153 Matching Results

Search Results

Advanced search parameters have been applied.

Solvolytic liquefaction of wood under mild conditions

Description: Conversion of wood to liquid products requires cleavage of bonds which crosslink the wood structure. This study examines a low-severity wood solubilization process utilizing a solvent medium consisting of a small amount of sulfuric acid and a potentially wood-derivable alcohol. In one half hour of reaction time at 250/sup 0/C under 15 psia starting nitrogen pressure, over 95% of the wood (maf) was rendered acetone-soluble. The product is a soft, black, bitumen-like solid at room temperature but readily softens at 140/sup 0/C. Between 25 and 50% of the original wood oxygen, depending on alcohol used, was removed as water. Approximately 2 to 17% of the alcohols were retained in the product. Gel permeation chromatography showed that the product's median molecular weight is around 300. Based on experimental and literature results, a mechanism for wood solubilization is proposed. This involves protonation of the etheric oxygen atoms, leading to subsequent bond scission to form carbonium ions which are stabilized by solvent alkoxylation. At severe conditions, polymerization and condensation reactions result in acetone-insoluble materials.
Date: April 1, 1982
Creator: Yu, S.M.
Partner: UNT Libraries Government Documents Department

Intense electron beams

Description: This paper is concerned with intense electron beams, typically of the order of a few kA in current and up to tens of MeV in beam energy. A beam of this kind can be produced from induction machines, examples of which are the ERA (4 MeV, 1 kA) at Lawrence Berkeley Laboratory and the ASTRON (5 MeV, 500A), ETA (5 MeV, 10 kA), and ATA (50 MeV, 10 kA) at Lawrence Livermore National Laboratory (LLNL). The emphasis of the paper is on the characteristics of these beams and some applications. 13 refs., 2 figs. (LSP)
Date: October 20, 1987
Creator: Yu, S.S.
Partner: UNT Libraries Government Documents Department

Atomic data needs for beam transport in gas

Description: Determination of the parameters Z/sub eff/, electrical conductivity, plasma density, and the plasma temperature is essential in the study of heavy ion beam transport in gas. The calculation of these parameters require input from atomic physics. This note is an attempt to make these needs known to atomic physicists.
Date: November 3, 1977
Creator: Yu, S.S.
Partner: UNT Libraries Government Documents Department

Boltzmann approach to conductivity calculations

Description: The basic set of equations is derived for the calculation of the time-dependent electrical conductivity of an initially unionized gas generated by a propagating relativistic electron beam. In contrast with previous work, which used thermal averages, conductivity is determined from the complete, nonequilibrium electron energy distribution. The evolution of the distribution function, resulting from interaction with gas molecules, ions, beam induced electric field, and beam current, is described by a Boltzmann equation.
Date: August 11, 1978
Creator: Yu, S.S.
Partner: UNT Libraries Government Documents Department

Modeling beam-front dynamics at low gas pressures

Description: The dynamics of space charge neutralization at the front of an intense self-focused electron beam pulse exhibits important differences in different gas pressure regimes. At very low pressures, the beam front is in the so-called ion-focused regime (IFR) where all secondary electrons are expelled from the beam region by the radial electric field without causing significant additional ionization. We estimate the upper pressure boundary of this regime by considering the distance scale length for cascade (avalanche) ionization. Data from the FX-25 diode experiments indicate a critical transition pressure (P/sub c/) that agrees with this estimate and with its scaling among various gas types. Normal mobility-limited treatments (local conductivity models) of the secondary electrons at the beam front are not justified until the gas pressure is 10 to 50 times higher than P/sub c/, due to runaway of these secondary electrons in the strong space-charge electric field at the lower pressures. The main conclusion of this study is that a non-local phase space (Boltzmann) treatment of the secondary electrons is required to accurately describe these different beam front regimes and the transitions between them; such a code model is currently under development.
Date: May 13, 1982
Creator: Briggs, R.J. & Yu, S.
Partner: UNT Libraries Government Documents Department

