because of a favorable phase shift that occurs in the focusing fields. Furthermore, particle-in-cell simulations show that the self-fields of the injected bunches actually tend to reduce the energy spread on the final beam. The final beam characteristics can be calculated using a computationally inexpensive Hamiltonian formulation when beam-loading effects are minimal.

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PubMed2004

Ultrashort laser pulses and electromagnetic pulse generation in air and on dielectric surfaces.

Intense, ultrashort laser pulses propagating in the atmosphere have been observed to emit sub-THz electromagnetic pulses (EMPS). The purpose of this paper is to analyze EMP generation from the interaction of ultrashort laser pulses with air and with dielectric surfaces and to determine the efficiency of conversion of laser energy to EMP energy. In our self-consistent model the laser pulse partially ionizes the medium, forms a plasma filament, and through the ponderomotive forces associated with the laser pulse, drives plasma currents which are the source of the EMP. The propagating laser pulse evolves under the influence of diffraction, Kerr focusing, plasma defocusing, and energy depletion due to electron collisions and ionization. Collective effects and recombination processes are also included in the model. The duration of the EMP in air, at a fixed point, is found to be a few hundred femtoseconds, i.e., on the order of the laser pulse duration plus the electron collision time. For steady state laser pulse propagation the flux of EMP energy is nonradiative and axially directed. Radiative EMP energy is present only for nonsteady state or transient laser pulse propagation. The analysis also considers the generation of EMP on the surface of a dielectric on which an ultrashort laser pulse is incident. For typical laser parameters, the power and energy conversion efficiency from laser radiation to EMP radiation in both air and from dielectric surfaces is found to be extremely small, < 10(-8). Results of full-scale, self-consistent, numerical simulations of atmospheric and dielectric surface EMP generation are presented. A recent experiment on atmospheric EMP generation is also simulated.

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PubMed2003

Stimulated Raman scattering of intense laser pulses in air.

Stimulated rotational Raman scattering (SRRS) is known to be one of the processes limiting the propagation of high-power laser beams in the atmosphere. In this paper, SRRS, Kerr nonlinearity effects, and group velocity dispersion of short laser pulses and pulse trains are analyzed and simulated. Fully time-dependent, three-dimensional, nonlinear propagation equations describing the Raman interaction, optical Kerr nonlinearity due to bound electrons, and group velocity dispersion are presented and discussed. The effective time-dependent nonlinear refractive index containing both Kerr and Raman processes is derived. Linear stability analysis is used to obtain growth rates and phase matching conditions for the SRRS, modulational, and filamentation instabilities. Numerical solutions of the propagation equations in three dimensions show the detailed evolution of the Raman scattering instability for various pulse formats. The dependence of the growth rate of SRRS on pulse duration is examined and under certain conditions it is shown that short (approximately psec) laser pulses are stable to the SRRS instability. The interaction of pulses in a train through the Raman polarization field is also illustrated.

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PubMed2003

Asymmetric self-phase modulation and compression of short laser pulses in plasma channels.

A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dramatic photon deceleration while propagating a distance on the order of a dephasing length. The deceleration of photons is localized to the back of the pulse and is accompanied by compression and explosive growth of the ponderomotive potential. Fully explicit particle-in-cell simulations are applied to the problem and are compared with ponderomotive guiding center simulations. A numerical Wigner transform is used to examine local frequency shifts within the pulse and to suggest an experimental diagnostic of plasma waves inside a capillary.

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PubMed2003

Electron distribution function in short-pulse photoionization.

Two well-known limiting regimes of photoionization, when a laser beam interacts with a gas, correspond to the tunneling and the multiphoton processes. The latter dominates in the low-intensity regime, while the former is appropriate at higher intensities. Electrons are born with negligible velocity in tunneling ionization, while in l-photon ionization they are born with a fixed energy determined by l, the photon energy and the ionization potential of the molecule. The transport equation for the distribution function of electrons can be integrated along the characteristics defined by the classical equations of motion in the laser field. Expressions for the distribution function have been obtained in the two regimes using the appropriate analytical form for the ionization rate. Results from two-dimensional particle-in-cell simulations and illustrative plots of the distribution function are presented and discussed.

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PubMed2002

Propagation of intense short laser pulses in the atmosphere.

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.

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PubMed2002

Raman forward scattering and self-modulation of laser pulses in tapered plasma channels.

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.

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PubMed2001

Seeding of the forward Raman instability by ionization fronts and Raman backscatter.

The self-modulated laser wakefield accelerator utilizes the forward Raman instability to drive a large-amplitude plasma wave. The effectiveness of this process could depend on how it competes with other processes such as self-focusing and cavitation, or other instabilities such as filamentation and hosing. The relative timing between the various processes is dependent on the nature of the seed for each instability. Both ionization fronts and Raman backscatter are capable of seeding the forward Raman instability. This causes the forward Raman instability to emerge much earlier than would otherwise be expected.

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PubMed2001

Apparent superluminal propagation of a laser pulse in a dispersive medium.

The distortion of a laser pulse propagating in a dispersive gain/absorptive medium is analyzed. The relationship between the distortion of the pulse and superluminal propagation is discussed. We present an analytical approach based on the laser envelope equation that is readily applicable to arbitrary input pulse shapes. This analysis is used to interpret recent experiments that claim to have observed distortionless superluminal laser pulse propagation.

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PubMed2001

Wakefield generation and GeV acceleration in tapered plasma channels.

