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B J Albright,
L Yin,
B M Hegelich,
K J Bowers,
C Huang,
A Henig,
J C Fernández,
K A Flippo,
S A Gaillard, T J T Kwan,
X Q Yan,
T Tajima,
D Habs
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ABSTRACT: A simple model has been derived for expansion of a thin (up to 100s of nm thickness) target initially of solid density irradiated by an ultraintense laser. In this regime, ion acceleration mechanisms, such as the Break-Out Afterburner (BOA) [1], emerge with the potential for dramatically improved energy, efficiency, and energy spread. Ion beams have been proposed [2] as drivers for fast ignition inertial confinement fusion [3]. Analysis of kinetic simulations of the BOA shows the period of enhanced acceleration occurs between times t1, when the target becomes relativistically transparent to the laser, and t2, when the target becomes classically underdense and the enhanced acceleration terminates. A simple model for target expansion has been derived that contains early, one-dimensional (1D) expansion of the target and three-dimensional (3D) expansion at late times. The model assumes expansion is slab-like at the instantaneous ion sound speed and requires as input target composition, laser intensity, laser spot area, and the efficiency of laser absorption into electron thermal energy.
Journal of Physics Conference Series 09/2010; 244(4):042022.
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ABSTRACT: VPIC [1], a first-principles 3d electromagnetic charge-conserving relativistic kinetic particle-in-cell code, was recently adapted to run on Los Alamos's Roadrunner [2], the first supercomputer to break a petaflop (1015 floating point operations per second) in the TOP500 supercomputer performance rankings. [3] We summarize VPIC's modeling capabilities, VPIC's optimization techniques and Roadrunner's computational characteristics. We then discuss three applications enabled by VPIC's unprecedented performance on Roadrunner: modeling laser plasma interaction in upcoming inertial confinement fusion experiments at the National Ignition Facility, modeling short-pulse laser GeV ion acceleration and modeling reconnection in space and laboratory plasmas.
Journal of Physics Conference Series 08/2009; 180(1):012055.
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ABSTRACT: Recent innovations in large-scale 3-D kinetic simulations along with advanced visualization techniques are facilitating new scientific discoveries into the basic physics of collisionless magnetic reconnection. Present supercomputers are now fully capable of exploring the dynamics of large-scale electron-positron plasmas, whereas the next generation will extend this capability to allow first-principle simulations of magnetic reconnection in electron-proton plasmas.
IEEE Transactions on Plasma Science 09/2008; · 1.17 Impact Factor
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ABSTRACT: A new laser-driven ion acceleration mechanism using ultrathin targets has been identified from particle-in-cell simulations. After a brief period of target normal sheath acceleration TNSA S. P. Hatchett et al., Phys. Plasmas 7, 2076 2000, two distinct stages follow: first, a period of enhanced TNSA during which the cold electron background converts entirely to hot electrons, and second, the "laser breakout afterburner" BOA when the laser penetrates to the rear of the target where a localized longitudinal electric field is generated with the location of the peak field co-moving with the ions. During this process, a relativistic electron beam is produced by the ponderomotive drive of the laser. This beam is unstable to a relativistic Buneman instability, which rapidly converts the electron energy into ion energy. This mechanism accelerates ions to much higher energies using laser intensities comparable to earlier TNSA experiments. At a laser intensity of 10 21 W/cm 2 , the carbon ions accelerate as a quasimonoenergetic bunch to 100 s of MeV in the early stages of the BOA with conversion efficiency of order a few percent. Both are an order of magnitude higher than those realized from TNSA in recent experiments Hegelich et al., Nature 441, 439 2006. The laser-plasma interaction then evolves to produce a quasithermal energy distribution with maximum energy of 2 GeV. © 2007 American Institute of Physics.