Analytic solution for the problem of gridded gap-electron flow interaction

Description: The present study is motivated by the modelling of high-power klystrons. The two basic components of a Klystron are the resonant cavities and the drift spaces. This paper addresses only the first of these two components. Modelling of the drift spaces is deferred to future work. The formulation is not restricted to Klystron modelling, but is applicable to any problem involving the interaction of an electron beam with a resonant cavity. While the theory of Klystrons has been worked out in detail in the small signal limit, the problem remains largely unsolved when the signals are large. In particular, the hydrodynamic models of electron beams used to derive the small signal theories fail when particle trajectories cross each other. In this paper, we employ a Vlasov description of the electron beam to study the Klystron problem. In the Vlasov formulation we follow the evolution of the electron distribution function in phase-space. The general framework can naturally accommodate particle crossing, and the beam dynamics is accurately described even when the signals are large.
Date: January 1, 1984
Creator: Kheifets, S.; Yu, S. & Jaeger, J.
Partner: UNT Libraries Government Documents Department

Analysis of valence-electron structures of Y/sub 3/Al/sub 5/O/sub 12/(YAG) and Al/sub 2/O/sub 3/, Cr/sub 2/O/sub 2/ (Ruby), a study of certain properties of these laser materials related to their valence-electron structures

Description: A direct method of determination of the valence-electron structure from its crystal structure has been presented by the writer at XIIth International Congress of Crystallography, at Ottawa, 1981. Here the method is applied to determine the valence-electron structures of Y/sub 3/Al/sub 5/O/sub 12/ (YAG) and Al/sub 2/O/sub 3/, Cr/sub 2/O/sub 3/ (ruby) to see how certain properties of these laser materials are related to their valence-electron structures. The first point observed is the very strong, continuous, but zig-zag three-dimensional fractional covalent M-O bond nets (M = Al or Cr) being connected with the high melting points, the great resistance against the puncture by the strong laser beam. In the case of Nd-YAG, the distortion caused by the replacement of the Y atoms by almost the same size Nd atoms is small. On the other hand, because of the similarity of the valence-electron structures of ..cap alpha..-Al/sub 2/O/sub 3/ and Cr/sub 2/O/sub 3/, the replacement of Al atoms by the little larger size Cr atoms in small amount is easily understood. The large atomic spins of magnetic moment m/sub B/ = 2.76 ..mu../sub B/ (= experiment value, the theoretical m/sub B/ = 2.70/sub 2/ ..mu../sub B/) of Cr atoms pointing parallel to the c axis of the hexagonal lattice, is found to be due to the direction of the spin situated in the space of lowest density space of the valence-electron cloud distribution. This agrees with the result of ..cap alpha..-Fe disclosed in the paper Electron theory of the magnetic moment structures of ..cap alpha..-Fe, epsilon-Co, Ni from neutron diffraction experiments presented at the Symposium on Neutron Scattering, Argonne National Laboratory, 1981. This will be found also true in many other oxides like ..cap alpha..-Fe/sub 2/O/sub 3/, FeTiO/sub 3/, MnTiO/sub 3/ and so on.
Date: January 1, 1982
Creator: Yu, S.H. & Yu, L.H.
Partner: UNT Libraries Government Documents Department

Is the two-term expansion valid for highly anisotropic systems. The Townsend ionization coefficient in strong runaway as a test case

Description: The Townsend ionization coefficient in the strong runaway regime is calculated within the framework of the two-term expansion. Results are compared to the 1-D model. General features of the two models are qualitatively similar, but quantitative differences by factors of approx. 2 are observed.
Date: July 15, 1982
Creator: Yu, S.S. & Melendez, R.E.
Partner: UNT Libraries Government Documents Department

Model of current enhancement at high pressure

Description: A model is proposed to account for the phenomenon of net current enhancement at high pressures recently observed on the Experimental Test Accelerator. The proposed mechanism involves energetic secondary electrons (delta rays) which are pushed forward by the self-magnetic field of the electron beam. For high current beams, the forward delta ray current can build up to a significant fraction of the beam current. Analytic calculations of the steady-state solution as well as the rate of buildup of the delta ray current are presented in this paper. In addition, numerical results from a nonlocal Boltzmann code, NUTS, are presented. The analytic and numerical results have many features which are in qualitative agreement with the experiments, but quantitative discrepancies still exist.
Date: April 5, 1983
Creator: Yu, S.S. & Melendez, R.E.
Partner: UNT Libraries Government Documents Department

Numerical simulation of secondary electron orbits near an electron beam propagating in a low pressure gas

