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Beam matching and halo control

Description: We present an overview of the status of ongoing work on physics models describing beam matching and halo control for particle accelerators, particularly high power ion linacs. We consider moments and various new variables that more naturally describe beam halo evolution. We compute matched beams and ``mode invariants`` (analogs of moment invariants) using primarily symbolic techniques.
Date: September 1, 1997
Creator: Lysenko, W. & Parsa, Z.
Partner: UNT Libraries Government Documents Department

Power Corrections in Charmless Nonleptonic B Decays: Annihilationis Factorizable and Real

Description: We classify {Lambda}{sub QCD}/m{sub b} power corrections to nonleptonic B {yields} M{sub 1}M{sub 2} decays, where M{sub 1,2} are charmless non-isosinglet mesons. Using recent developments in soft-collinear effective theory, we prove that the leading contributions to annihilation amplitudes of order {alpha}{sub s}(m{sub b}) {Lambda}{sub QCD}/m{sub b} are real. The leading annihilation amplitudes depend on twist-2 and the twist-3 three parton distributions. A complex nonperturbative parameter from annihilation first appears at {Omega}[{alpha}{sub s}{sup 2}({radical}{Lambda}m{sub b}){Lambda}{sub QCD}/m{sub b}]. 'Chirally enhanced' contributions are also factorizable and real at lowest order. Thus, incalculable strong phases are suppressed in annihilation amplitudes, unless the {alpha}{sub s}({radical}{Lambda}m{sub b}) expansion breaks down. Modeling the distribution functions, we find that (11 {+-} 9)% and (15 {+-} 11)% of the absolute values of the measured {bar B}{sup 0} {yields} K{sup -}{pi}{sup +} and B{sup -} {yields} K{sup -}K{sup 0} penguin amplitudes come from annihilation. This is consistent with the expected size of power corrections.
Date: October 10, 2006
Creator: Arnesen, Christian M.; Ligeti, Zoltan; Rothstein, Ira Z. & Stewart, Iain W.
Partner: UNT Libraries Government Documents Department

The Hamiltonian Mechanics of Stochastic Acceleration

Description: We show how to nd the physical Langevin equation describing the trajectories of particles un- dergoing collisionless stochastic acceleration. These stochastic di erential equations retain not only one-, but two-particle statistics, and inherit the Hamiltonian nature of the underlying microscopic equations. This opens the door to using stochastic variational integrators to perform simulations of stochastic interactions such as Fermi acceleration. We illustrate the theory by applying it to two example problems.
Date: July 17, 2013
Creator: Burby, J. W.
Partner: UNT Libraries Government Documents Department

Study of the uncertainty of the gluon distribution

Description: The uncertainty in the calculation of many important new processes at the Tevatron and LHC is dominated by that concerning the gluon distribution function. We investigate the uncertainty in the gluon distribution of the proton by systematically varying the gluon parameters in the global QCD analysis of parton distributions. The results depend critically on the parton momentum fraction x and the QCD scale Q{sup 2}. The uncertainties are presented for integrated gluon-gluon and gluon-quark luminosities for both the Tevatron and LHC as a function of {radical}{tau}={radical}x{sub 1}x{sub 2}={radical}{cflx s}/s, the most relevant quantity for new particle production. The uncertainties are reasonably small, except for large x.
Date: July 1, 1998
Creator: Huston, J., FERMI
Partner: UNT Libraries Government Documents Department

Magnetic field effects on plasma ionization balance

Description: Magnetic fields give rise to several phenomena that can significantly affect ionization balance in a plasma. Theoretical models commonly used to determine the charge state distribution (viz., <Z>) of ions in non-magnetized plasmas are reviewed first, for both equilibrium and non-equilibrium situations. Then, after a brief survey of laboratory and cosmic plasmas with strong fields, B > 10{sup 6} Gauss, some of the ways such magnetic fields influence <Z> are highlighted. Most key problems have yet to be tackled.
Date: December 31, 1995
Creator: Weisheit, J.C.
Partner: UNT Libraries Government Documents Department

