B. I. Cohen

Lawrence Livermore National Laboratory, Livermore, California, United States

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Publications (46)133.51 Total impact

  • [Show abstract] [Hide abstract] ABSTRACT: Kinetic simulations of nonlinear electron plasma waves (EPW) are presented in 2D with the Vlasov code LOKI (2 space and 2 velocity dimensions; Banks et al., Phys. Plasmas 18, 052102 (2011)). Propagating EPWs are created with an external wave potential with uniform transverse amplitude. The evolution of the plasma wave field and its self-consistent quasi-steady distribution of trapped electrons is studied after the external drive is turned off. For finite-amplitude EPWs, the onset of the trapped-electron-induced filamentation instability (H. Rose, Phys. Plasmas 15, 042311 (2008)) and trapped electron sideband instability (S. Brunner and E. Valeo, PRL 93, 145003 (2004)) are studied as a function of wave amplitude and k0λDe, where k0 is the wavenumber of the external potential. We extend the theory of Kruer et al PRL 23, 1969 to 2D to find growth rates of both instabilities and compare these to the ones obtained from the simulations. In the nonlinear state, the distribution of resonant electrons is dramatically modified
    No preview · Article · Oct 2012
  • [Show abstract] [Hide abstract] ABSTRACT: We present results on the kinetic filamentation of finite-width nonlinear electron plasma waves (EPW). Using 2D simulations with the PIC code BEPS, we excite a traveling EPW with a Gaussian transverse profile and a wavenumber k0λDe= 1/3. The transverse wavenumber spectrum broadens during transverse EPW localization for small width (but sufficiently large amplitude) waves, while the spectrum narrows to a dominant k as the initial EPW width increases to the plane-wave limit. For large EPW widths, filaments can grow and destroy the wave coherence before transverse localization destroys the wave; the filaments in turn evolve individually as self-focusing EPWs. Additionally, a transverse electric field develops that affects trapped electrons, and a beam-like distribution of untrapped electrons develops between filaments and on the sides of a localizing EPW. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Laboratory Research and Development Program at LLNL under project tracking code 12-ERD-061. Supported also under Grants DE-FG52-09NA29552 and NSF-Phy-0904039. Simulations were performed on UCLA's Hoffman2 and NERSC's Hopper.
    No preview · Article · Oct 2012
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    [Show abstract] [Hide abstract] ABSTRACT: Two-dimensional Vlasov simulations of nonlinear electron plasma waves are presented, in which the interplay of linear and nonlinear kinetic effects is evident. The plasma wave is created with an external traveling wave potential with a transverse envelope of width Delta y such that thermal electrons transit the wave in a "sideloss" time, t(sl) similar to Delta(y)/v(e). Here, v(e) is the electron thermal velocity. The quasisteady distribution of trapped electrons and its self-consistent plasma wave are studied after the external field is turned off. In cases of particular interest, the bounce frequency, omega(be) = k root e phi/m(e), satisfies the trapping condition omega(be)t(sl) > 2 pi such that the wave frequency is nonlinearly downshifted by an amount proportional to the number of trapped electrons. Here, k is the wavenumber of the plasma wave and phi is its electric potential. For sufficiently short times, the magnitude of the negative frequency shift is a local function of phi. Because the trapping frequency shift is negative, the phase of the wave on axis lags the off-axis phase if the trapping nonlinearity dominates linear wave diffraction. In this case, the phasefronts are curved in a focusing sense. In the opposite limit, the phasefronts are curved in a defocusing sense. Analysis and simulations in which the wave amplitude and transverse width are varied establish criteria for the development of each type of wavefront. The damping and trapped-electron-induced focusing of the finite-amplitude electron plasma wave are also simulated. The damping rate of the field energy of the wave is found to be about the sideloss rate, v(e) similar to t(sl)(-1). For large wave amplitudes or widths Delta y, a trapping-induced self-focusing of the wave is demonstrated. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3577784]
    Full-text · Article · Dec 2011 · Physics of Plasmas
  • D J Strozzi · D P Grote · M Tabak · B I Cohen · R P J Town · A J Kemp
    [Show abstract] [Hide abstract] ABSTRACT: This paper shows work at Lawrence Livermore National Lab (LLNL) devoted to modeling the propagation of, and heating by, a relativistic electron beam in a idealized dense fuel assembly for fast ignition [1]. The implicit particle-in-cell (PIC) code LSP is used. Experiments planned on the National Ignition Facility (NIF) in the next few years using the Advanced Radiography Capability (ARC) short-pulse laser motivate this work. We demonstrate significant improvement in the heating of dense fuel due to magnetic forces, increased beam collimation, and insertion of a finite-radius carbon region between the beam excitation and fuel regions.
