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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

Stimulated brillouin backscatter of a short-pulse laser

Description: Stimulated Brillouin backscattering (SBBS) from a short-pulse laser, where the pulse length is short compared to the plasma length, is found to be qualitatively different than in the long pulse regime, where the pulse length is long compared to the plasma length. We find that after an initial transient of order the laser pulse length transit time, the instability reaches a steady state in the variables x{prime} = x {minus} V{sub g}t, t{prime} = t, where V{sub g} is the pulse group velocity. In contrast, SBBS in a long pulse can be absolutely unstable and grows indefinitely, or until nonlinearities intervene. We find that the motion of the laser pulse induces Doppler related effects that substantially modify the backscattered spectrum at higher intensities, where the instability is strongly coupled (i.e. , has a growth rate large compared to the ion acoustic frequency).
Date: November 3, 1994
Creator: Hinkel, D.E.; Williams, E.A. & Berger, R.L.
Partner: UNT Libraries Government Documents Department

Crossed Beam Energy Transfer in the NIF ICF Target Design

Description: In the National Ignition Facility (NIF) ICF point design, the cylindrical hohlraum target is illuminated by multiple laser beams through two laser entrance holes on the ends. According to simulations by LASNEX and HYDRA plasma created inside the hohlraum will stream out of the LEH, accelerate to supersonic speeds and then fan out radially. Inside the hohlraum, flows are subsonic. Forward Brillouin scattering can transfer energy between pairs of laser beams (0 and 1) if the following frequency matching condition is satisfied: {omega}{sub 0} - {omega}{sub 1} = (k{sub 0} - k{sub 1}) {center_dot} V + |k{sub 0} - k{sub 1}| c{sub s} (1) where {omega}{sub 0.1} and k{sub 0.1} are the frequencies and wave-numbers of the two laser beams, V is the plasma flow velocity and c{sub s} is the local ion sound speed. In the nominal case of equal frequency beams, this requires the component of the plasma flow velocity transverse to the bisector of the beam directions to be sonic, with the resulting transfer being to the downstream beam. In the NIF beam geometry, this is from the outer to inner cones of beams. The physics of this transfer is the same as in beam bending; the difference being that in the case of beam bending the effect is to redistribute power to the downstream side of the single beam. Were significant power transfer to occur in the point design, the delicately tuned implosion symmetry would be spoiled. To directly compensate for the transfer, the incident beam powers would have to be adjusted. The greatest vulnerability in the point design thus occurs at 15.2ns, when the inner beams are at their peak power and are at their nominal design power limit. In this situation, some other means of symmetry control would be required, such as re-pointing. At ...
Date: August 27, 2003
Creator: Williams, E A; Hinkel, D E & Hittinger, J A
Partner: UNT Libraries Government Documents Department

Simulating Time-Dependent Energy Transfer Between Crossed Laser Beams in an Expanding Plasma

Description: A coupled mode system is derived to investigate a three-wave parametric instability leading to energy transfer between co-propagating laser beams crossing in a plasma flow. The model includes beams of finite width refracting in a prescribed transverse plasma flow with spatial and temporal gradients in velocity and density. The resulting paraxial light equations are discretized spatially with a Crank-Nicholson-type scheme, and these algebraic constraints are nonlinearly coupled with ordinary differential equations in time that describe the ion acoustic response. The entire nonlinear differential-algebraic system is solved using an adaptive, backward-differencing method coupled with Newton's method. A numerical study is conducted in two dimensions that compares the intensity gain of the fully time-dependent coupled mode system with the gain computed under the further assumption of a strongly-damped ion acoustic response. The results demonstrate a time-dependent gain suppression when the beam diameter is commensurate with the velocity gradient scale length. The gain suppression is shown to depend on time-dependent beam refraction and is interpreted as a time-dependent frequency shift.
Date: October 11, 2004
Creator: Hittinger, J F; Dorr, M R; Berger, R L & Williams, E A
Partner: UNT Libraries Government Documents Department

A Reduced Model of Kinetic Effects Related to the Saturation of Stimulated Brillouin Scattering

Description: We developed a reduced description of kinetic effects that is included in a fluid model of stimulated Brillouin backscattering (SBS) in low Z plasmas (e.g. He, Be). Following hybrid-PIC simulations, the modified ion distribution function is parametrized by the width {delta} of the plateau created by trapping around the phase velocity of the SBS-driven acoustic wave. An evolution equation is derived for {delta}, which affects SBS through a frequency shift and a reduced Landau damping. This model recovers the linear Landau damping value for small waves and the time-asymptotic nonlinear frequency shift calculated by Morales and O'Neil. Finally we compare our reduced model with Bzohar simulations of a Be plasma representative of experiments that have shown evidence of ion trapping.
Date: August 26, 2003
Creator: Divol, L; Williams, E A; Cohen, B I; Langdon, A B & Lasinski, B F
Partner: UNT Libraries Government Documents Department