Local Atomic Structure of Semiconductor Alloys Using Pair Distribution Function Analysis

Description: We have been taking advantage of recent experimental developments, which involve utilizing diffraction data from x-rays or neutrons out to very large wave-vectors, to obtain a detailed structural characterization of semiconductor alloys. This approach allows an accurate Pair Distribution Function (PDF) to be obtained to 20A and beyond and reveals the local structure of the alloy directly. These data can be modeled explicitly to learn about local correlations and short-range order in materials. We are combining theory, modeling and experiments to study a range of materials from semiconductors to thermoelectrics and proton conductors.
Date: June 24, 2002
Creator: Billinge, S.J.L. & Thorpe, M.F.
Partner: UNT Libraries Government Documents Department

Finite Size Effects on the Real-Space Pair Distribution Function of Nanoparticles

Description: The pair distribution function (PDF) method is a powerful approach for the analysis of the structure of nanoparticles. An important approximation used in nanoparticle PDF simulations is the incorporation of a form factor describing nanoparticle size and shape. The precise effect of the form factor on the PDF is determined by both particle shape and structure if these characteristics are both anisotropic and correlated. The correct incorporation of finite size effects is important for distinguishing and quantifying the structural consequences of small particle size in nanomaterials.
Date: October 1, 2008
Creator: Gilbert, Benjamin
Partner: UNT Libraries Government Documents Department

A comparison between Gauss-Newton and Markov chain Monte Carlo basedmethods for inverting spectral induced polarization data for Cole-Coleparameters

Description: We develop a Bayesian model to invert spectral induced polarization (SIP) data for Cole-Cole parameters using Markov chain Monte Carlo (MCMC) sampling methods. We compare the performance of the MCMC based stochastic method with an iterative Gauss-Newton based deterministic method for Cole-Cole parameter estimation through inversion of synthetic and laboratory SIP data. The Gauss-Newton based method can provide an optimal solution for given objective functions under constraints, but the obtained optimal solution generally depends on the choice of initial values and the estimated uncertainty information is often inaccurate or insufficient. In contrast, the MCMC based inversion method provides extensive global information on unknown parameters, such as the marginal probability distribution functions, from which we can obtain better estimates and tighter uncertainty bounds of the parameters than with the deterministic method. Additionally, the results obtained with the MCMC method are independent of the choice of initial values. Because the MCMC based method does not explicitly offer single optimal solution for given objective functions, the deterministic and stochastic methods can complement each other. For example, the stochastic method can first be used to obtain the means of the unknown parameters by starting from an arbitrary set of initial values and the deterministic method can then be initiated using the means as starting values to obtain the optimal estimates of the Cole-Cole parameters.
Date: May 15, 2008
Creator: Chen, Jinsong; Kemna, Andreas & Hubbard, Susan S.
Partner: UNT Libraries Government Documents Department

Cumalative Distribution Functions for the Relative Humidity Thresholds for the Onset of Carbon Steel Corrosion

Description: The purpose of this calculation is to process the cumulative distribution functions (CDFs) characterizing the relative humidity (RH) thresholds for the onset of carbon steel corrosion provided by expert elicitation and minimize the set of values to 200 points for use in WAPDEG.
Date: May 13, 1998
Creator: Mon, K.G.
Partner: UNT Libraries Government Documents Department

CUMULATIVE DISTRIBUTION FUNCTIONS FOR THE TEMPERATURE THRESHOLD FOR THE ONSET OF CARBON STEEL CORROSION

Description: The purpose of this calculation is to process the cumulative distribution functions (CDFs) characterizing the temperature threshold for the onset of corrosion provided by expert elicitation and minimize the set of values to 200 points for use in WAPDEG.
Date: May 15, 1998
Creator: Mon, K.G.
Partner: UNT Libraries Government Documents Department

