Showing papers by "Lin Yin published in 2009"

Transition from collisional to kinetic regimes in large-scale reconnection layers

Abstract: Using first-principles fully kinetic simulations with a Fokker-Planck collision operator, it is demonstrated that Sweet-Parker reconnection layers are unstable to a chain of plasmoids (secondary islands) for Lundquist numbers beyond S >{approx} 1000. The instability is increasingly violent at higher Lundquist number, both in terms of the number of plasmoids produced and the super-Alfvenic growth rate. A dramatic enhancement in the reconnection rate is observed when the half-thickness of the current sheet between two plasmoids approaches the ion inertial length. During this transition, the reconnection electric field rapidly exceeds the runaway limit, resulting in the formation of electron-scale current layers that are unstable to the continual formation of new plasmoids.

Enhanced laser-driven ion acceleration in the relativistic transparency regime.

Abstract: We report on the acceleration of ion beams from ultrathin diamondlike carbon foils of thickness 50, 30, and 10 nm irradiated by ultrahigh contrast laser pulses at intensities of $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. An unprecedented maximum energy of 185 MeV ($15\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$) for fully ionized carbon atoms is observed at the optimum thickness of 30 nm. The enhanced acceleration is attributed to self-induced transparency, leading to strong volumetric heating of the classically overdense electron population in the bulk of the target. Our experimental results are supported by both particle-in-cell (PIC) simulations and an analytical model.

Advances in petascale kinetic plasma simulation with VPIC and Roadrunner

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.

Progress and prospects of ion-driven fast ignition

Abstract: Fusion fast ignition (FI) initiated by laser-driven ion beams is a promising concept examined in this paper. FI based on a beam of quasi-monoenergetic ions (protons or heavier ions) has the advantage of a more localized energy deposition, which minimizes the required total beam energy, bringing it close to the ≈10 kJ minimum required for fuel densities ∼ 500 gc m −3 . High-current, laser-driven ion beams are most promising for this purpose. Because they are born neutralized in picosecond timescales, these beams may deliver the power density required to ignite the compressed DT fuel, ∼10 kJ/10 ps into a spot 20 µm in diameter. Our modelling of ion-based FI include high fusion gain targets and a proof of principle experiment. That modelling indicates the concept is feasible, and provides confirmation of our understanding of the operative physics, a firmer foundation for the requirements, and a better understanding of the optimization trade space. An important benefit of the scheme is that such a high-energy, quasi-monoenergetic ignitor beam could be generated far from the capsule (1 cm away), eliminating the need for a reentrant cone in the capsule to protect the ion-generation laser target, a tremendous practical benefit. This paper summarizes the ion-based FI concept, the integrated ion-driven FI modelling, the requirements on the ignitor beam derived from that modelling, and the progress in developing a suitable laser-driven ignitor ion beam.

Influence of Coulomb collisions on the structure of reconnection layers

Abstract: The influence of Coulomb collisions on the structure of reconnection layers is examined in neutral sheet geometry using fully kinetic simulations with a Monte Carlo treatment of the Fokker–Planck operator. The algorithm is first carefully benchmarked against key predictions from transport theory, including the parallel and perpendicular resistivities as well as the thermal force. The results demonstrate that the collisionality is accurately specified, thus allowing the initial Lundquist number to be chosen as desired. For modest Lundquist numbers S≲1000, the classic Sweet–Parker solution is recovered. Furthermore, a distinct transition to a faster kinetic regime is observed when the thickness of the resistive layer δSP falls below the ion inertial length di. For higher Lundquist numbers S≳1000, plasmoids (secondary islands) are observed within the elongated resistive layers. These plasmoids give rise to a measurable increase in the reconnection rate and for certain cases induce a transition to kinetic reg...

Onset and saturation of backward stimulated Raman scattering of laser in trapping regime in three spatial dimensions

Abstract: A suite of three-dimensional (3D) VPIC [K. J. Bowers et al., Phys. Plasmas 15, 055703 (2008)] particle-in-cell simulations of backward stimulated Raman scattering (SRS) in inertial confinement fusion hohlraum plasma has been performed on the heterogeneous multicore supercomputer, Roadrunner, presently the world’s most powerful supercomputer. These calculations reveal the complex nonlinear behavior of SRS and point to a new era of “at scale” 3D modeling of SRS in solitary and multiple laser speckles. The physics governing nonlinear saturation of SRS in a laser speckle in 3D is consistent with that of prior two-dimensional (2D) studies [L. Yin et al., Phys. Rev. Lett. 99, 265004 (2007)], but with important differences arising from enhanced diffraction and side loss in 3D compared with 2D. In addition to wave front bowing of electron plasma waves (EPWs) due to trapped electron nonlinear frequency shift and amplitude-dependent damping, we find for the first time that EPW self-focusing, which evolved from trap...

