Collisional Simulations Research Articles

Core heating analysis of fast ignition targets by integrated simulations

A coupled PIC and Fokker-Planck hierarchical simulation model for analysis of fast ignition core heating was checked by comparing the heating profile with collisional PIC simulations. The integrated simulations using FI 3 code were carried out for the recent fast heating experiments and showed that the dense core (ρ > 10 g/cc) is heated up to 0.45 keV by the fast electrons via the Coulomb interactions (not anomalous processes).

Molecular dynamics study of particle emission by reactive cluster ion impact

Molecular dynamics (MD) simulations of sputtering process with fluorine cluster impact onto silicon targets were performed. By iterating collisional simulations on a same target, accumulation of incident atoms and evolution of surface morphology were examined as well as emission process of precursors. When (F 2) 300 clusters were sequentially irradiated on Si(1 0 0) target at 6 keV of total incident energy, column-like surface structure covered with F atoms was formed. As the number of incident clusters increased, sputtering yield of Si atoms also increased because the target surface was well fluoridised to provide SiF x precursors. Size distribution of emitted particles showed that SiF 2 was the major sputtered particle, but various types of silicon-fluoride compounds such like Si 2F x , Si 3F x and very large molecules consists of 100 atoms were also observed. This size distribution and kinetic energy distribution of desorbed materials were studied, which showed that the sputtering mechanism with reactive cluster ions is similar to that under thermal equilibrium condition at high-temperature.

Interpretation of TOF–SIMS depth profiles from ultrashallow high-k dielectric stacks assisted by hybrid collisional computer simulation

The TRIDYN collisional computer simulation has been modified to account for emission of ionic species and molecules during sputter depth profiling, by introducing a power law dependence of the ion yield as a function of the oxygen surface concentration and by modelling the sputtering of monoxide molecules. The results are compared to experimental data obtained with dual beam TOF–SIMS depth profiling of ZrO2/SiO2/Si high-k dielectric stacks with thicknesses of the SiO2 interlayer of 0.5, 1, and 1.5 nm. Reasonable agreement between the experiment and the computer simulation is obtained for most of the experimental features, demonstrating the effects of ion-induced atomic relocation, i.e., atomic mixing and recoil implantation, and preferential sputtering. The depth scale of the obtained profiles is significantly distorted by recoil implantation and the depth-dependent ionization factor. A pronounced double-peak structure in the experimental profiles related to Zr is not explained by the computer simulation, and is attributed to ion-induced bond breaking and diffusion, followed by a decoration of the interfaces by either mobile Zr or O.

Collisional Simulations of Cometary Nuclei

AbstractCollisional simulations of cometary nuclei (icy dust aggregates) are conducted. Conditions for sticking are determined as a function of the impact velocity and mechanical properties of nuclei. Possible collisional outcomes in three evolutionary stages of icy-dust aggregates are discussed.

Particle-in-cell simulations of heat flux driven ion acoustic instability

The return current instability of ion acoustic waves in a laser heated plasma is studied by means of a collisional particle-in-cell code and theoretical analysis in the regime of nonlocal heat transport. The physical scenario of localized, inverse Bremsstrahlung heating in a single laser hot spot, electron thermal transport, return current of cold electrons, instability of ion acoustic waves, and resulting ion acoustic turbulence are examined in a self-consistent kinetic collisional particle simulation. The observed growth of the return current instability is in excellent agreement with predictions of a linear, nonlocal theory. Ion acoustic fluctuations contribute to the inhibition of thermal transport, which leads to the enhancement of the electron temperature in the center of a hot spot. Increased electron collisionality and hot ion tail production are the dominant saturation mechanisms of the return current instability in a one-dimensional geometry. The effects of the ion acoustic turbulence on other interaction processes are also discussed.

Mechanisms of target poisoning during magnetron sputtering as investigated by real-time in situ analysis and collisional computer simulation

The evolution of reactive gas uptake at the target surface has been investigated by real-time in situ diagnostics during magnetron sputtering. Using a planar circular dc magnetron for reactive sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture, the target uptake of nitrogen was determined at varying gas flows of nitrogen using the N14(d,α)C12 nuclear reaction, directly demonstrating the target “poisoning” effect. The expected hysteresis behavior at increasing/decreasing nitrogen gas flow is confirmed. Within the precision of the measurement, the nitrogen content remains unaltered after switching off the magnetron, indicating the absence of a significant mobile fraction of nitrogen in the target. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen. This is quantitatively reproduced by TRIDYN collisional computer simulations.

