SEARCH · Engineering Papers
Results for “Collision Rates”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Review of Early 21st-Century Monte Carlo Perturbation and Sensitivity Techniques for k -Eigenvalue Radiation Transport Calculations
Since the 1960s, Monte Carlo methods have been used to compute the effect of perturbations on system responses and for computing sensitivity coefficients. This review article focuses on 21st-century developments specific to k-eigenvalue calculations. The theory of correlated sampling, differential operator sampling, and adjoint-based approaches and their historical methods from the 20th century are briefly summarized. Specific focus is given to four recent and significant developments: fission source correction using the correlated sampling and differential operator sampling methods, adjoint-based perturbations for the k eigenvalue using the iterated fission probability method, an extension to reaction rate ratios using generalized perturbation theory, and a recent development using a collision history approach allowing for the calculation of sensitivity coefficients of bilinear ratios and generalized responses. Differences and similarities of the four methods are discussed along with a comparison to the 20th-century approaches. A perspective on future developments is also given.
Short-baseline electron antineutrino disappearance study by using neutrino sources from {sup 13}C + {sup 9}Be reaction
To investigate the existence of sterile neutrino, we propose a new neutrino production method using {sup 13}C beams and a {sup 9}Be target for short-baseline electron antineutrino (ν-bar {sub e} ) disappearance study. The production of secondary unstable isotopes which can emit neutrinos from the {sup 13}C + {sup 9}Be reaction is calculated with three different nucleus-nucleus (AA) reaction models. Different isotope yields are obtained using these models, but the results of the neutrino flux are found to have unanimous similarities. This feature gives an opportunity to study neutrino oscillation through shape analysis. In this work, expected neutrino flux and event rates are discussed in detail through intensive simulation of the light ion collision reaction and the neutrino flux from the beta decay of unstable isotopes followed by this collision. Together with the reactor and accelerator anomalies, the present proposed ν-bar {sub e} source is shown to be a practically alternative test of the existence of the Δ m {sup 2} ∼ 1 eV{sup 2} scale sterile neutrino.
Heavy quarkonium production at collider energies: Partonic cross section and polarization
We calculate the O(α³ s ) short-distance, QCD collinear-factorized coefficient functions for all partonic channels that include the production of a heavy quark pair at short distances. Thus, this provides the first power correction to the collinear-factorized inclusive hadronic production of heavy quarkonia at large transverse momentum, pT , including the full leading-order perturbative contributions to the production of heavy quark pairs in all color and spin states employed in NRQCD treatments of this process. We discuss the role of the first power correction in the production rates and the polarizations of heavy quarkonia in high-energy hadronic collisions. The consistency of QCD collinear factorization and nonrelativistic QCD factorization applied to heavy quarkonium production is also discussed.
Ephemeral Learning - Augmenting Triggers with Online-Trained Normalizing Flows
The large data rates at the LHC require an online trigger system to select relevant collisions. Rather than compressing individual events, we propose to compress an entire data set at once. We use a normalizing flow as a deep generative model to learn the probability density of the data online. The events are then represented by the generative neural network and can be inspected offline for anomalies or used for other analysis purposes. We demonstrate our new approach for a toy model and a correlation-enhanced bump hunt.
Temperature Dependence of the Rate Constants for the Reactions O plus plus O Sub 2 Yields O Sub 2 plus plus O and O plus plus N Sub 2 Yields NO plus plus N
Theoretical description of the reaction rate of an ion-molecule interchange reaction as a function of collision parameters
Low-perigee aerodynamic heating during orbital flight of an atmosphere Explorer
An extensive, low-perigee orbital aerodynamic heating study was undertaken in support of the Atmosphere Explorer-C Temperature Alarm. State of the art of low-density, high-speed flows, some models of the earth's atmosphere, external flow-field definition, thermodynamic and transport properties of atmospheric gases, the accommodation coefficient orbital thermal environment, and correlation of theory and measurements are discussed. Aerodynamic heating rates are determined for eight selected orbits by means of a reduced, analytical model verified by both ground test and flight data. These heating rates are compared with classical free-molecule and first-order collision regime values.
The influence of autoionization accompanied by excitation on dielectronic recombination and ionization equilibrium
In the process of dielectronic recombination, the doubly excited state formed by radiationless capture may autoionize preferentially into an excited state of the recombining ion. This additional autoionization process has not been discussed in previous treatments of dielectronic recombination. The dielectronic recombination rates for certain nonhydrogenic Fe ions, although still larger than the direct radiative recombination rates, are found to be substantially reduced by the inclusion of the additional autoionization rate in the branching ratio for the stabilizing radiative transition. Consequently, the temperatures of maximum equilibrium abundance are significantly lower than those predicted by recent calculations. Finally, the radiative energy loss rate coefficients are calculated for radiation processes involving electron Fe-ion collisions in high-temperature plasmas. Electron impact excitation of resonance line radiation is the dominant radiative cooling mechanism in steady-state plasmas at temperatures where ions with bound electrons are abundant. However, it is found that the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.
