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R A Fonseca

Publications and source records attributed to R A Fonseca.

14 recordsLinked to original sources
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

Hose instability and wake generation by an intense electron beam in a self-ionized gas.

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

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 wakefield acceleration in self-ionized gas or plasmas.

Tunnel ionizing neutral gas with the self-field of a charged particle beam is explored as a possible way of creating plasma sources for a plasma wakefield accelerator [Bruhwiler et al., Phys. Plasmas (to be published)]. The optimal gas density for maximizing the plasma wakefield without preionized plasma is studied using the PIC simulation code OSIRIS [R. Hemker et al., in Proceeding of the Fifth IEEE Particle Accelerator Conference (IEEE, 1999), pp. 3672-3674]. To obtain wakefields comparable to the optimal preionized case, the gas density needs to be seven times higher than the plasma density in a typical preionized case. A physical explanation is given.

Journal Article↗
PubMed2001

Optical coherence tomography of bleb-like subretinal lesions after retinal reattachment surgery.

PURPOSE: To report the optical coherence tomographic characteristics of persistent bleb-like subretinal lesions after retinal reattachment surgery, including scleral buckling and cryopexy. METHODS: Case reports. RESULTS: In two eyes of two patients yellow to yellowish-orange bleb-like lesions were found in the posterior pole after retinal reattachment surgery. The common aspects of the surgery were scleral buckling procedures and cryopexy. There were subretinal precipitates in some of the lesions. The lesions showed no associated leakage during fluorescein or indocyanine green angiography. Optical coherence tomography demonstrated that the bleb-like lesions were tiny retinal detachments. CONCLUSION: Optical coherence tomography examination of bleb-like subretinal lesions showed that they were actually retinal detachments. Our patients and previously reported patients developed these lesions after scleral buckling and cryopexy. These lesions may represent persistent retinal detachment because of the presence of protein in the subretinal fluid.

Adult↗
PubMed2001

Combined hamartoma of the retina and retinal pigment epithelium associated with juvenile nasopharyngeal angiofibroma.

PURPOSE: To report a patient who had concomitant combined hamartoma of the retinal pigment epithelium and retina and juvenile nasopharyngeal angiofibroma. METHOD: Case report. RESULTS: An 18-year-old man presented with a juvenile nasopharyngeal angiofibroma and a combined hamartoma of the retina and retinal pigment epithelium in the ipsilateral eye. The diagnosis of juvenile nasopharyngeal angiofibroma was confirmed by histopathology, and the diagnosis of combined hamartoma was established by typical ophthalmoscopic and fluorescein angiographic findings. CONCLUSION: Juvenile nasopharyngeal angiofibroma may be associated with combined hamartoma of the retina and retinal pigment epithelium.

Adolescent↗
PubMed2000

Role of reinsertion of the lower eyelid retractor on involutional entropion.

AIMS: To verify and evaluate the effect of reinsertion of the lower eyelid retractor aponeurosis to correct involutional entropion. METHODS: The involutional entropion is one affection that occurs mainly in the lower eyelid of patients over 60 years old. The surgical techniques proposed to correct this condition are based on correction of horizontal laxity-the preseptal orbicularis muscle overrides the pretarsal muscle, and the reinsertion of the lower eyelid retractor aponeurosis. 30 patients clinically diagnosed with involutional entropion and randomly selected underwent reinsertion of the lower eyelid retractor aponeurosis to the tarsal plate, without horizontal shortening or resection of the skin or orbicularis muscle. RESULTS: Good anatomical and functional correction was achieved in 96.6% of the patients and no recurrence was observed on 29 month follow up examination. The surgical result was very satisfactory. CONCLUSIONS: It was concluded that this procedure is effective and has low recurrence rate, showing the important role of the reinsertion of the lower eyelid retractor aponeurosis in this surgical correction.

Aged↗