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Conservative differencing of the electron Fokker-Planck transport equation

Description: We need to extend the applicability and improve the accuracy of kinetic electron transport codes. In this paper, special attention is given to modelling of e-e collisions, including the dominant contributions arising from anisotropy. The electric field and spatial gradient terms are also considered. I construct finite-difference analogues to the Fokker-Planck integral-differential collision operator, which conserve the particle number, momentum and energy integrals (sums) regardless of the coarseness of the velocity zoning. Such properties are usually desirable, but are especially useful, for example, when there are spatial regions and/or time intervals in which the plasma is cool, so that the collision operator acts rapidly and the velocity distribution is poorly resolved, yet it is crucial that gross conservation properties be respected in hydro-transport applications, such as in the LASNEX code. Some points are raised concerning spatial differencing and time integration.
Date: January 12, 1981
Creator: Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Implicit plasma simulation

Description: Implicit time integration methods have been used extensively in numerical modelling of slowly varying phenomena in systems that also support rapid variation. Examples include diffusion, hydrodynamics and reaction kinetics. This article discussed implementation of implicit time integration in plasma codes of the ''particle-in-cell'' family, and the benefits to be gained.
Date: March 3, 1985
Creator: Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Electromagnetic ''particle-in-cell'' plasma simulation

Description: ''PIC'' simulation tracks particles through electromagnetic fields calculated self-consistently from the charge and current densities of the particles themselves, external sources, and boundaries. Already used extensively in plasma physics, such simulations have become useful in the design of accelerators and their r.f. sources. 5 refs.
Date: April 22, 1985
Creator: Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Electromagnetic direct implicit PIC simulation

Description: Interesting modelling of intense electron flow has been done with implicit particle-in-cell simulation codes. In this report, the direct implicit PIC simulation approach is applied to simulations that include full electromagnetic fields. The resulting algorithm offers advantages relative to moment implicit electromagnetic algorithms and may help in our quest for robust and simpler implicit codes.
Date: March 29, 1983
Creator: Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Electromagnetic effects in relativistic electron beam plasma interactions

Description: Electromagnetic effects excited by intense relativistic electron beams in plasmas are investigated using a two-dimensional particle code. The simulations with dense beams show large magnetic fields excited by the Weibel instability as well as sizeable electromagnetic radiation over a significant range of frequencies. The possible relevance of beam plasma instabilities to the laser acceleration of particles is briefly discussed. 6 refs., 4 figs.
Date: February 13, 1985
Creator: Kruer, W.L. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Compact torus compression of microwaves

Description: The possibility that a compact torus (CT) might be accelerated to large velocities has been suggested by Hartman and Hammer. If this is feasible one application of these moving CTs might be to compress microwaves. The proposed mechanism is that a coaxial vacuum region in front of a CT is prefilled with a number of normal electromagnetic modes on which the CT impinges. A crucial assumption of this proposal is that the CT excludes the microwaves and therefore compresses them. Should the microwaves penetrate the CT, compression efficiency is diminished and significant CT heating results. MFE applications in the same parameters regime have found electromagnetic radiation capable of penetrating, heating, and driving currents. We report here a cursory investigation of rf penetration using a 1-D version of a direct implicit PIC code.
Date: May 17, 1985
Creator: Hewett, D.W. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Solution of the field equations for 2-D electromagnetic direct implicit plasma simulation

Description: A direct implicit particle-in-cell (PIC) simulation model with full electromagnetic (EM) effects has been implemented in 2-D Cartesian geometry. The model, implemented with the D/sub 1/ time differencing scheme, was first implemented in a 1-D electrostatic (ES) version to gain some experience with spatial differencing in forms suitable for extension to the full EM field in two dimensions. The implicit EM field solve is considerably different from the implicit ES code. The EM field calculation requires an inductive part as well as the electrostatic and the B field must be self-consistently advanced.
Date: January 2, 1985
Creator: Hewett, D.W. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Profile modification and hot electron temperature from resonant absorption at modest intensity