Rotational excitation of linear molecules by collisions with atoms - Comparison of classical and quantum methods
Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO-He, CS-H2, OCS-H2, HCl-He, and HCl-Ar at collision energies up to 500/cm. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.
Perceived threat and avoidance maneuvers in response to cockpit traffic displays
Airline pilots rated their perception of the danger of an air-to-air collision based on cockpit displays of traffic information while they monitored simulated departures. They selected avoidance maneuvers when necessary for separation. Most evasive maneuvers were turns rather than vertical maneuvers. Evasive maneuvers chosen for encounters with lowor moderate-collision danger were generally toward the intruding aircraft. This tendency lessened as the perceived threat level increased. In the highest threat situations, pilots turned toward the intruder only at chance levels. Intruders coming from positions in front of the pilot's ship were more frequently avoided by turns toward than when intruders approached laterally or from behind. Some of the implications of the pilot's turning-toward tendencies are discussed with respect to automatic collision avoidance systems and coordination of avoidance maneuvers of conflicting aircraft.
Theoretical studies of Resonance Enhanced Stimulated Raman Scattering (RESRS) of frequency-doubled Alexandrite laser wavelength in cesium vapor
The solutions for the imaginary susceptibility of the Raman field transition with arbitrary relaxation rates and field strengths are examined for three different sets of relaxation rates. These rates correspond to: (1) Far Infrared (FIR) Raman lasers in the diabatic collision regime without consideration of coupled population decay in a closed system, (2) Raman FIR lasers in the diabatic collision regime with coupled population conserving decay, and (3) IR Raman gain in cesium vapor. The model is further expanded to include Doppler broadening and used to predict the peak gain as a function of detuning for a frequency doubled Alexandrite laser-pumped cesium vapor gain cell.
Spatial distribution of H2O(+) in Comet P/Halley
Comet Halley spectroscopic and imaging data near the time of the Vega 1 encounter in March 1986 have been analyzed for H2O(+) distribution and near-nucleus ion physics, using a data-reduction procedure encompassing flat fielding, sky removal, continuum subtraction, contamination correction, absolute flux calibration, and conversion to ion-column densities. A 25,000 km-radius collision zone within which collisions dominate the solar wind's sweeping effect is verified. A production rate of H2O(+) ions is calculated on the basis of the average velocity and column density integral across the tail. An analysis of the tailward ion flow indicates constant acceleration beyond 130,000 km from the nucleus.
Enhanced stabilisation of trapped electron modes by collisional energy scattering in tokamaks
The collisional stabilisation via energy scattering and pitch-angle scattering of micro-instabilities in tokamak plasmas is investigated by means of gyrokinetic simulations with a special emphasis on the often neglected energy scattering operator. It is shown that in the linear regime energy scattering has a negligible effect on Ion Temperature Gradient (ITG) modes but enhances the stabilisation of Trapped Electron Modes (TEM) in presence of nonzero ion temperature and density gradients. This stabilisation is sensitive to the model used for the energy restoring term in the collision operator. The contributions of parallel and drift motion to the total growth rate in velocity space are used to characterize the complex stabilisation mechanisms behind pitch-angle and energy scattering for a range of relevant parameters such as the magnetic shear, the collisionality, the logarithmic density gradient, and the logarithmic ion temperature gradient. It is shown that depending on these parameters, energy scattering stabilisation of TEM can be either due to a decrease of the contribution from drifting trapped electrons or to an increase of the contribution from the parallel motion of passing electrons. Finally, for a standard ITG/TEM case, the effect of energy scattering on the nonlinear heat and particle fluxes is investigated.
Density drift instabilities and weak collisions
A model is developed which describes the effects of weak collisions on the linear kinetic theory of electrostatic density drift instabilities. A dispersion equation valid at all frequencies and wave numbers is derived using the assumptions of a weak, uniform density gradient; a uniform magnetic field; and the BGK collision operator with a modification of the local approximation. The properties of the universal and collisional density drift instabilities at maximum growth rates are examined in detail. The thresholds of the instabilities are examined for an ionospheric model which includes ion-neutral, electron-neutral, and electron-ion collisions, and are compared with the threshold of the lower hybrid density drift instability. It is concluded that the k to the -5th short wavelength density power spectra observed above 280 km in the PLUMEX experiment are due to the effects of the universal density drift instability.
