60 Matching Results

Search Results

Advanced search parameters have been applied.

Numerical simulation studies of toroidal pinches

Description: A time dependent two-dimensional MHD calculation for pinch experiments, previously developed at Columbia, is improved and modified so that very long calculations (until the plasma reaches a quasi-equilibrium state) are possible. The calculation method is presented, and the simulation results of the Jutphaas screw pinch SP-1 and the Los Alamos reverse-field pinches ZT-I, ZT-S and ZT-40 are studied.
Date: January 1, 1978
Creator: Park, W.
Partner: UNT Libraries Government Documents Department

Two-dimensional modeling of the formation of spheromak configurations

Description: A reduced set of two-dimensional MHD equations have been derived describing the axisymmetric time evolution of a MHD stable plasma evolving slowly due to resistive diffusion and changing boundary conditions. The equations are restricted to low ..beta.. but allow changing topology. They are integrated in time to demonstrate a possible spheromak formation method. External circuit equations are integrated simultaneously with the plasma equations to determine the electromagnetic boundary conditions self consistently. The effects of a finite conductivity vacuum chamber are included.
Date: September 1, 1980
Creator: Jardin, S.C. & Park, W.
Partner: UNT Libraries Government Documents Department

Nonlinear saturation of non-resonant internal instabilities in a straight spheromak

Description: An initial value numerical solution of the time dependent nonlinear ideal magnetohydrodynamic equations demonstrates that spheromak equilibria which are linearly unstable to nonresonant helical internal perturbations saturate at low amplitude without developing singularities. These instabilities thus represent the transition from an axisymmetric to a non-axisymmetric equilibrium state, caused by a peaking of the current density.
Date: April 1, 1982
Creator: Park, W. & Jardin, S.C.
Partner: UNT Libraries Government Documents Department

Sawtooth oscillation in tokamaks

Description: A three-dimensional nonlinear toroidal full MHD code, MH3D, has been used to study sawtooth oscillations in tokamaks. The profile evolution during the sawtooth crash phase compares well with experiment, but only if neoclassical resistivity is used in the rise phase. (Classical resistivity has been used in most of the previous theoretical sawtooth studies.) With neoclassical resistivity, the q value at the axis drops from 1 to about 0.8 before the crash phase, and then resets to 1 through a Kadomtsev-type complete reconnection process. This ..delta..q/sub 0/ approx. = 0.2 is much larger than ..delta..q/sub o/ approx. = 0.01, which is obtained if classical resistivity is used. The current profile is strongly peaked at the axis with a flat region around the singular surface, and is similar to the Textor profile. To understand this behavior, approximate formulas for the time behavior of current and q values are derived. A functional dependence of sawtooth period scaling is also derived. A semi-empirical scaling is found which fits the experimental data from various tokamaks. Some evidence is presented which indicates that the fast crash time is due to enhanced effective resistivity inside the singular current sheet, generated by, e.g., microinstability and electron parallel viscosity with stochastic fields at the x-point. 16 refs., 5 figs.
Date: March 1, 1989
Creator: Park, W. & Monticello, D.A.
Partner: UNT Libraries Government Documents Department

Assessment of eddy current effects on compression experiments in the TFTR tokamak

Description: The eddy current induced on the TFTR vacuum vessel during compression experiments is estimated based on a cylindrical model. It produces an error magnetic field that generates magnetic islands at the rational magnetic surfaces. The widths of these islands are calculated and found to have some effect on electron energy confinement. However, resistive MHD simulation results indicate that the island formation process can be slowed down by plasma rotation.
Date: May 1, 1986
Creator: Wong, K.L. & Park, W.
Partner: UNT Libraries Government Documents Department

Contribution to RFP workshop, session 5. Heating and diffusion

Description: Results are summarized of two-dimensional MHD simulations of the reversed field pinches Eta-Beta, ZT-1, ZT-S, and ZT-40. The model used is the single-fluid, time-dependent, axisymmetric (i.e., no toroidal variation) magnetohydrodynamic equations with dissipation. Initial conditions assume a fully ionized cold (approximately 1 eV) plasma-at-rest, with appropriate bias toroidal field. The equations are driven by time-dependent boundary conditions: the toroidal field B/sub z/(t), and the poloidal flux chi (t), prescribed as functions of time in agreement with the experimental conditions. The poloidal flux is indirectly extrapolated from the prescribed toroidal current I/sub z/(t).
Date: January 1, 1978
Creator: Park, W & Chu, C K
Partner: UNT Libraries Government Documents Department

