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Formation and Structure of Internal and Edge Transport Barriers

Description: The phenomenology of transport barrier formation is reviewed with a focus on physics that may be common to the edge and the core. To this end, the framework of E x B velocity shear reduction of turbulence-induced fluxes, applied to the edge for some time, is studied in light of measurements of core bifurcation dynamics and recent tests of causality. Also, the possible role of the magnetic shear structure in facilitating core barrier formation is examined. Experimental and theoretical challenges for developing predictive capability for reactor-relevant conditions are highlighted by recent observations of spontaneous electric field shear generation far removed from edge effects, and efforts to characterize the plasma edge at and across the L-H transition.
Date: February 1, 1998
Creator: Synakowski, E.J.
Partner: UNT Libraries Government Documents Department

Efficient single scatter electron Monte Carlo

Description: A single scatter electron Monte Carlo code (SSMC), CREEP, has been written which bridges the gap between existing transport methods and modeling real physical processes. CREEP simulates ionization,elastic and bremsstrahlung events individually. Excitation events are usually treated with an excitation-only stopping power, although simulation of individual excitation events is possible. Agreement of these quantities with experimental values is generally quite good.One application of this code is the generation of probability distribution functions (PDFs) to describe the phase space of a single electron emerging from a sphere of a given material and radius. A library of data sets for such spheres (or ``kugels``) is being computed for a variety of incident energies, material types, and sizes. The final goal of this work is to achieve extremely accurate transport results with an efficiency that is similar to that of condensed history methods.
Date: November 3, 1997
Creator: Svatos, M.M. & Rathkopf, J.A.
Partner: UNT Libraries Government Documents Department

Modelling TF ripple loss of alpha particles in TFTR DT experiments

Description: Modelling of TF ripple loss of alphas in DT experiments on TFTR now includes neoclassical calculations of first orbit loss, stochastic ripple diffusion, ripple trapping and collisional effects. A rapid way to simulate experiment has been developed which uses a simple stochastic domain model for TF ripple loss within the TRANSP analysis code, with the ripple diffusion threshold evaluated by comparison with more accurate but computationally expensive Hamiltonian coordinate guiding center code simulations. Typical TF collisional ripple loss predictions are 6-10% loss of alphas for TFTR D-T experiments at I{sub p} = 1.0-2.0 MA and R = 2.52 m.
Date: July 1, 1995
Creator: Redi, M.H.; Budny, R.V. & Darrow, D.S.
Partner: UNT Libraries Government Documents Department

Status of electron transport in MCNP{trademark}

Description: In recent years, an ongoing project within the radiation transport group (XTM) at Los Alamos National Laboratory has been the implementation and validation of an electron transport capability in the Monte Carlo code NICNP. In this paper the authors document the continuous-energy electron transport methods currently in use in MCNP, and describes a recent improvement of the energy-loss straggling algorithm. MCNP also supports electron transport calculations in a multigroup mode.
Date: September 1, 1995
Creator: Hughes, H.G.
Partner: UNT Libraries Government Documents Department

Energy and particle transport in the radiative divertor plasmas of DIII-D

Description: It has been argued that divertor energy transport dominated by parallel electron thermal conduction, or q{parallel} = -kT{sup 5/2}{sub 2} dT{sub e}/ds{parallel}, leads to severe localization of the intense radiating region and ultimately limits the fraction of energy flux that can be radiated before striking the divertor target. This is due to the strong T{sup 5/2}{sub e} dependence of electron heat conduction which results in very short spatial scales of the T{sub e} gradient at high power densities and low temperatures where deuterium and impurities radiate most effectively. However, we have greatly exceeded this constraint on DIII-D with deuterium gas puffing which reduces the peak heat flux to the divertor plate a factor of 5 while distributing the divertor radiation over a long length.
Date: June 1997
Creator: Leonard, A. W.; Allen, S. L. & Brooks, N. H.
Partner: UNT Libraries Government Documents Department

