569 research outputs found
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma
Elimination of the numerical Cerenkov instability for spectral EM-PIC codes
When using an electromagnetic particle-in-cell (EM-PIC) code to simulate a
relativistically drifting plasma, a violent numerical instability known as the
numerical Cerenkov instability (NCI) occurs. The NCI is due to the unphysical
coupling of electromagnetic waves on a grid to wave-particle resonances,
including aliased resonances, i.e., , where and refer to the time and space
aliases and the plasma is drifting relativistically at velocity in the
-direction. Recent studies have shown that an EM-PIC code which uses a
spectral field solver and a low pass filter can eliminate the fastest growing
modes of the NCI. Based on these studies a new spectral PIC code for studying
laser wakefield acceleration (LWFA) in the Lorentz boosted frame was developed.
However, we show that for parameters of relevance for LWFA simulations in the
boosted frame, a relativistically drifting plasma is susceptible to a host of
additional unstable modes with lower growth rates, and that these modes appear
when the fastest growing unstable modes are filtered out. We show that these
modes are most easily identified as the coupling between modes which are purely
transverse (EM) and purely longitudinal (Langmuir) in the rest frame of the
plasma for specific time and space aliases. We rewrite the dispersion relation
of the drifting plasma for a general field solver and obtain analytic
expressions for the location and growth rate for each unstable mode, i.e, for
each time and space aliased resonances. We show for the spectral solver that
when the fastest growing mode is eliminated a new mode at the fundamental
resonance () can be seen. (Please check the whole abstract in the
paper).Comment: 36 pages, 12 figure
Laser-driven shock acceleration of monoenergetic ion beams
We show that monoenergetic ion beams can be accelerated by moderate Mach
number collisionless, electrostatic shocks propagating in a long scale-length
exponentially decaying plasma profile. Strong plasma heating and density
steepening produced by an intense laser pulse near the critical density can
launch such shocks that propagate in the extended plasma at high velocities.
The generation of a monoenergetic ion beam is possible due to the small and
constant sheath electric field associated with the slowly decreasing density
profile. The conditions for the acceleration of high-quality, energetic ion
beams are identified through theory and multidimensional particle-in-cell
simulations. The scaling of the ion energy with laser intensity shows that it
is possible to generate MeV proton beams with state-of-the-art 100
TW class laser systems.Comment: 13 pages, 4 figures, accepted for publication in Physical Review
Letter
Mitigation of numerical Cerenkov radiation and instability using a hybrid finite difference-FFT Maxwell solver and a local charge conserving current deposit
A hybrid Maxwell solver for fully relativistic and electromagnetic (EM)
particle-in-cell (PIC) codes is described. In this solver, the EM fields are
solved in space by performing an FFT in one direction, while using finite
difference operators in the other direction(s). This solver eliminates the
numerical Cerenkov radiation for particles moving in the preferred direction.