Measurements of charmonium production in p+p, p+Au, and Au+Au collisions at s NN = 200 GeV with the STAR experiment
Here, we present the first results from the STAR MTD of mid-rapidity charmonium measurements via the di-muon decay channel in p+p, p+Au, and Au+Au collisions at √S NN = 200 GeV at RHIC. The inclusive J/Ψ production cross section in p+p collisions can be described by the Non-Relativistic QCD (NRQCD) formalism coupled with the color glass condensate e ective theory (CGC) at low transverse momentum (p T ) and next-to-leading order NRQCD at high p T . The nuclear modification factor in p+Au collisions for inclusive J/Ψ is below unity at low p T and consistent with unity at high p T , which can be described by calculations including both nuclear PDF and nuclear absorption e ects. The double ratio of inclusive J/Ψ and Ψ(2S) production rates for 0 < p T < 10 GeV/c at mid-rapidity between p+p and p+Au collisions is measured to be 1.37 0.42 0.19. The nuclear modification factor in Au+Au collisions for inclusive J/Ψ shows significant J/Ψ suppression at high p T in central collisions and can be qualitatively described by transport models including dissociation and regeneration contributions.
Comparison of Quantum Mechanical and Empirical Potential Energy Surfaces and Computed Rate Coefficients for N2 Dissociation
Physics-based modeling of hypersonic flows is predicated on the availability of chemical reaction rate coefficients and cross sections for the collisional processes. This approach has been built around the use of quantum mechanical calculations to describe the interaction between the colliding particles. In this approach a potential energy surface (PES) is computed by solving the electronic Schrödinger equation and collision cross sections are determined for that PES using classical, semiclassical or quantum mechanical scattering methods. The rate coefficients are computed by integrating the thermally weighted cross sections. State-to-state rate coefficients are determined by only integrating over a thermal distribution of collisional energies. Finally, thermal rate coefficients are determined by summation of the state-to-state rate coefficients for reactions of molecules in all relevant ro-vibrational energy levels. If the flow is in thermal non-equilibrium, the translational, vibrational and rotational energy modes can be represented in different ways: three unique temperatures can be used to describe the distributions, the populations of individual ro-vibrational energy levels can be determined by solving the Master Equation, or through the use of direct simulation in particle-based Monte Carlo sampling. The PES-to-rate coefficient approach had been proposed and attempted in the early days of digital computing, but it is only in the last 15 years that computer hardware and software have been up to the task of calculating accurate interatomic and intermolecular potentials.
Columbus proto-planetesimal dust aggregation experiment
A microgravity experiment to study the growth of dust particles which has been proposed to be flown on one of the Columbus precursor flights is described. The microgravity environment will allow for low collision velocities (of order of mm/s) of the dust grains and for a large Knudsen number of the embedding gas; conditions expected in the early solar nebula. The outcome of the experiment will yield estimates of the sticking efficiency and the critical velocity for agglomeration. The values of these two parameters will provide substantial improvements in the constraints for models of the formation of planetesimals. In particular, the questions related to growth rate and mode of the aggregation process will be answered. The range of material type, collision velocities, properties of the environment in which growth takes place, and other factors permit a natural extension of this experiment to take advantage of the capabilities of the Space Station Columbus. Other astrophysical applications, such as processes in Saturnian rings, with somewhat different regimes could also be investigated.
Avalanche mechanism for runaway electron amplification in a tokamak plasma
The avalanche of runaway electrons is thought to pose a significant obstacle to the success of reactor scale devices such as ITER. As a result, a significant effort has been devoted toward quantifying both the threshold for the initiation of the avalanche of runaway electrons and the efficiency of the avalanche mechanism. In this work, these two quantities are computed utilizing a guiding-center formulation with large-angle collision operators of varying physics fidelity. The use of a guiding-center formulation, while computationally more costly compared to bounce-averaged approaches, provides a conceptually straightforward means of incorporating tokamak geometry. It is found that while the avalanche threshold is only weakly impacted by toroidal geometry for fully ionized low- Z plasmas, it can be significantly impacted if high- Z impurities are present. Furthermore, it is shown that the efficiency of the avalanche mechanism depends sensitively on the impurity content, the charge state of the underlying impurities, and the radial profile of the seed electron population. Lastly, the commonly employed Møller secondary source term used to model the generation of secondary electrons is shown to yield avalanche growth rates and thresholds in good agreement with a more complete conservative large-angle collision operator.