Human myoglobin recognition of oxygen: dynamics of the energy landscape.

Femtosecond to nanosecond dynamics of O(2) rebinding to human WT myoglobin and its mutants, V68F and I107F, have been studied by using transient absorption. The results are compared with NO rebinding. Even though the immediate environment around the heme binding site is changed by the mutations, the picosecond geminate rebinding of oxygen is at most minimally affected. On the other hand, the V68F (E11) mutation causes drastic differences in rebinding on the nanosecond time scale, whereas the effect of the I107F (G8) mutation remains relatively small within our 10-ns time window. Unlike traditional homogeneous kinetics and molecular dynamics collisional simulations, we propose a "bifurcation model" for populations of directed and undirected dynamics on the ultrafast time scale, reflecting the distribution of initial protein conformations. The major mutation effect occurs on the time scale on which global protein conformational change is possible, consistent with transitions between the conformations of directed and undirected population playing a role in the O(2) binding. We discuss the relevance of these findings to the bimolecular function of the protein.

Scattering of Planetesimals from Young Planetary Disks: Application to the β Pictoris System

AbstractTransient redshifted events monitored in the spectrum of β Pictoris have been interpreted as resulting from the evaporation of numerous comet-like bodies in the vicinity of this star. The dynamical origin for this phenomenon is attributed to mean-motion resonances (4:1 and 3:1) with a Jovian-like planet. Numerical simulations of this phenomenon are able to correctly reproduce the dynamical characteristics of the star-grazers observed. The numerical study allows to estimate the density of the planetesimal disk from which the bodies are supposed to originate, i.e. ∼ a few 108 bodies per AU. A key issue with this model is the refilling of the resonances, as without refilling they should be cleared within a few 105 yr and the observed phenomenon should stop. Collisions among planetesimals are a plausible mechanism. Collisional simulations show that collisions are able to sustain the observed phenomenon over much more than 106 yr, provided the population of the disk is high enough. The mass density of this population is estimated to a few tens of Earth masses per AU, which is only marginally realistic. However, the mass estimate is very poorly constrained.

Numerical study of the nonlinear evolution of toroidicity-induced Alfvén eigenmodes

A simulation code for the nonlinear evolution of the toroidicity-induced Alfvén eigenmodes (TAE) is implemented based on a collisional δf simulation method [Phys. Plasmas 4, 3591 (1997)] and a Hamiltonian formulation of the guiding-center motion [Phys. Fluids 27, 2455 (1984)]. The nonlinear behavior of TAE in the regime of weak damping is studied numerically and compared with the predictions of theoretical analysis. The theoretical prediction for the scaling of the saturation amplitude with the linear growth rate and the collision rate is well confirmed. The enhancement of the saturation level due to resonance overlapping when there are multiple modes is observed and analyzed. The simulation code is applied to the alpha-driven TAE’s in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)]. An estimate of the background damping rate is obtained from the simulation result.

Collisional Simulations of Neptune's Ring Arcs

The currently accepted model for Neptune arc confinement relies on the radial and azimuthal confining perturbations due to the nearby satellite, Galatea. This model calls for arc particle orbits exhibiting a negative eccentricity gradient and crossing at quadrature, a configuration that paradoxically leads to collisions energetic enough to disrupt arc confinement. We confirm with numerical collisional N-body simulations that the confinement mechanism relying on a 42:43 corotation–inclination resonance and a 42:43 outer Lindblad resonance with Galatea is indeed capable of confining a large population of 10-m-size and bigger particles over short time scales. Moreover, we find that an 84:86 outer vertical resonance, also due to Galatea, falling within 20 m of the arcs' radial position, effectively reduces the collision frequency and relative collisional velocities and consequently stabilizes the arcs over long time scales against the disruptive effects of collisions.

Collisional deactivation of highly vibrationally excited pyrazine

The collisional deactivation of vibrationally excited pyrazine (C4N2H4) in the electronic ground state by 19 collider gases was studied using the time-resolved infrared fluorescence (IRF) technique. The pyrazine was photoexcited with a 308 nm laser and its vibrational deactivation was monitored following rapid radiationless transitions to produce vibrationally excited molecules in the electronic ground state. The IRF data were analyzed by a simple approximate inversion method, as well as with full collisional master equation simulations. The average energies transferred in deactivating collisions (〈ΔE〉d) exhibit a near-linear dependence on vibrational energy at lower energies and less dependence at higher energies. The deactivation of ground state pyrazine was found to be similar to that of ground state benzene [J. R. Barker and B. M. Toselli, Int. Rev. Phys. Chem. 12, 305 (1990)], but it is strikingly different from the deactivation of triplet state pyrazine [T. J. Bevilacqua and R. B. Weisman, J. Chem. Phys. 98, 6316 (1993)].