Stimulated Brillouin Scattering from Helium-Hydrogen Plasmas

Description: An extensive study of the stimulated Brillouin scattering (SBS) in helium-hydrogen plasmas has been performed using a gas jet at the Janus Laser Facility. We observe three regions of reflectivity by varying the probe intensity from 10{sup 14} to 10{sup 16}: saturated region, linear region, and near SBS threshold region. In the linear regime, adding small amounts of H to a He plasma reduces the SBS reflectivity by a factor of 4.
Date: August 22, 2003
Creator: Froula, D H; Divol, L; Price, D; Gregori, G; Williams, E A & Glenzer, S H
Partner: UNT Libraries Government Documents Department

Raman Generated Magnetic Fields in Laser Light Speckles

Description: In modern 2D and 3D PIC simulations relevant to National Ignition Facility (NIF) parameters, the low frequency magnetic fields associated with the localized fast electron currents generated by Stimulated Raman Scatter have been identified. We consider electron plasma densities from 0.1 to 0.2 of critical density (n{sub c}) and electron plasma temperatures (T{sub e}) from a few keV to over 10 keV in simulations with space scales corresponding to a laser speckle in modeling with our massively parallel PIC code 23. These magnetic fields are {approx} 1 MG, Then the electrons accelerated by the Raman process are magnetized with their Lamor radii on the order of a speckle width. The transport of these hot electrons out of the speckle then becomes a more complex process than generally assumed.
Date: September 2, 2003
Creator: Lasinski, B F; Still, C H; Langdon, A B; Hinkel, D E & Williams, E A
Partner: UNT Libraries Government Documents Department

Comparison of Raman Scattering Measurements and Modeling in NIF Ignition Experiments

Description: Recent NIF indirect-drive experiments have shown significant Raman scattering from the inner beams. NIF data has motivated improvements to rad-hydro modeling, leading to the 'high flux model' [M. D. Rosen et al., HEDP 7, 180 (2011)]. Cross-beam energy transfer [P. A. Michel et al., Phys. Plasmas 17, 056305 (2010] in the laser entrance hole is an important tool for achieving round implosions, and is uniformly distributed across the laser spot in rad-hydro simulations (but not necessarily in experiments). We find the Raman linear gain spectra computed with these plasma conditions agree well in time-dependent peak wavelength with the measured data, especially when overlapping laser-beam intensities are used. More detailed, spatially non-uniform modeling of the cross-beam transfer has been performed. The resulting gains better follow the time history of the measured backscatter. We shall present the impact of spatially non-uniform energy transfer on SRS gain. This metric is valid when amplification is in a linear regime, and so we shall also present an assessment of whether electron trapping in Langmuir waves can play a role in these shots.
Date: November 4, 2011
Creator: Strozzi, D J; Hinkel, D E; Williams, E A; Town, R J; Michel, P A; Divol, L 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

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

Efficient Coupling of 527 nm Laser Beam Power to a Long Scalelength Plasma

Description: We experimentally demonstrate that application of laser smoothing schemes including smoothing by spectral dispersion (SSD) and polarization smoothing (PS) increases the intensity range for efficient coupling of frequency doubled (527 nm) laser light to a long scalelength plasma with n{sub e}/n{sub cr} = 0.14 and T{sub e} = 2 keV.
Date: August 24, 2005
Creator: Moody, J. D.; Divol, L.; Glenzer, S. H.; MacKinnon, A. J.; Froula, D. H.; Gregori, G. et al.
Partner: UNT Libraries Government Documents Department

Observation of the saturation of Langmuir waves driven by ponderomotive force in a large scale plasma

Description: We report the observation of amplification of a probe laser beam (I {le} 1 {times} 10{sup 14} W/cm{sup 2}) in a large scale ({approximately} 1 mm) plasma by interaction with a pumping laser beam (I = 2 {times} 10{sup 15} W/cm{sup 2}) and a stimulated Langmuir wave. When the plasma density is adjusted to allow the Langmuir wave dispersion to match the difference frequency and wave number of the two beams, amplification factors as high as {times} 2.5 result. Interpretation of this amplification as scattering of pump beam energy by the Langmuir wave that is produced by the ponderomotive force of the two beams, allows the dependence of Langmuir wave amplitude on ponderomotive force to be measured. It is found that the Langmuir wave amplitude saturates at a level that depends on ion wave damping, and is generally consistent with secondary ion wave instabilities limiting its growth. 20 refs., 4 figs.
Date: June 22, 1997
Creator: Kirkwood, R. K.; Moody, J. D.; MacGowan, B. J.; Glenzer, S. H.; Kruer, W. L.; Estabrook, K. G. et al.
Partner: UNT Libraries Government Documents Department