Numerical Calculation of Neoclassical Distribution Functions and Current Profiles in Low Collisionality, Axisymmetric Plasmas

Description: A new code, the Neoclassical Ion-Electron Solver (NIES), has been written to solve for stationary, axisymmetric distribution functions (f ) in the conventional banana regime for both ions and elec trons using a set of drift-kinetic equations (DKEs) with linearized Fokker-Planck-Landau collision operators. Solvability conditions on the DKEs determine the relevant non-adiabatic pieces of f (called h ). We work in a 4D phase space in which Ψ defines a flux surface, θ is the poloidal angle, v is the total velocity referenced to the mean flow velocity, and λ is the dimensionless magnetic moment parameter. We expand h in finite elements in both v and λ#21; . The Rosenbluth potentials, φ#8; and ψ, which define the integral part of the collision operator, are expanded in Legendre series in cos χ , where #31;χ is the pitch angle, Fourier series in cos #18;θ , and finite elements in v . At each ψ , we solve a block tridiagonal system for hi (independent of fe ), then solve another block tridiagonal system for he (dependent on fi ). We demonstrate that such a formulation can be accurately and efficiently solved. NIES is coupled to the MHD equilibrium code JSOLVER [J. DeLucia, et al., J. Comput. Phys. 37 , pp 183-204 (1980).] allowing us to work with realistic magnetic geometries. The bootstrap current is calculated as a simple moment of the distribution function. Results are benchmarked against the Sauter analytic formulas and can be used as a kinetic closure for an MHD code (e.g., M3D-C1 [S.C. Jardin, et al ., Computational Science &amp; Discovery, 4 (2012).]).
Date: June 28, 2012
Creator: B.C. Lyons, S.C. Jardin, and J.J. Ramos
Partner: UNT Libraries Government Documents Department

Ion acceleration and cooling in gasless self-sputtering

Description: Copper plasma with hyperthermal directed velocity (8.8 eV) but very low temperature (0.6 eV) has been obtained using self-sputtering far above the runaway threshold. Ion energy distribution functions (IEDFs) were simultaneously measured at 34 locations. The IEDFs show the tail of the Thompson distribution near the magnetron target. They transform to shifted Maxwellians with the ions being accelerated and cooled. We deduce the existence of a highly asymmetric, pressure-driven potential hump which acts as a controlling&quot;watershed&quot; between the ion return flux and the expanding plasma.
Date: October 31, 2010
Creator: Horwat, David & Anders, Andre
Partner: UNT Libraries Government Documents Department

The sputter generation of negative ion beams

Description: A brief review is given of recent progress toward a quantitative understanding of negative ion formation by sputtering from surfaces covered with fractional layers of highly electropositive adsorbates. Practical models developed for estimating changes in work functions {Delta}{phi} by electropositive adsorbates are described. The secondary negative ion generation process is examined through the use of composite energy/velocity dependent analytical models. These models are used to illustrate the effect of work function on the energy distributions of negative ions sputter ejected from a polycrystalline molybdenum surface covered with fractional layers of cesium. Predictions are also made of the functional dependence of the probability for negative ion formation on cesium coverage. The models predict energy distributions which are in basic disagreement with experimental observations, implying their inappropriateness for describing the sputter negative ion generation process. We have also developed a model for calculating sputter ratios based on the use of simple scaling procedures to bring Sigmund theory into close agreement with experimental observation accounting for the threshold effect. Scaling factors for projectile energies E > 1000 eV are found to be independent of energy while those for projectile energies E{sub th} < E < 1000 eV were found to be energy dependent. In this study, the model and scaling techniques utilized to bring Sigmund theory into agreement with experiment are discussed in detail and several examples provided which illustrate the versatility, accuracy and utility of the model. In the present report, we describe the model and apply it to the case of sputtering a selected number of metals with energetic cesium ions. In particular, we present sputter ratio information for a number of Cs-projectile/metal-target combinations; the targets are bombarded at normal incidence to the surface.
Date: December 31, 1996
Creator: Alton, G.D.
Partner: UNT Libraries Government Documents Department