Description: A numerical simulation has been constructed to obtain a detailed, quantitative estimate of the electromagnetic fields generated in a recently-proposed collective accelerator scheme for electrons. The code treats the secondary electrons by particle simulation and the beam dynamics by a time-dependent envelope model. The simulation gives a fully relativistic description of secondary electrons moving in selfconsistent electromagnetic fields. The calculations are made using coordinates t, x, y, z for the electrons and t, ct-z, r for the axisymmetric electromagnetic fields and currents. Code results showing the axial electric field dependence on the configuration of the ultrashort U.V. laser pulse will be given. 4 refs., 4 figs.
Date: August 14, 1985
Creator: Teague, M.R. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Model of emittance growth in a self-pinched beam

Description: A semi-phenomenological formula is proposed for the change of emittance of a self-pinched beam which is not matched to its equilibrium radius. Near equilibrium this formula, coupled with an envelope equation, yields the damped sausage oscillations observed in simulation and experiments. For a beam which is injected cold (no transverse velocity spread), the formula coincides with the analytically calculated initial growth of emittance. The basic theory is developed here and used to compute the linear damping rate for several current profiles. The resultant non-linear increase in equilibrium quantities is also calculated in lowest order of the degree of mismatch.
Date: December 3, 1979
Creator: Lee, E.P. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Nonlocal Boltzmann theory of plasma channels

Description: The mathematical framework for the LLNL code NUTS is developed. This code is designed to study the evolution of an electron-beam-generated plasma channel at all pressures. The Boltzmann treatment of the secondary electrons presented include all inertial, nonlocal, electric and magnetic effects, as well as effects of atomic collisions. Field equations are advanced simultaneously and self-consistently with the evolving plasma currents.
Date: January 3, 1983
Creator: Yu, S.S. & Melendez, R.E.
Partner: UNT Libraries Government Documents Department

IFR code for secondary particle dynamics

Description: A numerical simulation has been constructed to obtain a detailed, quantitative estimate of the electromagnetic fields and currents existing in the Advanced Test Accelerator under conditions of laser guiding. The code treats the secondary electrons by particle simulation and the beam dynamics by a time-dependent envelope model. The simulation gives a fully relativistic description of secondary electrons moving in self-consistent electromagnetic fields. The calculations are made using coordinates t, x, y, z for the electrons and t, ct-z, r for the axisymmetric electromagnetic fields and currents. Code results, showing in particular current enhancement effects, will be given.
Date: October 11, 1985
Creator: Teague, M.R. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Soft X-ray Studies of Pu Electronic Structure: Past Lessons and Future Directions

Description: Photoelectron Spectroscopy (PES) and X-ray Absorption Spectroscopy (XAS, Figure 1) have contributed greatly to our improved understanding of Pu electronic structure. From these and related measurements, the following has been determined: (1) The Pu 5f spin-orbit splitting is large; (2) The number of Pu5f electrons is near 5; and (3) The Pu 5f spin-orbit splitting effect dominates 5f itineracy. Significant questions remain concerning the nature of Pu electronic structure. Perhaps the missing piece of the puzzle is the direct experimental determination of the unoccupied electronic structure using high energy inverse photoelectron spectroscopy or Bremstrahlung Isochromat Spectroscopy (BIS). Past BIS studies of Th and U indicate the feasibility and utility of Pu studies.
Date: February 7, 2008
Creator: Tobin, J. G. & Yu, S. W.
Partner: UNT Libraries Government Documents Department

A systems study of an RF power source for a 1 TeV next linear collider based upon the relativistic-klystron two-beam accelerator

Description: A systems study, including physics, engineering and costing, has been conducted to assess the feasibility of a relativistic-klystron two-beam-accelerator (RK-TBA) system as a RF power source candidate for a 1 TeV linear collider. Several key issues associated with a realizable RK-TBA system have been addressed, and corresponding schemes have been developed and examined quantitatively. A point design example has been constructed to present a concrete conceptual design which has acceptable transverse and longitudinal beam stability properties. The overall efficiency of RF production for such a power source is estimated to be 36%, and the cost of the full system is estimated to be less than 1 billion dollars.
Date: November 1, 1994
Creator: Yu, S.; Goffeney, N. & Deadrick, F.
Partner: UNT Libraries Government Documents Department