Shear flow effects on ion thermal transport in tokamaks

Description: From various laboratory and numerical experiments, there is clear evidence that under certain conditions the presence of sheared flows in a tokamak plasma can significantly reduce the ion thermal transport. In the presence of plasma fluctuations driven by the ion temperature gradient, the flows of energy and momentum parallel and perpendicular to the magnetic field are coupled with each other. This coupling manifests itself as significant off-diagonal coupling coefficients that give rise to new terms for anomalous transport. The authors derive from the gyrokinetic equation a set of velocity moment equations that describe the interaction among plasma turbulent fluctuations, the temperature gradient, the toroidal velocity shear, and the poloidal flow in a tokamak plasma. Four coupled equations for the amplitudes of the state variables radially extended over the transport region by toroidicity induced coupling are derived. The equations show bifurcations from the low confinement mode without sheared flows to high confinement mode with substantially reduced transport due to strong shear flows. Also discussed is the reduced version with three state variables. In the presence of sheared flows, the radially extended coupled toroidal modes driven by the ion temperature gradient disintegrate into smaller, less elongated vortices. Such a transition to smaller spatial correlation lengths changes the transport from Bohm-like to gyrobohm-like. The properties of these equations are analyzed. The conditions for the improved confined regime are obtained as a function of the momentum-energy deposition rates and profiles. The appearance of a transport barrier is a consequence of the present theory.
Date: March 1, 1995
Creator: Tajima, T.; Horton, W.; Dong, J.Q. & Kishimoto, Y.
Partner: UNT Libraries Government Documents Department

Kinetic Alfven waves and plasma transport at the magnetopause

Description: Large amplitude compressional type waves, with frequencies ranging from 10--500 mHz, are nearly always found in the magnetosheath near the magnetopause where there are large gradients in density, pressure and magnetic field. As compressional waves propagation to the magnetopause, there gradients efficiently couple them with shear/kinetic Alfven waves near the Alfven field-line resonance location ({omega} = k{sub {parallel}} v{sub A}). The authors present a solution of the kinetic-MHD wave equations for this process using a realistic equilibrium profile including full ion Larmor radius effects and wave-particle resonance interactions for electrons and ions to model the dissipation. For northward IMF a KAW propagates backward to the magnetosheath. For southward IMF the wave remains in the magnetopause but can propagate through the k{sub {parallel}} = 0 location. The quasi-linear theory predicts that KAWs produce plasma transport with a diffusion coefficient D{sub {perpendicular}} {approximately} 10{sup 9} m{sup 2}/s and plasma convection on the order of 1 km/s. However, for southward IMF additional transport can occur because magnetic islands form at the k{sub {parallel}} = 0 location. Due to the broadband nature of the observed waves these islands can overlap leading to stochastic transport which is much larger than that due to quasilinear effects.
Date: May 1, 1997
Creator: Johnson, J.R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Energetic Particle Transport in Compact Quasi-axisymmetric Stellarators

Description: Hamiltonian coordinate, guiding-center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the attractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle-following code is used to predict ion loss rates and particle confinement for thermal and neutral beam ions in a small experiment with R = 145 cm, B = 1-2 T and for alpha particles in a reactor-size device. In contrast to tokamaks, it is found that high edge poloidal flux has limited value in improving ion confinement in QAS, since collisional pitch-angle scattering drives ions into ripple wells and stochastic field regions, where they are quickly lost. The necessity for reduced stellarator ripple fields is emphasized. The high neutral beam ion loss predicted for these configurations suggests that more interesting physics could be explored with an experiment of less constrained size and magnetic field geometry.
Date: March 1, 1999
Creator: Redi, M.H.; Mynick, H.E.; Suewattana, M.; White, R.B.; Zarnstorff, M.C.; Isaev, M.Yu. et al.
Partner: UNT Libraries Government Documents Department