Blackbox and non-blackbox optimization: A common perspective

Description: The SEARCH (Search Envisioned As Relation & Class Hierarchizing) framework developed elsewhere offered an alternate perspective toward blackbox optimization (BBO)-optimization in absence of domain knowledge. This paper argues that the fundamental concepts are also applicable to non-blackbox optimization (NBBO)-optimization in presence of information about the search domain and objective function. The SEARCH framework investigates the conditions essential for transcending the limits of random enumerative search using a framework developed in terms of relations, classes and partial ordering. This paper reviews some of the main results of that work and describes its generality by ordering different popular BBO and NBBO algorithms.
Date: September 1, 1996
Creator: Karguptra, H.
Partner: UNT Libraries Government Documents Department

Measuring longitudinal distribution and bunch length of femtosecond bunches with RF zero-phasing method

Description: Recently there has been increasing interest in applications of very short electron bunches. Accurately measuring bunch length and profiles becomes essential for characterizing, commissioning, and operating such short bunch machines. The RF zero-phasing method is the only technique that is able to measure bunch length and longitudinal density distribution in the femtosecond regime. In this paper, analytical formulas for calculating longitudinal profile and bunch length are given. using a such technique, bunch lengths as short as 84 fs rms have been measured.
Date: June 1, 1997
Creator: Wang, D.X. & Krafft, G.A.
Partner: UNT Libraries Government Documents Department

Theory of Resonance Influence of Sawtooth Crashes on Ions with Large Orbit Width

Description: The role of resonances in the sawtooth-crash-induced redistribution of fast ions is investigated. In particular, the conditions of wave-particle resonant interaction in the presence of the equilibrium electric field and the mode rotation are obtained, and effects of sawteeth on the resonant particles with arbitrary width of non-perturbed orbits are studied. It is found that resonances play the dominant role in the transport of ions having sufficiently high energy. It is shown that the resonance regions may overlap, in which case the resonant particles may constitute the main fraction of the fast ion population in the sawtooth mixing region. The behavior of the resonant particles is studied both by constructing a Poincaré map and analytically, by means of the adiabatic invariant derived in this paper and calculation of the characteristic frequencies of the particle motion.
Date: February 1, 1998
Creator: White, R.B.; Lutsenko, V.V.; Kolesnichenko, Y.I. & Yakovenko, Y.V.
Partner: UNT Libraries Government Documents Department

Conceptual foundation of the Fokker-Planck approach to space-charge effects

Description: An rms-mismatched beam can evolve rapidly to a configuration of quasiequilibrium under the influence of space-charge forces. As sit evolves, its emittance grows and a diffuse halo forms. The beam's distribution function accounts for all the complicated dynamics. Unfortunately, the distribution function is difficult to calculate in as much as the physics lies at the interface between classical mechanics and thermodynamics. This paper presents the foundation for a statistical theory of the dynamics of nonequilibrium space-charge-dominated beams. Within certain approximations, the theory takes on a Fokker-Planck form. Key questions arise concerning the nature of the dynamical friction and diffusion in the beam's phase space and of the quasiequilibrium configuration that ensues.
Date: July 1, 1996
Creator: Bohn, C.L.
Partner: UNT Libraries Government Documents Department

Generation of time histories with a specified auto spectral density and probability density function