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. One of the purposes of this paper is to evaluate the stability properties of intense laser pulses propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects such as group velocity dispersion, as well as wakefield and relativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. This cancellation can result in pulse propagation over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides the guiding necessary for long distance propagation. Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial modification due to Raman and modulation instabilities. For both short and long pulses the seed for instability growth is inherently determined by the pulse shape and not by background noise. These results would indicate that the self-modulated LWFA is not the optimal configuration for achieving high energies. The standard LWFA, although having smaller accelerating fields, can provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the accelerating field behind the laser pulse can be made equal to the speed of light. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Depending on the tapering gradient, this luminous wakefield phase velocity is obtained several plasma wavelengths behind the laser pulse. Simulations of laser pulses propagating in a tapered plasma channel are presented. Experimental techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented.

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PubMed2001

Simulation and design of stable channel-guided laser wakefield accelerators.

Most laser wakefield accelerator (LWFA) experiments to date have operated in the self-modulated (SM) regime and have been self-guided. A channel-guided LWFA operating in the standard or resonant regime is expected to offer the possibility of high electron energy gain and high accelerating gradients without the instabilities and poor electron beam quality associated with the SM regime. Plasma channels such as those produced by a capillary discharge have demonstrated guiding of intense laser pulses over distances of several centimeters. Optimizing the performance in a resonant LWFA constrains the on-axis plasma density in the channel to a relatively narrow range. A scaling model is presented that quantifies resonant LFWA performance in terms of the maximum accelerating gradient, dephasing length, and dephasing-limited energy gain. These performance quantities are expressed in terms of laser and channel experimental parameters, clearly illustrating some of the tradeoffs in the choice of parameters. The predicted energy gain in this model is generally lower than that indicated by simpler scaling models. Simulations agree well with the scaling model in both low and high plasma density regimes. Simulations of a channel-guided, self-modulated LWFA are also presented. Compared with the resonant LWFA regime, the requirements on laser and channel parameters in the SM regime are easier to achieve, and a channel-guided SM-LWFA is likely to be less unstable than a self-guided SM-LWFA.

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PubMed2000

Stable laser-pulse propagation in plasma channels for GeV electron acceleration

To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in plasma without disruption. A 3D envelope equation for a laser pulse in a tapered plasma channel is derived, which includes wakefields and relativistic and nonparaxial effects, such as finite pulse length and group velocity dispersion. It is shown that electron energies of approximately GeV in a plasma-channel LWFA can be achieved by using short pulses where the forward Raman and modulation nonlinearities tend to cancel. Further energy gain can be achieved by tapering the plasma density to reduce electron dephasing.

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PubMed2000

Laser pulse modulation instabilities in plasma channels

In this paper the modulational instability associated with propagation of intense laser pulses in a partially stripped, preformed plasma channel is analyzed. In general, modulation instabilities are caused by the interplay between (anomalous) group velocity dispersion and self-phase modulation. The analysis is based on a systematic approach that includes finite-perturbation-length effects, nonlinearities, group velocity dispersion, and transverse effects. To properly include the radial variation of both the laser field and plasma channel, the source-dependent expansion method for analyzing the wave equation is employed. Matched equilibria for a laser beam propagating in a plasma channel are obtained and analyzed. Modulation of a uniform (matched) laser beam equilibrium in a plasma channel leads to a coupled pair of differential equations for the perturbed spot size and laser field amplitude. A general dispersion relation is derived and solved. Surface plots of the spatial growth rate as a function of laser beam power and the modulation wave number are presented.

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PubMed2000

Ultrashort free-electron laser pulse

Three-dimensional characteristics of short free-electron laser pulses are analyzed. When the optical pulse length is short, the growth rate and optical guiding will vary among the Fourier components comprising the pulse. Matched beam solutions of the wave equation, including diffraction and nonparaxial effects, are discussed. In certain limits a front to back asymmetry develops along the pulse as well as a frequency spread across it. In these limits the asymmetry and the frequency spread are relatively small unless the number of optical cycles in the pulse approaches unity.

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PubMed2000

Transverse modulation of an electron beam generated in self-modulated laser wakefield accelerator experiments

Low energy electron beams (E approximately 300 keV) generated in a self-modulated laser wakefield accelerator experiment were observed to filament and be deflected away from the laser axis forming radial jets in the electron beam profile. At higher energies (E>900 keV), the filamentation and jets were suppressed and smooth electron beams copropagating with the laser were observed. The observed electron beam filamentation likely results from laser beam filamentation in the plasma due to relativistic self-focusing effects. The radial jets of low energy electrons are likely caused by transverse ejection of the electrons due to the radial structure of the wakefield and space charge deflection of electrons as they exit the laser focus.

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PubMed2000

Relativistic focusing and ponderomotive channeling of intense laser beams

The ponderomotive force associated with an intense laser beam expels electrons radially and can lead to cavitation in plasma. Relativistic effects as well as ponderomotive expulsion of electrons modify the refractive index. An envelope equation for the laser spot size is derived, using the source-dependent expansion method with Laguerre-Gaussian eigenfunctions, and reduced to quadrature. The envelope equation is valid for arbitrary laser intensity within the long pulse, quasistatic approximation and neglects instabilities. Solutions of the envelope equation are discussed in terms of an effective potential for the laser spot size. An analytical expression for the effective potential is given. For laser powers exceeding the critical power for relativistic self-focusing the analysis indicates that a significant contraction of the spot size and a corresponding increase in intensity is possible.

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