Physics of Plasmas 05/2007; 14(14):056706. · 2.15 Impact Factor
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ABSTRACT: A new laser-driven ion acceleration mechanism has been identified in particle-in-cell simulations of high-contrast-ratio ultra-intense lasers with very thin (10s of nm) solid targets [Yin et al., Laser and Particle Beams 24, 291 (2006); Yin et al., "Monoenergetic and GeV ion acceleration from the laser break-out afterburner using ultrathin targets," Phys. Plasmas (in press)]. After a brief period of target normal sheath acceleration (TNSA), "enhanced" TNSA follows. In this stage, the laser rapidly heats all the electrons in the target as the target thickness becomes comparable to the skin depth and enhanced acceleration of the ions results. Then, concomitant with the laser penetrating the target, a large accelerating longituidinal electric field is generated that co-moves with the ions. This last phase has been termed the laser "break-out afterburner" (BOA). Earlier work suggested that the BOA was associated with the Buneman instability that efficiently converts energy from the drift of the electrons into the ions. In this Letter, this conjecture is found to be consistent with particle-in-cell simulation data and the analytic dispersion relation for the relativistic Buneman instability.
Physics of Plasmas 03/2007; 14(14):094501. · 2.15 Impact Factor
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ABSTRACT: Experiments at the LANL Trident facility demonstrated the production of monoenergetic ion beams from the interaction of an ultraintense laser with a target comprising a heavy ion substrate and thin layer of light ions. An analytic model is obtained that predicts how the mean energy and quality of monoenergetic ion beams and the energy of substrate ions vary with substrate material and light-ion layer composition and thickness. Dimensionless parameters controlling the dynamics are derived and the model is validated with particle-in-cell simulations and experimental data.
Physical Review Letters 10/2006; 97(11):115002. · 7.37 Impact Factor
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ABSTRACT: In this work, two-dimensional particle-in-cell simulations are used to examine the electron physics in the rod-pinch diode, a device that can be used to produce a relatively low-energy (a few MeV) radiographic electron source. It is found that with diode parameters for which the electrons’ dominant dynamics are approximated well as a magnetized fluid, the diode produces an electron source with a desired small spot size as the electrons drift to and impinge on the anode tip. However, for a large cathode-to-anode radius ratio, a population of electrons that consists predominantly of electrons emitted from the downstream surface of the cathode is found to propagate in the upstream direction and the diode may perform anomalously as a consequence. A method is proposed for improving the quality of the electron source by suppressing electron emission from the downstream cathode surface to reduce the presence of unmagnetized electrons.
Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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W. S. Koh, L. K. Ang,
S. P. Lau, T. J. T. Kwan
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ABSTRACT: This paper presents a quantum model of space-charge limited (SCL) bipolar flow in a nano-sized planar gap, including the effects of electron tunneling and exchange-correlation. It is found that the classical scaling of the SCL bipolar flow is no longer valid when the gap spacing D is comparable or smaller than the electron's de Broglie wavelength at gap voltage Vg. The classical value of the SCL bipolar electron flow is greatly enhanced due to the electron tunneling through the space-charge electric potential created by both the electrons and ions. The space-charge effect of ions is less significant (compared to electron tunneling) in the deep quantum regime that the quantum SCL bipolar flow is nearly identical to the unipolar electron flow (or quantum Child-Langmuir law).
Applied Physics Letters - APPL PHYS LETT. 01/2005; 87.
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ABSTRACT: A simple derivation of the new scaling of Child-Langmuir law in the quantum regime is presented. Based on a dimensional argument of the Schrodinger equation and the Poisson equation, the limiting current in the deeply quantum regime is found to be proportional to the square root of the gap voltage and to the inverse fourth power of gap spacing. The importance of electron exchange-correlation interactions in the quantum regime is discussed.
IEEE Transactions on Plasma Science 05/2004; · 1.17 Impact Factor
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ABSTRACT: This paper presents a consistent quantum mechanical model of Child-Langmuir (CL) law, including electron exchange-correlation interaction, electrode's surface curvature, and finite emitter area. The classical value of the CL law is increased by a larger factor due to the electron tunneling through the space-charge potential, and the electron exchange-correlation interaction becomes important when the applied gap voltage Vg and the gap spacing D are, respectively, on the order of Hartree energy level, and nanometer scale. It is found that the classical scaling of Vg(3/2) and D(-2) is no longer valid in the quantum regime, and a new scaling of Vg(1/2) and D(-4) is established. The smooth transition from the classical regime to the quantum regime is also demonstrated.