Description: It is recognized that some dynamic and noise environments are characterized by time histories which are not Gaussian. An example is high intensity acoustic noise. Another example is some transportation vibration. A better simulation of these environments can be generated if a zero mean non-Gaussian time history can be reproduced with a specified auto (or power) spectral density (ASD or PSD) and a specified probability density function (pdf). After the required time history is synthesized, the waveform can be used for simulation purposes. For example, modem waveform reproduction techniques can be used to reproduce the waveform on electrodynamic or electrohydraulic shakers. Or the waveforms can be used in digital simulations. A method is presented for the generation of realizations of zero mean non-Gaussian random time histories with a specified ASD, and pdf. First a Gaussian time history with the specified auto (or power) spectral density (ASD) is generated. A monotonic nonlinear function relating the Gaussian waveform to the desired realization is then established based on the Cumulative Distribution Function (CDF) of the desired waveform and the known CDF of a Gaussian waveform. The established function is used to transform the Gaussian waveform to a realization of the desired waveform. Since the transformation preserves the zero-crossings and peaks of the original Gaussian waveform, and does not introduce any substantial discontinuities, the ASD is not substantially changed. Several methods are available to generate a realization of a Gaussian distributed waveform with a known ASD. The method of Smallwood and Paez (1993) is an example. However, the generation of random noise with a specified ASD but with a non-Gaussian distribution is less well known.
Date: August 1, 1996
Creator: Smallwood, D.O.
Partner: UNT Libraries Government Documents Department

Bootstrap current close to magnetic axis in tokamaks

Description: It is shown that the bootstrap current density close to the magnetic axis in tokamaks does not vanish in simple electron-ion plasmas because the fraction of the trapped particles is finite. The magnitude of the current density could be comparable to that in the outer core region. This may reduce or even eliminate the need of the seed current.
Date: December 1, 1996
Creator: Shaing, K.C. & Hazeltine, R.D.
Partner: UNT Libraries Government Documents Department

Constrained noninformative priors

Description: The Jeffreys noninformative prior distribution for a single unknown parameter is the distribution corresponding to a uniform distribution in the transformed model where the unknown parameter is approximately a location parameter. To obtain a prior distribution with a specified mean but with diffusion reflecting great uncertainty, a natural generalization of the noninformative prior is the distribution corresponding to the constrained maximum entropy distribution in the transformed model. Examples are given.
Date: October 1, 1994
Creator: Atwood, C.L.
Partner: UNT Libraries Government Documents Department

Production asymmetries in x{sub f} and P{sub t}{sup 2} for D{sup {plus_minus}} mesons

Description: We present differences in leading and non-leading charged D meson production as a doubly-differential function of both P{sup 2}{sub t} and {chi}{integral}. Comparisons to specific models are made. This information is from the analysis of half the data from Fermilab experiment E791, taken during the 1991-2 fixed target run with a 500 GeV/c {pi}{sup {minus}} beam incident on a segmented target.
Date: November 1, 1994
Creator: Carter, T. & Collaboration, E791
Partner: UNT Libraries Government Documents Department

Beam matching and halo control

Description: The authors present an overview of the status of ongoing work on physics models describing beam matching and halo control for particle accelerators, particularly high power ion linacs. They consider moments and various new variables that more naturally describe beam halo evolution. The authors compute matched beams and {open_quotes}mode invariants{close_quotes} (analogs of moment invariants) using primarily symbolic techniques.
Date: May 1, 1997
Creator: Lysenko, W. & Parsa, Z.
Partner: UNT Libraries Government Documents Department

Fast ion physics from neutral particle analysis on the large helical device

Description: Time-resolved measurements of the energetic ion distribution from different viewing angles allow study of some important aspects of stellarator physics. Examples from the Advanced Toroidal Facility (ATF) of measurements of the ion distribution function and the variation of orbits with pitch {nu}{parallel}/{nu} and electric field are used to illustrate some of these points; ATF`s magnetic configuration was similar to that of the Large Helical Device (LHD). Silicon ion-implanted detectors should allow measurements of the ion distribution function that are not practical with conventional neutral particle analyzers (NPAs) on LHD. Initial implementation of a silicon-detector NPA array on LHD is discussed.
Date: December 1, 1997
Creator: Lyon, J.F. & Spong, D.A.
Partner: UNT Libraries Government Documents Department