Biomedical subjects

L O Silva

Publications and source records attributed to L O Silva.

12 recordsLinked to original sources

PubMed2006

Kinetics of the collisionless expansion of spherical nanoplasmas.

The collisionless expansion of spherical plasmas composed of cold ions and hot electrons is analyzed using a novel kinetic model, with special emphasis on the influence of the electron dynamics. Simple, general laws are found, relating the relevant expansion features to the initial conditions of the plasma, determined from a single dimensionless parameter. A transition is identified in the behavior of the ion energy spectrum, which is monotonic only for high electron temperatures, otherwise exhibiting a local peak far from the cutoff energy.

Journal Article↗

PubMed2006

Space-charge effects in the current-filamentation or Weibel instability.

We consider how an unmagnetized plasma responds to an incoming flux of energetic electrons. We assume a return current is present and allow for the incoming electrons to have a different transverse temperature than the return current. To analyze this configuration we present a nonrelativistic theory of the current-filamentation or Weibel instability for rigorously current-neutral and nonseparable distribution functions, f(0)(p(x), p(y), p(z)) is not equal to f(x)(p(x))f(y)(p(y))f(z)(p(z)). We find that such distribution functions lead to lower growth rates because of space-charge forces that arise when the forward-going electrons pinch to a lesser degree than the colder, backward-flowing electrons. We verify the growth rate, range of unstable wave numbers, and the formation of the density filaments using particle-in-cell simulations.

Journal Article↗

PubMed2006

Very high Mach-number electrostatic shocks in collisionless plasmas.

The kinetic theory of collisionless electrostatic shocks resulting from the collision of plasma slabs with different temperatures and densities is presented. The theoretical results are confirmed by self-consistent particle-in-cell simulations, revealing the formation and stable propagation of electrostatic shocks with very high Mach numbers (M>>10), well above the predictions of the classical theories for electrostatic shocks.

Journal Article↗

PubMed2005

Quasiparticle approach to the modulational instability of drift waves coupling to zonal flows.

The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach. Simulations have been conducted in which a particle-in-cell representation is used for the quasiparticles, while a fluid model is employed for the plasma. The interactions have been studied in a plasma edge configuration which has applications in both tokamak physics and magnetopause boundary layer studies. Simulation results show the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients.

Journal Article↗

PubMed2005

Dynamics and control of shock shells in the coulomb explosion of very large deuterium clusters.

The explosion dynamics of very large (approximately 10(6)-10(7) atoms) deuterium clusters irradiated by ultraintense laser pulses (I approximately 10(18) W/cm(2)) is analyzed self-consistently with one-to-one three-dimensional and two-dimensional fully relativistic particle-in-cell simulations. Small-scale shock shells in the expanding ion cloud are observed. A technique to induce the formation of large shock shells inside a single cluster, increasing the probability of intracluster nuclear reactions, is proposed and demonstrated.

Journal Article↗

PubMed2004

Ion acceleration from the shock front induced by hole boring in ultraintense laser-plasma interactions.

Ion-acceleration processes have been studied in ultraintense laser plasma interactions for normal incidence irradiation of solid deuterated targets via neutron spectroscopy. The experimental neutron spectra strongly suggest that the ions are preferentially accelerated radially, rather than into the bulk of the material from three-dimensional Monte Carlo fitting of the neutron spectra. Although the laser system has a 10(-7) contrast ratio, a two-dimensional magnetic hydrodynamics simulation shows that the laser pedestal generates a 10 mum scale length in the coronal plasma with a 3 mum scale-length plasma near the critical density. Two-dimensional particle-in-cell simulations, incorporating this realistic density profile, indicate that the acceleration of the ions is caused by a collisionless shock formation. This has implications for modeling energy transport in solid density plasmas as well as cone-focused fast ignition using the next generation PW lasers currently under construction.

Journal Article↗

PubMed2004

Global simulation for laser-driven MeV electrons in fast ignition.

A comprehensive examination of the interaction of a picosecond-long ignition pulse on high-density (40 times critical density) pellets using a two-dimensional particle-in-cell model is described. The global geometry consists of a 50 mum diameter pellet surrounded by a corona which is isolated by a vacuum region from the boundary. For cone-attached targets, as much as 67% of the incident laser energy is absorbed with 12% sent forward as fast electrons in a 23 degrees cone. The current filaments are driven by the Weibel instability of the forward-going fast electron flux and its return current with the ions playing an important role of neutralizing the space charge. No global current filament coalescence has been observed. The electron distribution function obeys a power law, which begins at E approximately 0.2 MeV and falls off as E-(2-3).

Journal Article↗

PubMed2004

Near-GeV-energy laser-wakefield acceleration of self-injected electrons in a centimeter-scale plasma channel.

The first three-dimensional, particle-in-cell (PIC) simulations of laser-wakefield acceleration of self-injected electrons in a 0.84 cm long plasma channel are reported. The frequency evolution of the initially 50 fs (FWHM) long laser pulse by photon interaction with the wake followed by plasma dispersion enhances the wake which eventually leads to self-injection of electrons from the channel wall. This first bunch of electrons remains spatially highly localized. Its phase space rotation due to slippage with respect to the wake leads to a monoenergetic bunch of electrons with a central energy of 0.26 GeV after 0.55 cm propagation. At later times, spatial bunching of the laser enhances the acceleration of a second bunch of electrons to energies up to 0.84 GeV before the laser pulse intensity is significantly reduced.

Journal Article↗

PubMed2003

Plasma channels produced by a laser-triggered high-voltage discharge.

A plasma waveguide scheme for high-intensity laser guiding with densities and lengths suitable for laser-plasma particle accelerators is presented. This scheme uses a laser-triggered high-voltage discharge, presents negligible jitter, allows full access to the plasma, and can be scaled to large distances. Experimental results showing the feasibility of this scheme are presented.

Journal Article↗

PubMed2002

Photon acceleration of ultrashort laser pulses by relativistic ionization fronts.

We present experimental results from the collision of weak ultrashort pulses with relativistic ionization fronts in copropagation and counterpropagation. The observed frequency upshifts of the probe pulses provide not only information about the electron density of the ionization front but also reveal the fine structure of the front. The connection between the correlation lengths for copropagation and counterpropagation and the longitudinal and transverse dimensions of the ionization front is also demonstrated thus showing the feasibility of using the frequency upshift experienced by short probe pulses to fully characterize relativistic ionization fronts and other relativistic coherent structures in laser-produced plasmas.

Journal Article↗

PubMed2002

Two-dimensional collision of probe photons with relativistic ionization fronts.

The collision of a probe laser pulse with a relativistic ionization front is analyzed via two-dimensional ray-tracing theory and simulations. It is shown that collisions in higher dimensions lead to new regimes for the frequency upshift of the probe photons; the frequency upshift can be considerably higher for particular collision angles that maximize the interaction length with the ionization front gradient. Finite ionization fronts also lead to angle-dependent frequency upshifts, thus acting as diffraction gratings.

Journal Article↗

PubMed2001

Generation of ultra-intense single-cycle laser pulses by using photon deceleration.

A scheme to generate single-cycle laser pulses is presented based on photon deceleration in underdense plasmas. This robust and tunable process is ideally suited for lasers above critical power because it takes advantage of the relativistic self-focusing of these lasers and the nonlinear features of the plasma wake. The mechanism is demonstrated by particle-in-cell simulations in three and 2(1/2) dimensions, resulting in pulse shortening up to a factor of 4, thus making it feasible to generate few-femtosecond single-cycle pulses in the optical to IR domain with intensities I > 10(20) W/cm(2) by using present-day laser technology.

Lasers↗