Description: Resonant absorption is investigated in expanding plasmas. The momentum deposition associated with the ejection of hot electrons toward low density via wavebreaking readily exceeds that of the incident laser radiation and results in significant modification of the density profile at critical. New scaling of hot electron temperature with laser and plasma parameters is presented.
Date: October 13, 1980
Creator: Albritton, J.R. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Transport of a partially-neutralized ion beam in a heavy-ion fusion reactor chamber

Description: In a heavy-ion driven, inertial confinement fusion power plant, a space-charge dominated beam of heavy ions must be transported through a reactor chamber and focused on a 2-3 mm spot at the target. The spot size at the target is determined by the beam emittance and space charge, plus chromatic aberrations in the focusing lens system and errors in aiming the beam. The gain of the ICF capsule depends on the focal spot size. We are investigating low density, nearly-ballistic transport using an electromagnetic, r-z particle-in-cell code. Even at low density (n {approx} 5 {times} 10{sup 13} cm{sup {minus}3}), beam stripping may be important. To offset the effects of stripping and reduce the space charge, the beam is partially charge neutralized via a pre-formed plasma near the chamber entrance. Additional electrons for charge neutralization come from ionization of the background gas by the beam. Simulations have shown that stripping can greatly increase the spot size; however, partial neutralization can offset most of this increase.
Date: April 25, 1995
Creator: Callahan, D.A. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Effects of Ion-Ion Collisions and Inhomogeneity in Two-Dimensional Kinetic Ion Simulations of Stimulated Brillouin Backscattering

Description: Two-dimensional simulations with the BZOHAR [B.I. Cohen, B.F. Lasinski, A.B. Langdon, and E.A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability including the effects of ion-ion collisions and inhomogeneity. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities tend to decrease with increasing collisionality in the simulations. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models used the collisions are functions of the local ion temperature and density, but the collisions have no velocity dependence in the first model. In the second model, the collisions are also functions of the energy of the ion that is being scattered so as to represent a Fokker-Planck collision operator. Collisions decorrelate the ions from the acoustic waves in SBS, which disrupts ion trapping in the acoustic wave. Nevertheless, ion trapping leading to a hot ion tail and two-dimensional physics that allows the SBS ion waves to nonlinearly scatter remain robust saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. SBS backscatter in the presence of a spatially nonuniform plasma flow is also investigated. Simulations show that depending on the sign of the spatial gradient of the flow relative to the backscatter, ion trapping effects that produce a nonlinear frequency shift can enhance (auto-resonance) or decrease (anti-auto-resonance) reflectivities in agreement with theoretical arguments.
Date: October 17, 2005
Creator: Cohen, B I; Divol, L; Langdon, A B & Williams, E A
Partner: UNT Libraries Government Documents Department

Kinetic Enhancement of Raman Backscatter, and Electron Acoustic Thomson Scatter

Description: 1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. For the first time, a low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single hot spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a Gauss-Hermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly-damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.
Date: September 1, 2006
Creator: Strozzi, D J; Williams, E A; Langdon, A B & Bers, A
Partner: UNT Libraries Government Documents Department

Vlasov Simulations of Trapping and Inhomogeneity in Raman Scattering

Description: We study stimulated Raman scattering (SRS) in laser-fusion conditions with the Eulerian Vlasov code ELVIS. Back SRS from homogeneous plasmas occurs in subpicosecond bursts and far exceeds linear theory. Forward SRS and re-scatter of back SRS are also observed. The plasma wave frequency downshifts from the linear dispersion curve, and the electron distribution shows flattening. This is consistent with trapping and reduces the Landau damping. There is some acoustic ({omega} {proportional_to} {kappa}) activity and possibly electron acoustic scatter. Kinetic ions do not affect SRS for early times but suppress it later on. SRS from inhomogeneous plasmas exhibits a kinetic enhancement for long density scale lengths. More scattering results when the pump propagates to higher as opposed to lower density.
Date: August 9, 2005
Creator: Strozzi, D; Shoucri, M M; Williams, E A & Langdon, A B
Partner: UNT Libraries Government Documents Department