The transmission of differing energy beta particles through various materials

Description: The transmission of beta particles is frequently calculated in the same fashion as that of gamma rays, where the mass attenuation coefficient is defined by the slope of the exponential function. Numerous authors have used this approximation including Evans (1955), Loevinger (1952), and Chabot et. al. (1988). Recent work by McCarthy et. al. (1995) indicated that the exponential function seemed to fit well over a particular region of the transmission curve. Upon further investigation, the author decided to verify McCarthy`s results by the use of different absorber materials and attempt to reproduce the experiments. A theoretical method will be used to estimate the transmission of the beta particles through the three absorbers, aluminum, zirconium, and iron. An alternate Monte Carlo code, the Electron Gamma Shower version 4 code (EGS4) will also be used to verify that the experiment is approximating a pencil beam of beta particles. Although these two methods offer a good cross check for the experimental data, they pose a conflict in regards to the type of beam that is to be generated. The experimental lab setup uses a collimated beam of electrons that will impinge upon the absorber, while the codes are written using a pencil beam. A minor discrepancy is expected to be observed in the experimental results and is currently under investigation by McCarthy. The results of this project supported the theory that the beta mass attenuation coefficient was accurately represented by the slope of an exponential function, but only for that particular region of the transmission curve that has a minimal absorber thickness. By fitting the data beyond 50% of the beta particle range this theory does not hold true. The theory generated by McCarthy (1995) and the EGS4 Monte Carlo code indicated that the transmission curve for a pencil beam was not accurately ...
Date: April 1, 1996
Creator: Quayle, D.R.
Partner: UNT Libraries Government Documents Department

Precise measurements of the energy losses of heavy ions

Description: Accurate measurements of the energy loss of all charged particles are needed to determine the reliability of the Bethe theory of stopping power. Few measurements have been made for particles with energies greater than 20 MeV/u. A first step to accurate measurements is to establish the precision of an experimental method. The authors report here about the recent energy loss measurements for 290 MeV/u carbon ions from the HIMAC. They have been made with the method used for 70 MeV protons. The ion beam traverses an absorber of thickness t and the residual range of the ions is measured with a water container of adjustable thickness (``range gauge``).
Date: December 31, 1995
Creator: Bichsel, H. & Hiraoka, T.
Partner: UNT Libraries Government Documents Department

Plasma turbulence

Description: The origin of plasma turbulence from currents and spatial gradients in plasmas is described and shown to lead to the dominant transport mechanism in many plasma regimes. A wide variety of turbulent transport mechanism exists in plasmas. In this survey the authors summarize some of the universally observed plasma transport rates.
Date: July 1, 1998
Creator: Horton, W. & Hu, G.
Partner: UNT Libraries Government Documents Department

Mathematical modeling plasma transport in tokamaks

Description: In this work, the author applied a systematic calibration, validation and application procedure based on the methodology of mathematical modeling to international thermonuclear experimental reactor (ITER) ignition studies. The multi-mode plasma transport model used here includes a linear combination of drift wave branch and ballooning branch instabilities with two a priori uncertain constants to account for anomalous plasma transport in tokamaks. A Bayesian parameter estimation method is used including experimental calibration error/model offsets and error bar rescaling factors to determine the two uncertain constants in the transport model with quantitative confidence level estimates for the calibrated parameters, which gives two saturation levels of instabilities. This method is first tested using a gyroBohm multi-mode transport model with a pair of DIII-D discharge experimental data, and then applied to calibrating a nominal multi-mode transport model against a broad database using twelve discharges from seven different tokamaks. The calibrated transport model is then validated on five discharges from JT-60 with no adjustable constants. The results are in a good agreement with experimental data. Finally, the resulting class of multi-mode tokamak plasma transport models is applied to the transport analysis of the ignition probability in a next generation machine, ITER. A reference simulation of basic ITER engineering design activity (EDA) parameters shows that a self-sustained thermonuclear burn with 1.5 GW output power can be achieved provided that impurity control makes radiative losses sufficiently small at an average plasma density of 1.2 X 10{sup 20}/m{sup 3} with 50 MW auxiliary heating. The ignition probability of ITER for the EDA parameters, can be formally as high as 99.9% in the present context. The same probability for concept design activity (CDA) parameters of ITER, which has smaller size and lower current, is only 62.6%.
Date: December 31, 1995
Creator: Quiang, Ji
Partner: UNT Libraries Government Documents Department