Collisional simulations of particles in a narrow planetary ring

In order to better understand the effect of collisions on short-term features in narrow planetary rings, we have developed a model to investigate how particle-particle collisions might modify some consequences of satellite perturbations. To mimic the effect of collisional scattering, we use a Monte Carlo-like simulation that alters the velocities of model ring particles in the same manner as collisions with actual particles might. In this way we can simulate collisions while tracking individual particles in our model. Using parameters (collision frequency, coefficient of restitution, and orbital elements of colliders) appropriate for the F ring in a two-dimensional numerical model that incorporates the collisional simulation as well as the perturbations of adjacent satellites, we can reproduce periodic features seen in the Voyager images. Not surprisingly, we find that collisions wash out periodic features within approximately a collisional relaxation time (the interval over which the random component of velocity introduced by collisions equals the systematic velocity introduced by the perturbers). The induced random velocity depends on both the amount of energy dissipation per collision and the collision frequency. Our simulation should not be applied to long-term phenomena because it is not self-consistent, in that the modeled ring particles do not transfer angular momentum back to the perturbing moons.

Collisional simulations of satellite Lindblad resonances

The influence of a perturbing satellite on a planetary ring at isolated Lindblad resonances is studied with numerical computer simulations, combining Aarseth's force polynomial method for orbit integrations with the calculation of particle-particle impacts. Observed torque exchange between the satellite and the dissipative non-self-gravitating ring agrees with the theoretical torque for a gravitating ring to within 20%. This verifies that angular momentum exchange is not very sensitive to the details of the dominant physical processes. The theoretical estimate for the formation of a gap at a 2:1 inner Lindblad resonance is also shown to be of the correct order of magnitude. The theoretically predicted flux- reversal phenomenon due to streamline distortion is also verified. The reversal of angular momentum luminosity occurs already for the nonlinearity parameter q ⋍ 0.20–0.30 in our simulations with low optical depth, τ ⋍ 0.02.

Collisional Simulations of Isolated Lindblad Resonances

The influence of the perturbing satellite on the planetary ring at isolated Lindblad resonances is studied with numerical computer simulations, combining the Aarseth's force polynomial method for orbit integrations with the calculation of particle-particle impacts. Observed angular momentum exchange between the satellite and the dissipative, non-selfgravitating ring agrees with the Goldreich-Tremaine formula for gravitating ring within 20%, verifying that the exerted torque is not sensitive to the details of the dominant physical processes. The theoretically predicted angular momentum luminosity reversal was also observed.

Investigation of collisional effects in the plasma erosion opening switch

During the conduction phase of the plasma erosion opening switch (PEOS), magnetic field has been observed experimentally to penetrate completely through plasmas up to 30 cm long. Current channels in the main body of the plasma have been observed that are more than 10 cm (or more than about 60 collisionless skin depths) wide. In addition, the maximum current carried by the switch before opening (the conduction current) seems to scale roughly linearly with plasma density n and switch length l. Collisionless pic code simulations of the plasma switch show current conducted in skin-depth-like channels, with the conduction current scaling close to l 2/5 and n1/4. In this paper, the effect of collisions on the behavior of the PEOS is investigated and is shown to bring the pic simulations and experimental results in closer agreement. In collisional simulations, current channels as wide as those in experiments are observed, and the conduction current scales linearly with l and as n1/2 in the anode-dominated case. In the cathode-dominated case, linear scaling with both length and density can be inferred from the cathode penetration distance versus time.

Thresholds of Convective Raman Backscattering: Effects of∇n, Thomson Scattering, Noise, Collisions, and Landau Damping

Using kinetic simulations we find that Raman backscattering can exist for laser intensities a factor $2\ensuremath{\pi}$ lower than the density gradient ($\ensuremath{\nabla}n$) threshold and that Thomson scattering can account for backscattering signals even lower. In addition, kinetic, collisional simulations show that laser intensities \ensuremath{\gtrsim}3-10 times the electron-ion collisional threshold are needed for significant Raman absorption. Collisions can also reduce the heated electron temperatures for $\ensuremath{\nabla}n\ensuremath{\ne}0$.