Biomass-based alcohol fuels: the near-term potential for use with gasoline

Description: This report serves as an introduction to the requirements and prospects for a nationwide alcohol-gasoline fuel system based on alcohols derived from biomass resources. Technological and economic factors of the production and use of biomass-based methanol and ethanol fuels are evaluated relative to achieving 5 or 10 percent alcohol-gasoline blends by 1990. It is concluded the maximum attainable is a nationwide 5 percent methanol or ethanol-gasoline system replacing gasoline by 1990. Relative to existing gasoline systems, costs of alcohol-gasoline systems will be substantial.
Date: August 1, 1978
Creator: Park, W.; Price, G. & Salo, D.
Partner: UNT Libraries Government Documents Department

Reconnection rates of magnetic fields

Description: The Sweet-Parker and Petschek scalings of magnetic reconnection rate are modified to include the effect of the viscosity. The modified scalings show that the viscous effect can be important in high-..beta.. plasmas. The theoretical reconnection scalings are compared with numerical simulation results in a tokamak geometry for three different cases: a forced reconnection driven by external coils, the nonlinear m = 1 resistive internal kink, and the nonlinear m = 2 tearing mode. In the first two cases, the numerical reconnection rate agrees well with the modified Sweet-Parker scaling, when the viscosity is sufficiently large. When the viscosity is negligible, a steady state which was assumed in the derivation of the reconnection scalings is not reached and the current sheet in the reconnection layer either remains stable through sloshing motions of the plasma or breaks up to higher m modes. When the current sheet remains stable, a rough comparison with the Sweet-Parker scaling is obtained. In the nonlinear m = 2 tearing mode case where the instability is purely resistive, the reconnection occurs on the slower dissipation time scale (Psi/sub s/ approx. eta). In addition, experimental data of the nonlinear m = 1 resistive internal kink in tokamak discharges are analyzed and are found to give reasonable agreement with the modified Sweet-Parker scaling.
Date: May 1, 1983
Creator: Park, W.; Monticello, D.A. & White, R.B.
Partner: UNT Libraries Government Documents Department

Three-dimensional stellarator equilibrium as an ohmic steady state

Description: A stable three-dimensional stellarator equilibrium can be obtained numerically by a time-dependent relaxation method using small values of dissipation. The final state is an ohmic steady state which approaches an ohmic equilibrium in the limit of small dissipation coefficients. We describe a method to speed up the relaxation process and a method to implement the B vector . del p = 0 condition. These methods are applied to obtain three-dimensional heliac equilibria using the reduced heliac equations.
Date: July 1, 1985
Creator: Park, W.; Monticello, D.A.; Strauss, H. & Manickam, J.
Partner: UNT Libraries Government Documents Department

Stochasticity and the m = 1 mode in tokamaks. [Sawtooth oscillations]

Description: It has recently been proposed that stochasticity resulting from toroidal coupling could lead to a saturation of the m = 1 internal mode in tokamaks. We present results from the nonlinear evolution of the m = 1 mode with toroidal coupling that show that stochasticity is not enough to cause saturation of the m = 1 mode.
Date: May 1, 1986
Creator: Izzo, R.; Monticello, D.A.; Stodiek, W. & Park, W.
Partner: UNT Libraries Government Documents Department

Diagonal Pade approximations for initial value problems

Description: Diagonal Pade approximations to the time evolution operator for initial value problems are applied in a novel way to the numerical solution of these problems by explicitly factoring the polynomials of the approximation. A remarkable gain over conventional methods in efficiency and accuracy of solution is obtained. 20 refs., 3 figs., 1 tab.
Date: June 1, 1987
Creator: Reusch, M.F.; Ratzan, L.; Pomphrey, N. & Park, W.
Partner: UNT Libraries Government Documents Department

Recent regulatory experience of low-Btu coal gasification. Volume III. Supporting case studies