MONTE CARLO ADVANCES FOR THE EOLUS ASCI PROJECT

Description: The Eolus ASCI project includes parallel, 3-D transport simulation for various nuclear applications. The codes developed within this project provide neutral and charged particle transport, detailed interaction physics, numerous source and tally capabilities, and general geometry packages. One such code is MCNPW which is a general purpose, 3-dimensional, time-dependent, continuous-energy Monte Carlo fully-coupled N-Particle transport code. Significant advances are also being made in the areas of modern software engineering and parallel computing. These advances are described in detail.
Date: January 1, 2000
Creator: HENDRICK, J. S.; MCKINNEY, G. W. & COX, L. J.
Partner: UNT Libraries Government Documents Department

A Rgularized Boltzmann Scattering Operator for Highly Forward Peaked Scattering

Description: Extremely short collision mean free paths and near-singular elastic and inelastic differential cross sections (DCS) make analog Monte Carlo and deterministic computational approaches impractical for charged particle transport. The widely used alternative, the condensed history method, while efficient, also suffers from several limitations arising from the use of precomputed infinite medium distributions for sampling particle directions and energies.
Date: April 13, 2005
Creator: Prinja, Anil K.
Partner: UNT Libraries Government Documents Department

In-Plane Magnetic Field Effect on the Transport Properties in a Quasi-3D Quantum Well Structure

Description: The transport properties of a quasi-three-dimensional, 200 layer quantum well structure are investigated at integer filling in the quantum Hall state. We find that the transverse magnetoresistance R<sub>xx</sub>, the Hall resistance R<sub>xy</sub>, and the vertical resistance R<sub>zz</sub> all follow a similar behavior with both temperature and in-plane magnetic field. A general feature of the influence of increasing in-plane field B<sub>in</sub> is that the Hall conductance quantization first improves, but above a characteristic value B<sup>C</sup><sub>in</sub>, the quantization is systematically removed. We consider the interplay of the chid edge state transport and the bulk (quantum Hall) transport properties. This mechanism may arise from the competition of the cyclotron energy with the superlattice band structure energies. A comparison of the resuIts with existing theories of the chiral edge state transport with in-plane field is also discussed.
Date: May 25, 1999
Creator: Brooks, J.; Clark, R.; Lumpkin, N.; O'Brien, J.; Reno, J.; Simmons, J. et al.
Partner: UNT Libraries Government Documents Department

Treating electron transport in MCNP{sup trademark}

Description: The transport of electrons and other charged particles is fundamentally different from that of neutrons and photons. A neutron, in aluminum slowing down from 0.5 MeV to 0.0625 MeV will have about 30 collisions; a photon will have fewer than ten. An electron with the same energy loss will undergo 10{sup 5} individual interactions. This great increase in computational complexity makes a single- collision Monte Carlo approach to electron transport unfeasible for many situations of practical interest. Considerable theoretical work has been done to develop a variety of analytic and semi-analytic multiple-scattering theories for the transport of charged particles. The theories used in the algorithms in MCNP are the Goudsmit-Saunderson theory for angular deflections, the Landau an theory of energy-loss fluctuations, and the Blunck-Leisegang enhancements of the Landau theory. In order to follow an electron through a significant energy loss, it is necessary to break the electron`s path into many steps. These steps are chosen to be long enough to encompass many collisions (so that multiple-scattering theories are valid) but short enough that the mean energy loss in any one step is small (for the approximations in the multiple-scattering theories). The energy loss and angular deflection of the electron during each step can then be sampled from probability distributions based on the appropriate multiple- scattering theories. This subsumption of the effects of many individual collisions into single steps that are sampled probabilistically constitutes the ``condensed history`` Monte Carlo method. This method is exemplified in the ETRAN series of electron/photon transport codes. The ETRAN codes are also the basis for the Integrated TIGER Series, a system of general-purpose, application-oriented electron/photon transport codes. The electron physics in MCNP is similar to that of the Integrated TIGER Series.
Date: December 31, 1996
Creator: Hughes, H.G.
Partner: UNT Libraries Government Documents Department