Electron thermal transport and short-pulsed laser experiments

Description: The goal of this LDRD project is to provide theory for the LLNL ultra-short pulse laser experiments. The goal includes analysis of the experiments performed and help with planning new experiments. this final report we describe, the technical challenges we faced and he success we had with this project.
Date: March 18, 1996
Creator: More, R.M.; Rosen, M.D. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Collisionless absorption of light waves incident on overdense plasmas with steep density gradients

Description: Collisionless absorption of laser light incident on overdense plasmas with steep density gradients is studied analytically and numerically. For the normal incidence case, it is shown that both sheath inverse bremsstrahlung and the anomalous skin effect are limiting cases of the same collisionless absorption mechanism. Using particle-in-cell (PIC) plasma simulations, the effects of finite sheath-transit time and finite density gradient are investigated. The analyses are extended to oblique incident cases. For p-polarized obliquely incident light, the results are significantly different from those for the normal incidence case. Most noticeable is the absorption enhancement for the p-polarized light due to the interaction of the electrons with the normal (parallel to the density gradient) component of the laser electric field in the sheath region.
Date: July 31, 1995
Creator: Yang, T. Y. B.; Kruer, W. L. & Langdon, A. B.
Partner: UNT Libraries Government Documents Department

Correction of longitudinal errors in accelerators for heavy-ion fusion

Description: Longitudinal space-charge waves develop on heavy-ion inertial-fusion pulse from initial mismatches or from inappropriately timed or shaped accelerating voltages. Without correction, waves moving backward along the beam can grow due to the interaction with their resistively retarded image fields, eventually degrading the longitudinal emittance. A simple correction algorithm is presented here that uses a time-dependent axial electric field to reverse the direction of backward-moving waves. The image fields then damp these forward-moving waves. The method is demonstrated by fluid simulations of an idealized inertial-fusion driver, and practical problems in implementing the algorithm are discussed.
Date: June 10, 1993
Creator: Sharp, W. M.; Callahan, D. A.; Barnard, J. J.; Langdon, A. B. & Fessenden, T. J.
Partner: UNT Libraries Government Documents Department

Laser-Plasma Interactions in High-Energy Density Plasmas

Description: Laser-plasma interactions (LPI) have been studied experimentally in high-temperature, high-energy density plasmas. The studies have been performed using the Omega laser at the Laboratory for Laser Energetics (LLE), Rochester, NY. Up to 10 TW of power was incident upon reduced-scale hohlraums, distributed in three laser beam cones. The hot hohlraums fill quickly with plasma. Late in the laser pulse, most of the laser energy is deposited at the laser entrance hole, where most of the LPI takes place. Due to the high electron temperature, the stimulated Raman scattering (SRS) spectrum extends well beyond {omega}{sub 0}/2, due to the Bohm-Gross shift. This high-temperature, high-energy density regime provides a unique opportunity to study LPI beyond inertial confinement fusion (ICF) conditions.
Date: August 24, 2005
Creator: Constantin, C G; Baldis, H A; Schneider, M B; Hinkel, D E; Langdon, A B; Seka, W et al.
Partner: UNT Libraries Government Documents Department

Development and Application of a Predictive Computational Tool for Short-Pulse, High-Intensity Target Interactions

Description: The widely differing spatial, temporal, and density scales needed to accurately model the fast ignition process and other short-pulse laser-plasma interactions leads to a computationally challenging project that is difficult to solve using a single code. This report summarizes the work performed on a three year LDRD to couple together three independent codes using PYTHON to build a new integrated computational tool. An example calculation using this new model is described.
Date: January 26, 2007
Creator: Town, R J; Chung, H; Langdon, A B; Lasinski, B F; Lund, S M; McCandless, B C et al.
Partner: UNT Libraries Government Documents Department