Description: The MITRE Corporation conducted a five-month study for the Office of Resource Applications in the Department of Energy on the regulatory requirements of low-Btu coal gasification. During this study, MITRE interviewed representatives of five current low-Btu coal gasification projects and regulatory agencies in five states. From these interviews, MITRE has sought the experience of current low-Btu coal gasification users in order to recommend actions to improve the regulatory process. This report is the third of three volumes. It contains the results of interviews conducted for each of the case studies. Volume 1 of the report contains the analysis of the case studies and recommendations to potential industrial users of low-Btu coal gasification. Volume 2 contains recommendations to regulatory agencies.
Date: February 1, 1980
Creator: Ackerman, E.; Hart, D.; Lethi, M.; Park, W. & Rifkin, S.
Partner: UNT Libraries Government Documents Department

Performance evaluation of the Enraf-Nonius Model 872 radar gage

Description: There are indications that the Enraf-Nonius Radar Gage installed in Tank 241-SY-101 may not be providing an accurate reading of the true surface level in the waste tank. The Pacific Northwest Laboratory (PNL) performed an initial study to determine the effect of the following items on the distance read by the gage: Tank riser; Material permittivity and conductivity Foam; Proportion of supernatant to solid material in the field of view of the instrument; Physical geometry of the supernatant and solid material changing in the field of view with respect to time; and Varying water content in the solid material. The results of the tests indicate that distance measured by the radar gage is affected by the permittivity, conductivity, and angle of the target surface. These parameters affect the complex input impedance of the signal received by the radar gage to measure the distance to the target. In Tank 101-SY, the radar gage is placed on top of a 12 in. diameter riser. The riser affects the field of view of the instrument, and a much smaller target surface is detected when the radar beam propagates through a riser. In addition, the riser acts as a waveguide, and standing waves are enhanced between the target surface and the radar gage. The result is a change in the level measured by the radar gage due to changing properties of the target surface even when the distance to the target does not change. The test results indicate that the radar will not detect dry crust or foam. However, if the crust or foam is stirred so that it becomes wet, then the crust or foam became detectable. The level read using the radar gage decreased as the moisture in the crust or foam evaporated.
Date: December 1, 1992
Creator: Peters, T.J. & Park, W.R.
Partner: UNT Libraries Government Documents Department

Stabilization of sawtooty oscillation by island heating

Description: Using the compressible resistive MHD equations in a finite aspect ratio cylinder, it is found that the m = 1 mode (the sawtooth oscillation) can saturate when the pressure inside the magnetic island is higher than that of the original core plasma. The saturation condition is of the form ..delta beta../sub p/ greater than or equal to 8 epsilon/sup -1//sub q = 1/ (1 - q/sub 0/)/sup 2/. This saturation effect can be used to actively stabilize sawteeth by heating the island and/or by cooling the core plasma. This mechanism together with a stabilizing toroidal effect may also explain recent lower-hybrid-wave-driven tokamak experiments where the saturation of sawteeth has been observed.
Date: October 1, 1986
Creator: Park, W.; Monticello, D.A. & Chu, T.K.
Partner: UNT Libraries Government Documents Department

Off-axis sawteeth and double-tearing reconnection in reversed magnetic shear plasmas in TFTR

Description: Off-axis sawteeth are often observed in reversed magnetic shear plasmas when the minimum safety factor q is near or below 2. Fluctuations with m/n = 2/1 (m and n are the poloidal and toroidal mode numbers) appear before and after the crashes. Detailed comparison has been made between the measured T{sub e} profile evolution during the crash and a nonlinear numerical magnetohydrodynamics (MHD) simulation. The good agreement between the observation and simulation indicates that the off-axis sawteeth are due to a double-tearing magnetic reconnection process.
Date: June 1, 1996
Creator: Chang, Z.; Park, W. & Fredrickson, E.D.
Partner: UNT Libraries Government Documents Department