    No preview · Article · Sep 2010 · Journal of Physics Conference Series
  • B.I. Cohen · A.J. Kemp · L. Divol
    [Show abstract] [Hide abstract] ABSTRACT: A new framework is introduced for kinetic simulation of laser–plasma interactions in an inhomogeneous plasma motivated by the goal of performing integrated kinetic simulations of fast-ignition laser fusion. The algorithm addresses the propagation and absorption of an intense electromagnetic wave in an ionized plasma leading to the generation and transport of an energetic electron component. The energetic electrons propagate farther into the plasma to much higher densities where Coulomb collisions become important. The high-density plasma supports an energetic electron current, return currents, self-consistent electric fields associated with maintaining quasi-neutrality, and self-consistent magnetic fields due to the currents. Collisions of the electrons and ions are calculated accurately to track the energetic electrons and model their interactions with the background plasma. Up to a density well above critical density, where the laser electromagnetic field is evanescent, Maxwell’s equations are solved with a conventional particle-based, finite-difference scheme. In the higher-density plasma, Maxwell’s equations are solved using an Ohm’s law neglecting the inertia of the background electrons with the option of omitting the displacement current in Ampere’s law. Particle equations of motion with binary collisions are solved for all electrons and ions throughout the system using weighted particles to resolve the density gradient efficiently. The algorithm is analyzed and demonstrated in simulation examples. The simulation scheme introduced here achieves significantly improved efficiencies.
    No preview · Article · Jun 2010 · Journal of Computational Physics
  • A. J. Kemp · B. I. Cohen · L. Divol
    [Show abstract] [Hide abstract] ABSTRACT: We present new results on the physics of short-pulse laser-matter interaction of kilojoule-picosecond pulses at full spatial and temporal scale using a new approach that combines a three-dimensional collisional electromagnetic particle-in-cell code with a magnetohydrodynamic-hybrid model of high-density plasma. In the latter, collisions damp out plasma waves, and an Ohm’s law with electron inertia effects neglected determines the electric field. In addition to yielding orders of magnitude in speed-up while avoiding numerical instabilities, this allows us to model the whole problem in a single unified framework: the laser-plasma interaction at subcritical densities, energy deposition at relativistic critical densities, and fast- electron transport in solid densities. Key questions such as the multipicosecond temporal evolution of the laser energy conversion into hot electrons, the impact of return currents on the laser-plasma interaction, and the effect of self-generated electric and magnetic fields on electron transport will be addressed. We will report applications to current experiments.
    No preview · Article · Mar 2010 · Physics of Plasmas
  • A. M. Dimits · C. Wang · R. Calfish · B. I. Cohen · Y. Huang
    No preview · Article · Jan 2009
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    B I Cohen · E A Williams · H. X. Vu
    [Show abstract] [Hide abstract] ABSTRACT: An investigation of the possible inflation of stimulated Brillouin backscattering (SBS) due to ion kinetic effects is presented using electromagnetic particle simulations and integrations of three-wave coupled-mode equations with linear and nonlinear models of the nonlinear ion physics. Electrostatic simulations of linear ion Landau damping in an ion acoustic wave, nonlinear reduction of damping due to ion trapping, and nonlinear frequency shifts due to ion trapping establish a baseline for modeling the electromagnetic SBS simulations. Systematic scans of the laser intensity have been undertaken with both one-dimensional particle simulations and coupled-mode-equations integrations, and two values of the electron-to-ion temperature ratio (to vary the linear ion Landau damping) are considered. Three of the four intensity scans have evidence of SBS inflation as determined by observing more reflectivity in the particle simulations than in the corresponding three-wave mode-coupling integrations with a linear ion-wave model, and the particle simulations show evidence of ion trapping. Comment: 56 pages, 20 figures
    Full-text · Article · Jun 2007 · Physics of Plasmas
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    [Show abstract] [Hide abstract] ABSTRACT: A first set of shock timing, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP). In parallel, a robust set of optical and X-ray spectrometers, interferometer, calorimeters and imagers have been activated. The experiments have been undertaken with laser powers and energies of up to 8TW and 17kJ in flattop and shaped 1–9ns pulses focused with various beam smoothing options. The experiments have demonstrated excellent agreement between measured and predicted laser-target coupling in foils and hohlraums, even when extended to a longer pulse regime unattainable at previous laser facilities, validated the predicted effects of beam smoothing on intense laser beam propagation in long scale-length plasmas and begun to test 3Dcodes by extending the study of laser driven hydrodynamic jets to 3Dgeometries.