Moreover, the numerical Cerenkov instability (NCI) induced by the
relativistically drifting plasma and beam can be eliminated using this hybrid
solver by applying strategies that are similar to those recently developed for
pure FFT solvers. A current correction is applied for the charge conserving
current deposit to correctly account for the EM calculation in hybrid Yee-FFT
solver. A theoretical analysis of the dispersion properties in vacuum and in a
drifting plasma for the hybrid solver is presented, and compared with PIC
simulations with good agreement obtained. This hybrid solver is applied to both
2D and 3D Cartesian and quasi-3D (in which the fields and current are
decomposed into azimuthal harmonics) geometries. Illustrative results for laser
wakefield accelerator simulation in a Lorentz boosted frame using the hybrid
solver in the 2D Cartesian geometry are presented, and compared against results
from 2D UPIC-EMMA simulation which uses a pure spectral Maxwell solver, and
from OSIRIS 2D lab frame simulation using the standard Yee solver. Very good
agreement is obtained which demonstrates the feasibility of using the hybrid
solver for high fidelity simulation of relativistically drifting plasma with no
evidence of the numerical Cerenkov instability
Collisionless shock acceleration of narrow energy spread ion beams from mixed species plasmas using 1 m lasers
Collisionless shock acceleration of protons and C ions has been
achieved by the interaction of a 10 W/cm, 1 m laser with a
near-critical density plasma. Ablation of the initially solid density target by
a secondary laser allowed for systematic control of the plasma profile. This
enabled the production of beams with peaked spectra with energies of 10-18
MeV/a.m.u. and energy spreads of 10-20 with up to 3x10 particles within
these narrow spectral features. The narrow energy spread and similar velocity
of ion species with different charge-to-mass ratio are consistent with
acceleration by the moving potential of a shock wave. Particle-in-cell
simulations show shock accelerated beams of protons and C ions with
energy distributions consistent with the experiments. Simulations further
indicate the plasma profile determines the trade-off between the beam charge
and energy and that with additional target optimization narrow energy spread
beams exceeding 100 MeV/a.m.u. can be produced using the same laser conditions.Comment: Accepted for publication in Physical Review Accelerators and Beam
One-to-one full scale simulations of laser wakefield acceleration using QuickPIC
We use the quasi-static particle-in-cell code QuickPIC to perform full-scale,
one-to-one LWFA numerical experiments, with parameters that closely follow
current experimental conditions. The propagation of state-of-the-art laser
pulses in both preformed and uniform plasma channels is examined. We show that
the presence of the channel is important whenever the laser self-modulations do
not dominate the propagation. We examine the acceleration of an externally
injected electron beam in the wake generated by 10 J laser pulses, showing that
by using ten-centimeter-scale plasma channels it is possible to accelerate
electrons to more than 4 GeV. A comparison between QuickPIC and 2D OSIRIS is
provided. Good qualitative agreement between the two codes is found, but the 2D
full PIC simulations fail to predict the correct laser and wakefield
amplitudes.Comment: 5 pages, 5 figures, accepted for publication IEEE TPS, Special Issue
- Laser & Plasma Accelerators - 8/200
HISTÓRIA SOCIAL DA HANSENÍASE: UM ESTUDO COM EX-PACIENTES DA COLÔNIA SANTA MARTA/GOIÁS
Introdução e objetivos: A hanseníase é uma doença infectocontagiosa causada pelo Mycobacterium leprae. Nos nervos periféricos, o bacilo se aloja e provoca lesões que causam perda de sensibilidade térmica, dolorosa e tátil, podendo evoluir para incapacidades físicas permanentes3. O isolamento compulsório em Hospitais-Colônia foi a principal medida de controle da doença no passado1. O trabalho objetivou contextualizar a história social da hanseníase pelo viés de ex-pacientes que viveram na Colônia Santa Marta, correlacionando o isolamento social e questionar a responsabilidade do Estado perante aos ex-internos. Metodologia: Questionários com perguntas semiestruturadas, fotografias e diário de campo com observação participante. Resultados e discussões: A Colônia foi reestruturada em Hospital de Dermatologia Sanitária e as moradias em que alojam ex-pacientes da Colônia são organizadas na Vila Santa Marta. Dos ex-pacientes entrevistados, 80% apresentaram sequelas incapacitantes e 100% alegaram ter no preconceito sua principal dificuldade em viver a Hanseníase. Observou-se que o isolamento compulsório contribuiu para a consolidação do estigma da doença, que impediu a reinserção destes na sociedade mesmo após a cura da doença2. Verificou-se que as pessoas com sequelas incapacitantes têm necessidades especiais e encontram em estado de abandono, tanto pelo Estado como pela Santa Marta, sem assistência médico-hospitalar. Infelizmente as ações do Ministério da Saúde são apenas vinculadas a prevenção e tratamento da hanseníase e não contemplam os hospitais-colônia e as pessoas que viveram confinadas por décadas4. Conclusões: O isolamento foi uma medida de Estado e este deveria se responsabilizar pelo cuidado dos ex-internos, assim como estabelecer Políticas Públicas de enfretamento do preconceito. O trabalho é um compromisso com a dignidade das pessoas que são hoje idosas e se encontram em estado de abandono pelo poder público
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