ONEBFP development issues

Description: ONEBFP is a 1-D discrete-ordinates coupled electron-photon transport code that was recently developed to replace the ONELD code. The reasons for this replacement are explained in terms of a comparison of the programming structure and numerics of the two codes.
Date: October 1, 1996
Creator: Morel, J.E. & Walters, W.F.
Partner: UNT Libraries Government Documents Department

Electron transport phenomena and dense plasmas produced by ultra-short pulse laser interaction

Description: Recent experiments with femtosecond lasers provide a test bed for theoretical ideas about electron processes in hot dense plasmas. We briefly review aspects of electron conduction theory likely to prove relevant to femtosecond laser absorption. We show that the Mott-Ioffe-Regel limit implies a maximum inverse bremsstrahlung absorption of about 50% at temperatures near the Fermi temperature. We also propose that sheath inverse bremsstrahlung leads to a minimum absorption of 7-10% at high laser intensity.
Date: July 8, 1994
Creator: More, R.M.
Partner: UNT Libraries Government Documents Department

Electronic excitation in transmission of relativistic H{sup {minus}} ions through thin foils

Description: The authors describe a theoretical model to study the transmission of relativistic H{sup {minus}} ions through thin carbon foils. The approach is based on a Monte Carlo solution of the Langevin equation describing electronic excitations of the atoms during the transport through the foil. Calculations for the subshell populations of outgoing hydrogen atoms are found to be in good agreement with recent experimental data on an absolute scale and show that there exists a propensity for populating extreme Stark states.
Date: May 7, 1998
Creator: Reinhold, C.O.; Kuerpick, P.; Burgdoerfer, J.; Yoshida, S. & Gervais, B.
Partner: UNT Libraries Government Documents Department

950809 Charged particle transport updated multi-group diffusion

Description: In 1974, a charged particle transport scheme was introduced which utilized a multi-group diffusion method for the spatial transport and slowing down of energetic ions in a hot plasma. In this treatment a diffusion coefficient was used which was flux-limited to provide, hopefully, some degree of accuracy when the slowing down of an energetic charged particle is dominated by Coulomb collisions with thermal ions and electrons in a plasma medium. An advantage of this method was a very fast, memory-contained program for calculating the behavior of energetic charged particles which resulted in smoothly varying particle number densities and energy depositions. The main limitation of the original multi-group charged particle diffusion scheme is its constraint to a basic ten group structure; the same ten group structure for each of the five energetic ions tracked. This is regarded as a severe limitation, inasmuch as more groups would be desired to simulate more accurately the corresponding Monte Carlo results of energies deposited over spatial zones from a charged particle source. More generally, it seems preferable to have a different group structure for each particle type since they are created at inherently different energies. In this paper, the basic theory and multi-group description will be given. This is followed by the specific techniques that were used to solve the resultant equations. Finally, the modifications that were made to the cross section data as well as the methods used for energy and momentum deposition are described.
Date: September 1, 1995
Creator: Corman, E.G.; Perkins, S.T. & Dairiki, N.T.
Partner: UNT Libraries Government Documents Department

Ergodic mixing for turbulent drift motion

Description: The statistical properties of the long-time chaotic two-dimensional (2D) drift motion of a charged particle in an inhomogeneous magnetic field {beta}(x,y) and a time-dependent electrostatic potential {phi}(x,y,t) are studied by numerical symplectic integration. For a conditionally periodic potential with two or more incommensurate frequencies, an ergodic behavior is demonstrated in which the probability density of the particle position is proportional to the magnetic field {beta}. The accuracy of this prediction is found to be independent of the number N{sub {omega}} of the incommensurate frequencies for N{sub {omega}} {ge}2.
Date: February 16, 1995
Creator: Isichenko, M.B. & Petviashvili, N.V.
Partner: UNT Libraries Government Documents Department