Preliminary design report of a relativistic-Klystron two-beam-accelerator based power source for a 1 TeV center-of-mass next linear collider

Description: A preliminary point design for an 11.4 GHz power source for a 1 TeV center-of-mass Next Linear Collider (NLC) based on the Relativistic-Klystron Two-Beam-Accelerator (RK-TBA) concept is presented. The present report is the result of a joint LBL-LLNL systems study. consisting of three major thrust areas: physics, engineering, and costing. The new RK-TBA point design, together with our findings in each of these areas, are reported.
Date: February 22, 1995
Creator: Yu, S.; Goffeney, N. & Henestroza, E.
Partner: UNT Libraries Government Documents Department

Heavy ion fusion 2 MV injector

Description: A heavy-ion-fusion driver-scale injector has been constructed and operated at Lawrence Berkeley Laboratory. The injector has produced 2.3 MV and 950 mA of K{sup +}, 15% above original design goals in energy and current. Normalized edge emittance of less than 1 {pi} mm-mr was measured over a broad range of parameters. The head-to-tail energy flatness is less than {+-} 0.2% over the 1 {micro}s pulse.
Date: April 1, 1995
Creator: Yu, S.; Eylon, S. & Henestroza, E.
Partner: UNT Libraries Government Documents Department

Using traveling wave structures to extract power from relativistic klystrons

Description: The purpose of this note is to analyze the excitation of traveling wave (TW) output structures by an RF current. Such structures are being used in relativistic klystron experiments at Lawrence Livermore National Laboratory. First we will preset a set of difference equations that describes the excitation of the cells of a TW structure. Next we will restrict our attention to structures that have identical cells, except possibly for the first and last cells. Under these circumstances one can obtain difference equations that have constant coefficients, and we will present the general solution of these equations. Lastly we will apply our results to the analysis of a TW output structure. We will show that, by appropriate choice of the quality factors (Qs) and eigenfrequencies of the first and last cells, it is possible to obtain a traveling wave solution for which there is no reflected wave and where the excitation grows linearly with cell number.
Date: September 19, 1990
Creator: Ryne, R.D. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Relativistic kylstron simulations using RKTW2D

Description: We are developing a two-dimensional, time-dependent computer code for the simulation of relativistic klystrons. We will discuss the main features of our code. We will also present an example simulation of a relativistic klystron with both a standing wave output cavity and a traveling wave output structure.
Date: September 19, 1990
Creator: Ryne, R.D. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Beam-tracking studies with RINGBEARER II

Description: This report presents results from the RINGBEARER II linearized monopole/dipole particle simulation for an intense relativistic electron beam propagating in a gas near three types of channels: (1) pre-existing conductivity, (2) density, and (3) density with pre-existing conductivity. Comparisons are made with earlier analytic results for the initial conditions for the pre-existing conductivity channel.
Date: November 22, 1982
Creator: Masamitsu, J.A.; Yu, S.S. & Chambers, F.W.
Partner: UNT Libraries Government Documents Department

The stability of free-electron lasers against filamentation

Description: In inertial confinement fusion (ICF) experiments, the high electromagnetic fields propagating through a relatively dense plasma can result in a transverse instability, causing the matter and light to form filaments oriented parallel to the light beam. We examine whether a similar instability exists in the electron beam of a free-electron laser, where such an instability could interfere with the transfer of beam kinetic energy into optical wave energy. We heuristically examine the instability in a relativistic beam through which an intense laser beam is propagating. We ignore the FEL effects. We estimate how the altered index of refraction in an FEL affects the dispersion relation. Finally, we estimate the effect that the instability could have on the phase coherence of a particle as it transits an FEL. 10 refs., 2 tabs.
Date: September 15, 1987
Creator: Barnard, J.J.; Scharlemann, E.T. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Aspects of three field approximations: Darwin, frozen, EMPULSE

Description: The traditional approach used to study high energy beam propagation relies on the frozen field approximation. A minor modification of the frozen field approximation yields the set of equations applied to the analysis of the hose instability. These models are constrasted with the Darwin field approximation. A statement is made of the Darwin model equations relevant to the analysis of the hose instability.
Date: May 25, 1985
Creator: Boyd, J.K.; Lee, E.P. & Yu, S.S.
Partner: UNT Libraries Government Documents Department