    Full-text · Article · Jan 2007 · The European Physical Journal D
  • B. I. Cohen · L. Divol · AB Langdon · EA Williams
    [Show abstract] [Hide abstract] ABSTRACT: 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. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models 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 more physical 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 important saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities decrease with increasing collisionality in the simulations for velocity-independent collisions and very weakly decrease for the range of Fokker-Planck collisionality considered. 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 (autoresonance) reflectivities relative to anti-autoresonant configurations, in agreement with theoretical arguments.
    No preview · Article · Feb 2006 · Physics of Plasmas
  • [Show abstract] [Hide abstract] ABSTRACT: Experiments show that power is transferred between two copropagating 351 nm laser beams crossing in an Al plasma when the frequency of the driven ion wave is shifted by a Mach 1 flow. The resonant amplification of a low-intensity ( ⩽ 2.5×1014 W/cm2) beam intersected by a high-intensity (7.0×1014 W/cm2) pump beam is determined by comparing the transmitted beam power to that measured in experiments where the plasma flow direction is reversed and the ion wave is evidently detuned. The polarization of the amplified light is also observed to align to the pump polarization consistent with ion-wave scattering. The amplification is found to reduce with probe-beam intensity demonstrating a nonlinear saturation mechanism that is effective when the ion-wave damping is weak, which is modeled with a calculation including both the nonlinear ion-wave frequency shifts due to ion trapping and whole-beam pump depletion.
    No preview · Article · Nov 2005 · Physics of Plasmas
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    [Show abstract] [Hide abstract] ABSTRACT: The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2 × 1015 W cm−2. The targets were filled with 1 atm of CO2 producing up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 1020 cm−3 and temperatures of Te = 2 keV. The high energy in an NIF quad of beams of 16 kJ, illuminating the target from one direction, creates unique conditions for the study of laser–plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keV x-rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ~1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (~2 mm), increasing to 10–12% for ~7 mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modelling of the laser–plasma interactions at ignition-size scale lengths.
    Full-text · Article · Nov 2004 · Nuclear Fusion
  • B. I. Cohen · L. Divol · AB Langdon · EA Williams
    [Show abstract] [Hide abstract] ABSTRACT: Two-dimensional (2D) simulations with the BZOHAR[1] hybrid code (kinetic PIC ions and Boltzmann fluid electrons)are used to study saturation of stimulated Brillouin backscatter (SBBS). The simulations give a first-principles description of SBBS nonlinearities: ion wave breaking and trapping (and the nonlinear frequency shift and relaxation of the collisionless dissipation), two-ion-wave-decay instability, harmonic generation, and pump depletion.[1] The simulations address the affects of these nonlinearities on SBBS as a function of ZTe/Ti for a single ion species. Laser transverse nonuniformity, the spatially non-uniform detuning of the SBBS ion wave due to ion trapping[2], and ponderomotive filamentation have influence. Peak SBBS reflectivities in 2D are less than in 1D. High 2D reflectivities and ion wave amplitudes relax to small values in times corresponding to less than 40 ps in experimentally relevant conditions, while in 1D with the same parameters high reflectivities and ion wave amplitudes are sustained for longer times. Ion wave dissipation is higher in 2D. [1] B.I. Cohen, et al., Phys. Plas. 4, 956 (1997). [2] L. Divol, et al., Phys. Plas. 10, 1822 (2003).
    No preview · Article · Oct 2004 · Physics of Plasmas
  • [Show abstract] [Hide abstract] ABSTRACT: We present the first direct experimental observation of the parametric two-ion decay instability of ion-acoustic waves driven by a high intensity (5 x 10(15) W cm(-2)) laser beam in a laser produced high-Z plasma. Using two separate Thomson scattering diagnostics simultaneously, we directly measure the scattering from thermal ion-acoustic fluctuations, the primary ion waves that are driven to large amplitudes by the high intensity beam, and the two-ion decay products. The decay products are shown to be present only where the interaction takes place and their k spectrum is broad.
    No preview · Article · Aug 2004 · Physical Review Letters
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    S. E. Parker · Y. Chen · W. Wan · B. I. Cohen · W. M. Nevins
    [Show abstract] [Hide abstract] ABSTRACT: A new electromagnetic kinetic electron δf particle simulation model has been demonstrated to work well at large values of plasma β times the ion-to-electron mass ratio [Y. Chen and S. E. Parker, J. Comput. Phys. 198, 463 (2003)]. The simulation is three-dimensional using toroidal flux-tube geometry and includes electron-ion collisions. The model shows accurate shear Alfvén wave damping and microtearing physics. Zonal flows with kinetic electrons are found to be turbulent with the spectrum peaking at zero and having a width in the frequency range of the driving turbulence. This is in contrast with adiabatic electron cases where the zonal flows are near stationary, even though the linear behavior of the zonal flow is not significantly affected by kinetic electrons. Zonal fields are found to be very weak, consistent with theoretical predictions for β below the kinetic ballooning limit. Detailed spectral analysis of the turbulence data is presented in the various limits. © 2004 American Institute of Physics.