Physical Review Letters 12/2003; 91(20):208303. · 7.37 Impact Factor
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ABSTRACT: The chain model for x-ray flash radiography (Ref. 1) developed at Los Alamos is an integrated simulation capability consisting of linked codes for the various physical processes that model an en$= radiographic event. Two new features have been added to the computational chaiwmodel: (1) a link between accelerator and particle-in-cell codes, enabling accelerated electrons to be injected into a 2-D, relativistic, fully electromagnetic particle-in-cell (PIC) code and propagated to a bremsstrahlung converter target, and (2) a distribution-functio'n capability to create electron sources from PIC simulations for use in Monte Carlo electroxdphoton transport calculations to produce synthetic radiographs. Physical variables of electrons from PIC calculations are binned to produce distribution functions, which can be randomly sampled to obtain source particles for Monte Carlo transport calculations through a bremsstrahlung converter target. Several methods of binning have been used to construct both correlated and uncorrelated distributions. We will present end-to-end simulations of the radiographic process in order to compare synthetic radiographs produced using several electron distribution functions and analoglike links. In addition, we studied the effects of different electron distributions on photon spectra, doses, and spot sizes produced from a converter target. Advantages and disadvantages of the different techniques will be discussed, and applications of the chain model will be presented.
12/2001
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T.J.T. Kwan
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ABSTRACT: We have revisited the physical mechanisms, which lead to a time
delay of the onset of the undesirable spot size growth in high-dose
X-ray radiography. In our theoretical model, the delay time is partially
due to the time needed for the electron beam to thermally ionize the
target spot resulting in target ions and partially due to the time
needed for the ions to form a column of at least a quarter of the
betatron wavelength of the electron beam, Based on our model, an
analytic formula is derived, and it has been found to be consistent with
experimental observations for different target materials and beam
parameters. Our analysis shows that the delay time is directly
proportional to the beam spot size at the target and inversely
proportional to the beam current. Our calculations further show the spot
size of the DARHT-1 beam with 2 kA current should be stable because the
delay time of about 69 ns is longer then the beam pulse length
IEEE Transactions on Plasma Science 03/2000; · 1.17 Impact Factor
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ABSTRACT: Summary form only given. Next generation X-ray radiography
machines must use high electron beam current to provide the necessary
dose and very small spot size to achieve the desired optical resolution.
However, this combination of high current in a small area leads to
undesirable side effects that were not important in old machines.
Specifically, the intense local energy deposition from the high
intensity electron beam causes vaporization of the bremsstrahlung
target. The hot plasma thus generated provides a copious source of
positive ions that are rapidly accelerated into the negative potential
well of the incoming electron beam. As the ions propagate upstream, they
partially charge neutralize the electron beam, causing its spot size to
increase. We have studied the electron beam spot stabilization for the
Dual Axis Radiographic Hydrotest Facility (DARHT)
Plasma Science, 1999. ICOPS '99. IEEE Conference Record - Abstracts. 1999 IEEE International Conference on; 02/1999
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M.J. Burns,
B.E. Carlsten, T.J.T. Kwan,
D.C. Moir,
D.S. Prono,
S.A. Watson,
E.L. Burgess,
H.L. Rutkowski,
G.J. Caporaso,
Y.-J. Chen,
S. Sampayan,
G. Westenskow
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ABSTRACT: The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility
will use two perpendicular electron linear induction accelerators to
produce intense, bremsstrahlung X-ray pulses for flash radiography. We
intend to produce measurements containing 3D information with
sub-millimeter spatial of the interior features of very dense
explosively driven objects. The facility will be completed in two phases
with the first operational by June 1999 utilizing a single-pulse,
19.8-MeV, 2 to 4-kA, 60-ns accelerator (activated in March 1999), a
high-resolution electro-optical X-ray imaging system, and other
hydrodynamics testing systems. The second phase will be operational by
Sept. 2002 and features the addition of a 20-MeV, 2 to 4-kA,
2-microsecond accelerator. Four short electron micropulses of variable
pulse-width and spacing will be chopped out of the original, long
accelerator pulse for producing time-resolved X-ray images. The second
phase also features an extended, high-resolution electro-optical X-ray
system with a framing speed of about 2-MHz. In this paper we present a
Figure-Of-Merit for a X-ray based flash radiography system to motivate
the selection of accelerator parameters. We will then present sub-system
performance measurements from Phase 1, the physics of the interaction of
our high-intensity beams with the X-ray conversion target, initial Phase
1 accelerator measurements (if available), and plans for operation. We
will also discuss designs and prototype testing results for the
2-microsecond Phase 2 accelerator, including prototype induction cells
and pulsed power, prototype kicker magnet performance to chop the beam,
and design considerations for a multipulse X-ray conversion target
Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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ABSTRACT: The onset of the growth of the electron beam spot due to partial
charge neutralization by ions extracted from the target plasma has been
predicted by theory and simulation. The concept of an electrically
self-biased target was developed to control the length of the ion column
and experiments were fielded on the Integrated Test Stand (ITS) for the
Dual Axis Radiographic Hydro Test (DARHT) facility at Los Alamos
National Laboratory. The experimental results confirmed the stability of
the spot size when the target is self-biased at a potential of 350 kV.