Filamentation and Forward Brillouin Scatter of Entire Smoothed and Aberrated Laser Beams

Description: Laser-plasma interactions are sensitive to both the fine-scale speckle and the larger scale envelope intensity of the beam. For some time, simulations have been done on volumes taken from part of the laser beam cross-section, and the results from multiple simulations extrapolated to predict the behavior of the entire beam. However, extrapolation could very well miss effects of the larger scale structure on the fine-scale. The only definitive method is to simulate the entire beam. These very large calculations have been infeasible until recently, but they are now possible on massively parallel computers. Whole beam simulations show the dramatic difference in the propagation and break up of smoothed and aberrated beams.
Date: October 29, 1999
Creator: Still, C.H.; Berger, R.L.; Langdon, A.B.; Hinkel, D.E. & Williams, E.A.
Partner: UNT Libraries Government Documents Department

An MPP hydrocode to study laser-plasma interactions

Description: Because of the increased size and power inherent in a laser-AGEX on NIF, laser-plasma interactions (LPI) observed in NOVA AGEX play an increasingly important role. The process by which filamentation and stimulated backscatter grow is complex. Furthermore, there is a competition among the instabilities so that lessening one can increase another. Therefore, simulating them is an integral part to successful experiments on NIF. In this paper, we present a massively parallel hydrocode to simulate laser-plasma interactions in NIF-relevant AGEX regimes.
Date: October 1, 1998
Creator: Berger, R L; Langdon, A B; Langer, S H; Still, C H; Suter, L J & A, Williams E
Partner: UNT Libraries Government Documents Department

Propagation of realistic beams in underdense plasma

Description: The effect of beam structure on propagation through underdense plasma is examined in two different examples. First, it is shown that the distribution of intensities within a laser beam affects how the beam deflects in the presence of transverse plasma flow. A detailed analysis of beam deflection shows that the rate scales linearly with intensity and plasma density, and inversely with plasma temperature. When the plasma flow is subsonic, the deflection rate is proportional to the ion damping decrement, and scales as M/(1 - M{sup 2}){sup 3/2}, where M is the transverse flow Mach number. When the plasma flow is supersonic, the deflection rate scales as 1/[M(M{sup 2} - 1){sup 1/2}]. Next, the effect of beam structure on channel formation by very intense laser beer is studied. A diffraction-limited beam with 40 TW of input power forms a channel through 4OOpm of plasma, whereas when this beam is phase aberrated, channel formation does not occur.
Date: November 10, 1997
Creator: Hinkel, D.E.; Williams, E.A.; Berger, R.L.; Powers, L.V.; Langdon, A.B. & Still, C.H.
Partner: UNT Libraries Government Documents Department

Light absorption and scattering mechanisms in laser fusion plasmas

Description: The picture of laser light absorption and scattering which is emerging from theory and computer simulation studies of laser-plasma interactions is described. On the subject of absorption, we discuss theoretical and experimental evidence that resonance absorption in a steepened density profile is a dominant absorption mechanism. Recent work also indicates the presence of critical surface ripples, which we study using two and three dimensional computer simulations. Predictions of hot electron spectra due to resonance absorption are described, as are effects of plasma outflow. We then discuss two regimes where stimulated scattering may occur. Brillouin scattering is expected in the underdense target blow-off, for long laser pulses, and is limited by ion heating. Raman scattering in the background gas of a reactor target chamber is predicted to be at most a 10 percent effect for 1 ..mu..m lasers.
Date: October 4, 1977
Creator: Barnes, C.; Estabrook, K.G.; Kruer, W.L.; Langdon, A.B.; Lasinski, B.F.; Max, C.E. et al.
Partner: UNT Libraries Government Documents Department