    Preview · Article · Apr 2004 · Physics of Plasmas
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    [Show abstract] [Hide abstract] ABSTRACT: A new electromagnetic kinetic electron simulation model that uses a generalized split-weight scheme, where the adiabatic part is adjustable, along with a parallel canonical momentum formulation has been developed in three-dimensional toroidal flux-tube geometry. This model includes electron–ion collisional effects and has been linearly benchmarked. It is found that for H-mode parameters, the nonadiabatic effects of kinetic electrons increase linear growth rates of the ion-temperature-gradient-driven (ITG) modes, mainly due to trapped-electron drive. The ion heat transport is also increased from that obtained with adiabatic electrons. The linear behaviour of the zonal flow is not significantly affected by kinetic electrons. The ion heat transport decreases to below the adiabatic electron level when finite plasma β is included due to finite-β stabilization of the ITG modes. This work is being carried out using the 'Summit Framework'.
    Preview · Article · Sep 2003 · Nuclear Fusion
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    L. Divol · B. I. Cohen · EA Williams · AB Langdon · B. F. Lasinski
    [Show abstract] [Hide abstract] ABSTRACT: The nonlinear saturation of stimulated Brillouin scattering (SBS) in long beryllium plasmas (500 μm) and for long time (500 ps) is studied in detail through well-diagnosed one-dimensional (1D) hybrid particle-in-cell (PIC) simulations done using BZOHAR [B. I. Cohen, B. F. Lasinski, A. B. Langdon, and E. A. Williams, Phys. Plasmas 4, 956 (1997)]. Under conditions of interest, when the linear gain associated with the SBS growth is large, it is shown that following a first phase of large and bursty SBS reflectivity, SBS is suppressed by a self-induced spatial detuning due to inhomogeneous modifications of the (locally averaged) ion distribution function. This nonlinear evolution was observed over a range of laser intensities and plasma parameters. © 2003 American Institute of Physics.
    Preview · Article · Aug 2003 · Physics of Plasmas
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    [Show abstract] [Hide abstract] ABSTRACT: Thomson scattering has been used to measure the time resolved spectrum of ion wave decay products from two instabilities which can limit the growth of stimulated Raman scattering (SRS). This experiment detected ion wave decay products far above the thermal level and demonstrates that SRS produced Langmuir waves undergo the Langmuir decay instability in ignition relevant plasmas. Product waves of the electromagnetic decay instability were not detected. © 2003 American Institute of Physics.
    Full-text · Article · Jul 2003 · Physics of Plasmas
  • [Show abstract] [Hide abstract] ABSTRACT: An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I<1.5×1015 W cm−2, SBS is in a linear regime and is moderated primarily by velocity gradients, while for intensities above this threshold, nonlinear trapping is relevant. Direct evidence of detuning of SBS by a velocity gradient was achieved by directly measuring the frequency of the SBS-driven acoustic wave relative to the local resonant acoustic frequency. The frequency and amplitude of the ion-acoustic wave directly responsible for SBS has been measured as a function of space using a 3ω 200 ps Thomson-scattering probe beam. Furthermore, direct evidence of kinetic effects associated with the SBS process in the nonlinear regime has been investigated through a novel use of Thomson scattering. Specifically, a measured twofold increase in the ion temperature has been linked with ion-acoustic waves that have been driven to large amplitudes by the SBS instability. Ion-acoustic waves were excited to large amplitude with a 2ω 1.2-ns-long interaction beam with intensities up to 7×1015 W cm−2. The measured twofold increase in the ion temperature and its correlation with SBS reflectivity measurements provides quantitative evidence of hot ions created by ion trapping in laser plasmas. These detailed and accurate measurements in well-characterized plasma conditions allow a direct test of linear and nonlinear models of the saturation of SBS. © 2003 American Institute of Physics.
    No preview · Article · Apr 2003 · Physics of Plasmas
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    [Show abstract] [Hide abstract] ABSTRACT: Experiments demonstrate energy and power transfer between copropagating, same frequency, beams crossing at a small angle in a plasma with a Mach 1 flow. The process is interpreted as amplification of the low intensity probe beam by the stimulated scatter of the high intensity pump beam. The observed probe amplification increases slowly with pump intensity and decreases with probe intensity, indicative of saturation limiting the energy and power transfer due to ion-wave nonlinearities and localized pump depletion. The results are consistent with numerical modeling including ion-wave nonlinearities.
    Full-text · Article · Dec 2002 · Physical Review Letters