Our analyses and quantitative comparison between computer simulations
and experiments show that the ions generated by the electron beam were
from target materials and that the delay for onset of the spot growth
was governed by the time needed for the ionization of target material
and formation of the ion column with sufficient length
Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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ABSTRACT: Three methods of variable-weight statistical enhancement for Monte
Carlo semiconductor device simulation are compared. The steady-state
statistical errors and figures of merit for implementations of the
multicomb, cloning-rouletting, and splitting-gathering enhancement
methods are obtained for bulk silicon simulations. The results indicate
that all methods enhance the high-energy distribution tail with
comparable accuracy, but that the splitting-gathering method achieves a
lower error at low energies by automatically preserving a peak in the
bin populations at the peak of the particle energy distribution
IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems 01/1999; · 1.27 Impact Factor
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ABSTRACT: We adapt a multicomb variance reduction technique used in neutral
particle transport to Monte Carlo micro-electronic device modeling. We
implement the method in a two-dimensional (2-D) MOSFET device simulator
and demonstrate its effectiveness in the study of hot electron effects.
Our simulations show that the statistical variance of hot electrons is
significantly reduced with minimal computational cost. The method is
efficient, versatile, and easy to implement in existing device
simulators
IEEE Transactions on Electron Devices 05/1998; · 2.32 Impact Factor
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ABSTRACT: Summary form only given. The conversion of an intense relativistic
electron beam into X-rays for radiographic imaging is achieved through
the bremsstrahlung process of electrons in a target of optimal
thickness. To achieve desirable resolution for thick objects, an
extremely high-brightness electron beam is used, and a significant
amount of beam energy can be deposited in a small area of the target.
Vaporization of the target material and expansion of the resultant
plasma can occur. In a multi-pulsing design, which will resolve the
dynamic behavior of the object, the expanding plasma can have an effect
on the quality of subsequent electron beam pulses. The evolution of the
plasma was investigated using a two-dimensional Eulerian
magnetohydrodynamic code
Plasma Science, 1997. IEEE Conference Record - Abstracts., 1997 IEEE International Conference on; 06/1997
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ABSTRACT: Summary form only given. The conversion of an intense relativistic electron beam into X-rays for radiographic imaging is achieved through the bremsstrahlung process of electrons in a tantalum or tungsten target of some optimal thickness. A high-dose radiographic source with small spot size is needed to achieve desirable resolution for thick objects. Consequently, an extremely high brightness electron beam is used and a significant amount of electron beam energy can be deposited in a small area of the target. Vaporization of the target material and plasma generation can result. We describe a computational methodology used to model the beam-target interaction and the evolution of the resultant plasma. Several codes, including particle-in-cell (PIC), Monte Carlo transport, and magnetohydrodynamic (MHD) codes, contribute to simulate different parts of the problem in a linked fashion. Multi-dimensional PIC calculations provide detailed characterization of the beam transmitted to the target foil. Electron-photon Monte Carlo transport codes calculate beam scattering and energy deposition in the target. These energy source conditions are used in 1-D and 2-D MHD codes to model the foil expansion and the evolution of the target plasma issues addressed by the calculations include: the effects of the time dependence of the energy profile deposited in the target; the influence of the external magnetic field on plasma expansion; the influence of the expanding plasma on the guide magnetic field (and, consequently, on the beam quality); radiation effects; and multi-dimensional effects
Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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Plasma Science, 1994. Conference Record - Abstracts., 1994 IEEE International Conference on; 07/1994