3D simulation studies of tokamak plasmas using MHD and extended-MHD models

Description: The M3D (Multi-level 3D) tokamak simulation project aims at the simulation of tokamak plasmas using a multi-level tokamak code package. Several current applications using MHD and Extended-MHD models are presented; high-{beta} disruption studies in reversed shear plasmas using the MHD level MH3D code, {omega}{sub *i} stabilization and nonlinear island saturation of TAE mode using the hybrid particle/MHD level MH3D-K code, and unstructured mesh MH3D{sup ++} code studies. In particular, three internal mode disruption mechanisms are identified from simulation results which agree which agree well with experimental data.
Date: December 31, 1996
Creator: Park, W.; Chang, Z.; Fredrickson, E. & Fu, G.Y.
Partner: UNT Libraries Government Documents Department

Plasma simulation studies using multilevel physics models

Description: The question of how to proceed toward ever more realistic plasma simulation studies using ever increasing computing power is addressed. The answer presented here is the M3D (Multilevel 3D) project, which has developed a code package with a hierarchy of physics levels that resolve increasingly complete subsets of phase-spaces and are thus increasingly more realistic. The rationale for the multilevel physics models is given. Each physics level is described and examples of its application are given. The existing physics levels are fluid models (3D configuration space), namely magnetohydrodynamic (MHD) and two-fluids; and hybrid models, namely gyrokinetic-energetic-particle/MHD (5D energetic particle phase-space), gyrokinetic-particle-ion/fluid-electron (5D ion phase-space), and full-kinetic-particle-ion/fluid-electron level (6D ion phase-space). Resolving electron phase-space (5D or 6D) remains a future project. Phase-space-fluid models are not used in favor of delta f particle models. A practical and accurate nonlinear fluid closure for noncollisional plasmas seems not likely in the near future.
Date: January 19, 2000
Creator: Park, W.; Belova, E.V. & Fu, G.Y.
Partner: UNT Libraries Government Documents Department

High-{beta} disruption in tokamaks

Description: Three dimensional MHD simulations of high-{beta} plasmas show that toroidally localized high-n ballooning modes can be driven unstable by the local pressure steepening which arises from the evolution of low-n modes. Nonlinearly, the high-n mode becomes even more localized and produces a strong local pressure bulge which destroys the flux surfaces resulting in a thermal quench. The flux surfaces then recover temporarily but now contain large magnetic islands. This scenario is supported by experimental data.
Date: July 1, 1995
Creator: Park, W.; Fredrickson, E.D. & Janos, A.
Partner: UNT Libraries Government Documents Department

Simulation Studies of the Role of Reconnection in the ''Current Hole'' Experiments in the Joint European Torus

Description: Injection of lower-hybrid current drive into the current ramp-up phase of the Joint European Torus (JET) plasma discharges has been observed to produce an annular current distribution with a core region of essentially zero current density [Hawkes, et al., Phys. Rev. Lett. 87 (2001) 115001]. Similar ''current holes'' have been observed in the Japan Atomic Energy Research Institute (JAERI) Tokamak 60 Upgrade (JT-60U) plasma discharges with off-axis current drive supplied by the bootstrap current [T. Fujita, et al., Phys. Rev. Lett. 87 (2001) 245001]. In both cases, the central current does not go negative although current diffusion calculations indicate that there is sufficient noninductive current drive for this to occur. This is explained by the Multi-level 3-D code (M3D) nonlinear 2-D and 3-D resistive magnetohydrodynamic (MHD) simulations in toroidal geometry, which predict that these plasma discharges undergo n = 0 reconnection events--''axisymmetric sawteeth''--that redistribute th e current to hold its core density near zero. Unlike conventional sawteeth, these events retain the symmetry of the equilibrium, and thus are best viewed as a transient loss of equilibrium caused when an iota = 0 rational surface enters the plasma. If the current-density profile has a central minimum, this surface will enter on axis; otherwise it will enter off-axis. In the first case, the reconnection is limited to a small region around the axis and clamps the core current at zero. In the second case, more typical of the JET experiments, the core current takes on a finite negative value before the iota = 0 surface appears, resulting in discrete periodic axisymmetric sawtooth events with a finite minor radius. Interpretation of the simulation results is given in terms of analytic equilibrium theory, and the relation to conventional sawteeth and to a recent reduced-MHD analysis of this phenomenon in cylindrical geometry [Huysmans, et al., ...
Date: January 21, 2003
Creator: Breslau, J.A.; Jardin, S.C. & Park, W.
Partner: UNT Libraries Government Documents Department