Investigation of stimulated Raman scattering using a short-pulse diffraction limited laser beam near the instability threshold

Abstract: Short pulse laser plasma interaction experiments using diffraction limited beams provide an excellent platform to investigate the fundamental physics of Stimulated Raman Scattering. Detailed understanding of these laser plasma instabilities impacts the current inertial confinement fusion ignition designs and could potentially impact fast ignition when higher energy lasers are used with longer pulse durations ( > 1 kJ and> 1 ps). Using short laser pulses, experiments can be modeled over the entire interaction time of the laser using particle-in-cell codes to validate our understanding quantitatively. Experiments have been conducted at the Trident laser facility and the LULI (Laboratoire pour l'Utilisation des Lasers Intenses) to investigate stimulated Raman scattering near the threshold of the instability using 527 nm and 1059 nm laser light respectively with 1.5-3.0 ps pulses. In both experiments, the interaction beam was focused into a pre-ionized He gas-jet plasma. Measurements of the reflectivity as a function of intensity and k{lambda}{sub D} were completed at the Trident laser facility. At LULI, a 300 fs Thomson scattering probe is used to directly measure the density fluctuations of the driven electron plasma and ion acoustic waves. Work is currently underway comparing the results of the experiments with simulations using the VPICmore » [K. J. Bowers, et at., Phys. Plasmas, 15 055703 (2008)] particle-in-cell code. Details of the experimental results are presented in this manuscript.« less

Theory of laser ion acceleration from a foil target of nanometers

Abstract: A theory for laser ion acceleration is presented to evaluate the maximum ion energy in the interaction of ultrahigh contrast (UHC) intense laser with a nanometer-scale foil. In this regime the energy of ions may be directly related to the laser intensity and subsequent electron dynamics. This leads to a simple analytical expression for the ion energy gain under the laser irradiation of thin targets. Significantly, higher energies for thin targets than for thicker targets are predicted. Theory is concretized to the details of recent experiments which may find its way to compare with these results.

Simulation of the Cygnus Rod-Pinch Diode Using the Radiographic Chain Model

Abstract: The Cygnus radiographic machine is a relatively compact low-energy (<3 MV) X-ray source with some extremely desirable features for radiographic applications. These features include small spot size, which is critical for high-spatial resolution, and high dose in a low-energy range. The X-ray source is based on bremsstrahlung production in a small-diameter (~0.75 mm) tungsten rod by a high-current (~60 kA) electron beam converging at the tip of the rod. For quantitative analysis of radiographic data, it is essential to determine the bremsstrahlung spectrum accurately. We have used the radiographic chain model to self-consistently model the diode with a 2-D particle-in-cell (PIC) code (Merlin) linked to an electron-photon Monte Carlo code to obtain the spectrum under three different situations: a steady-state spectrum using a voltage pulse of 2.25 MV, a time-integrated spectrum using a time-dependent experimental voltage pulse, and the spectrum resulting from inclusion of reflexing electrons around the anode rod in our PIC simulation. Detailed electron dynamics were obtained. We conclude that the time-integrated bremsstrahlung spectrum is significantly softer than that of the steady state. Including the effects of reflexing electrons using a Monte Carlo transport method in Merlin produced a spectrum in better agreement with experimental data.

Recent progress on ion-driven fast ignition

Abstract: We report on the encouraging progress from research on fusion fast ignition (FI) initiated by laser-driven ion beams. Compared to electrons, FI based on a beam of quasi-mono-energetic ions (including protons and especially heavier ions such as C) has the advantage of a more localized energy deposition, which minimizes the required total beam energy. High-current, laser-driven ion beams are very promising for this purpose, and because of their ultra-low transverse emittance, these beams may be focused to the required dimension, ∼ tens of microns. Because they are created in ps timescales, these beams can deliver the power required to ignite the compressed D-T fuel, ∼ 10 kJ / 50 ps. Our recent integrated calculations of ion-based FI include high fusion gain targets and a proof of principle experiment, which indicate the concept is feasible. The beam requirements derived from high-gain calculations using realistic DT fuel implosion are presented. The scientific issues and technical issues in the generation of the required laser-driven ion beams, and recent progress in their realization, are summarized.

Generation of 0.5GEV C6+ ions from irradiation of ultra-thin foils with high contrast, high intensity laser pulses

Abstract: Laser-driven particle acceleration has been under intense investigation for the last decade and is of particular interest for key applications such as medical physics, fast ignition and inertial confinement fusion. The dominant ion acceleration mechanism is known as target normal sheath acceleration (TNSA); although easy accessible it suffers from a low energy conversion efficiency. Recently reported maximum ion energies1 are in the range of a few tens of MeV, which is well below the energies needed for most applications. Recently proposed acceleration mechanisms utilize ultra-high contrast laser pulses and ultra-thin solid targets allowing for generation of ions in the GeV range from intense laser-plasma interaction. A high laser contrast suppresses pre-ionization of the solid target by any pre-pulses or laser pedestals and in theory enables acceleration beyond TNSA in regimes such as the break-out afterburner (BOA)2 or radiation pressure acceleration (RPA).