The Efforts to Utilize High-Temperature Melting Technologies for ILLW and the Development of Guidelines for their Technical Assessment

Description: A couple of domestic institutions have been investigating the application of vitrification technology to treat low- and intermediate-level radioactive wastes in Korea. In the case that such investigations prove to be successful, it is expected that commercial vitrification plants will be constructed. The safety insuring on vitrification plants could not be compatible with criterion on radioactive waste management because the facilities are at high temperature and contain a variety of accommodations for the exhaust gases and residual products. Therefore, it is necessary to suggest a new strategy or modifications of criterion of radioactive waste management on considerations related with the vitrification technology. In order to ensure the safety of vitrification plants, a technical guideline or standard for design and operation of vitrification plants must be established too. A study on the safety assessment of vitrification plants in consideration with general items as an industrial facility, safety and technical requirements as a nuclear facility is needed to be ready before using and permitting them. Also, the stability of vitrified waste forms produced by vitrification plants must be analyzed to ensure their acceptance in final repositories, which includes chemical durability as one of the main considerable items. This paper introduces the status on the utilization of vitrification technology for treating LILW and efforts to develop technical guides with basic study results on chemical durability of forms.
Date: February 25, 2003
Creator: Lee, K. S.; Choi, Y. C.; Seo, Y. C.; Jeong, C. W. & Park, W. J.
Partner: UNT Libraries Government Documents Department

Feedback control of resistive instabilities

Description: Resistive instabilities are responsible for much of the global behavior and the determination of the possible domains of operation of tokamaks. Their successful control could have definite advantages, even making available new regimes of operation. Elimination of sawtoothing might allow operation with higher currents and more peaked current profiles, with q on axis well below unity. In this work different feedback schemes are explored. Simple analytical derivations of the effects of local heating and current drive feedback are presented. Although control of modes with m greater than or equal to 2 is fairly straightforward, the control of the m = 1 mode is more difficult because of its proximity to ideal instability. The most promising scheme utilizes high energy trapped particles. 20 refs., 3 figs.
Date: December 1, 1985
Creator: White, R.B.; Rutherford, P.H.; Furth, H.P.; Park, W. & Chen, L.
Partner: UNT Libraries Government Documents Department

Fishbone and internal kinks

Description: The internal-kink mode, combined with neutral-beam heating and beam losses, appears to be responsible for the fishbone soft x-ray oscillations in PDX. Nonlinear simulations of both ideal and resistive kinks are presented and shown to be consistent with experimental observations. The internal kink may also be important in low-beta internal disruptions.
Date: July 1, 1983
Creator: Strauss, H.; Park, W.; Monticello, D.; Izzo, R.; White, R.; McGuire, K. et al.
Partner: UNT Libraries Government Documents Department

Nonlinear saturation of the internal kink mode

Description: A numerical study shows that in a cylindrical tokamak the internal kink mode (m = 1) develops nonlinearly into a helical equilibrium state that possesses a singular current sheet. In the large aspect ratio limit, the neighboring equilibria obtained agree well with the asymptotic analytic theory of Rosenbluth et al.
Date: February 1, 1980
Creator: Park, W.; Monticello, D.A.; White, R.B. & Jardin, S.C.
Partner: UNT Libraries Government Documents Department

Finite pressure effects on sawtooth oscillation

Description: Nonlinear 3-D simulation of sawtooth oscillations shows that the helically twisted hot spot has a prominent toroidal bulge at the large major radius side, characteristic of a high {epsilon}{beta}{sub p} instability. This bulge drives other m/n islands just outside the mixing radius resulting in a stochastic annular region. A similar effect is also found in shaped plasmas, even at low {beta}. These effects agree with experimental data, giving detailed support to the Kadomtsev reconnection model. This effect can also be used to explain the experimental sawtooth heat pulse anomaly.' At very high {epsilon}{beta}{sub p}, the stochastic region can essentially fill the whole plasma, and can result in a high {beta} disruption.' 16 refs., 6 figs.
Date: August 1, 1990
Creator: Park, W.; Monticello, D.A.; Fredrickson, E. & McGuire, K.
Partner: UNT Libraries Government Documents Department