38 Matching Results

Search Results

Advanced search parameters have been applied.
in creator/contributor: "Cohen, B. I."  
Results: 1 - 24 next
open access

Nonlinear ion-cyclotron waves in mirror machines. Paper IAEA-CN-38/S-1

Description: Experimental results on ion-cyclotron waves observed in the 2XIIB mirror machine are reviewed, and relevant theoretical work is discussed. The work reported generally substantiates the quasilinear diffusion model of mirror-plasma confinement, but also suggests alternatives. The end-loss current required by theory of the drift-cyclotron loss-cone (DCLC) instability agrees with measurements of this current. The experiment indicates that an increased ratio of plasma radius to ion gyroradius improv… more
Date: May 22, 1980
Creator: Cohen, B. I.; Smith, G. R. & Berk, H. L.
Partner: UNT Libraries Government Documents Department
open access

Comparisons Between Experimental Measurments and Numerical Simulations of Spheromak Formation in SSPX

Description: Data from a recently installed insertable magnetic probe array in the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper et al., Nucl. Fusion 39, 863 (1999)] is compared against NIMROD [C. R. Sovinec et al., J. Comp. Phys. 195, 355 (2004)], a full 3D resistive magnetohydrodynamic code that is used to simulate SSPX plasmas. The experiment probe consists of a linear array of chip inductors arranged in clusters that are spaced every 2 cm, and spans the entire machine radius at the flux co… more
Date: March 15, 2006
Creator: Romero-Talam?s, C. A.; Hooper, E. B.; Hill, D. N.; Cohen, B. I.; McLean, H. S.; Wood, R. D. et al.
Partner: UNT Libraries Government Documents Department
open access

NIMROD Resistive Magnetohydrodynamic Simulations of Spheromak Physics

Description: The physics of spheromak plasmas is addressed by time-dependent, three-dimensional, resistive magneto-hydrodynamic simulations with the NIMROD code. Included in some detail are the formation of a spheromak driven electrostatically by a coaxial plasma gun with a flux-conserver geometry and power systems that accurately model the Sustained Spheromak Physics Experiment (SSPX) (R. D. Wood, et al., Nucl. Fusion 45, 1582 (2005)). The controlled decay of the spheromak plasma over several milliseconds … more
Date: December 11, 2007
Creator: Hooper, E. B.; Cohen, B. I.; McLean, H. S.; Wood, R. D.; Romero-Talamas, C. A. & Sovinec, C. R.
Partner: UNT Libraries Government Documents Department
open access

Edge Gyrokinetic Theory and Continuum Simulations

Description: No Description Available.
Date: September 13, 2007
Creator: Xu, X. Q.; Xiong, Z.; Dorr, M. R.; Hittinger, J. A.; Bodi, K.; Candy, J. et al.
Partner: UNT Libraries Government Documents Department
open access

Theoretical studies in tandem mirror physics

Description: Recent developments in six areas of tandem-mirror theory are explored. Specifically, FLR terms (including electric-field drift) have been added to our 3-D paraxial MHD equilibrium code. Our low-frequency MHD stability analysis with FLR, which previously included only m/sub theta/ = 1 rigid perturbations, has been extended to incorporate moderate m/sub theta/, rotational drive, finite-beta effects on wall stabilization, and the well-digging effect of energetic electrons by using three computatio… more
Date: July 17, 1984
Creator: Cohen, R. H.; Auerbach, S. P.; Baldwin, D. E.; Byers, J. A.; Chen, Y. J.; Cohen, B. I. et al.
Partner: UNT Libraries Government Documents Department
open access

Free-electron laser experiments in the microwave tokamak experiment

Description: Microwave pulses have been injected from a free electron-laser (FEL) into the Microwave Tokamak Experiment (MTX) at up to 0.2 GW at 140 GHz in short pulses (10-ns duration) with O-mode polarization. The power transmitted through the plasma was measured in a first experimental study of high power pulse propagation in the plasma; no nonlinear effects were found at this power level. Calculations indicate that nonlinear effects may be found at the higher power densities expected in future experimen… more
Date: August 1, 1990
Creator: Allen, S. L.; Brown, M. D.; Byers, J. A.; Casper, T. A.; Cohen, B. I.; Cohen, R. H. et al.
Partner: UNT Libraries Government Documents Department
open access

Gyrokinetic and global fluid simulations of tokamak microturbulence and transport

Description: Results are presented from the first systematic nonlinear kinetic simulation study of the swings and parameter dependences of toroidal ion-temperature-gradient (ITG) turbulence and transport, and from the first such study that includes sheared toroidal flows. Key results include the observation of clear gyroBohm scaling of the turbulent transport and of a surprisingly weak dependence of the transport on toroidal flow shear. Based on the simulation results, a parameterization of the transport is… more
Date: August 30, 1994
Creator: Dimits, A. M.; Byers, J. A.; Williams, T. J.; Cohen, B. I.; Xu, W. Q.; Cohen, R. H. et al.
Partner: UNT Libraries Government Documents Department
open access

Field and Current Amplification in the SSPX Spheromak

Description: Results are presented from experiments relating to magnetic field generation and current amplification in the SSPX spheromak. The SSPX spheromak plasma is driven by DC coaxial helicity injection using a 2MJ capacitor bank. Peak toroidal plasma currents of up to 0.7MA and peak edge poloidal fields of 0.3T are produced; lower current discharges can be sustained up to 3.5msec. When edge magnetic fluctuations are reduced below 1% by driving the plasma near threshold, it is possible to produce plasm… more
Date: October 8, 2002
Creator: Hill, D. N.; Blumer, R. H.; Cohen, B. I.; Hooper, E. B.; McLean, H. S.; Moller, J. et al.
Partner: UNT Libraries Government Documents Department
open access

Experiments on nonlinear absorption of ECH waves in MTX and a comparison with theory

Description: Intense pulse electron cyclotron heating (ECH) experiments have been carried out on the MTX tokamak. Rf pulses at 140 GHz, peak power of 1--2 GW, and 25 ns pulse length were generated by the ETA-III/IMP FEL and transported quasi-optically to MTX for o-mode launch. Because of the intense rf electric fields ({approximately}250 keV/cm), reduction of plasma absorption by nonlinear effects were expected and several rf beam geometries (k{sub {parallel}} gradient) were investigated to study their effe… more
Date: July 1, 1993
Creator: Stallard, B. W.; Allen, S. L.; Byers, J. A.; Casper, T. A.; Cohen, B. I.; Cohen, R. H. et al.
Partner: UNT Libraries Government Documents Department
open access

A Reduced Model of Kinetic Effects Related to the Saturation of Stimulated Brillouin Scattering

Description: We developed a reduced description of kinetic effects that is included in a fluid model of stimulated Brillouin backscattering (SBS) in low Z plasmas (e.g. He, Be). Following hybrid-PIC simulations, the modified ion distribution function is parametrized by the width {delta} of the plateau created by trapping around the phase velocity of the SBS-driven acoustic wave. An evolution equation is derived for {delta}, which affects SBS through a frequency shift and a reduced Landau damping. This model… more
Date: August 26, 2003
Creator: Divol, L.; Williams, E. A.; Cohen, B. I.; Langdon, A. B. & Lasinski, B. F.
Partner: UNT Libraries Government Documents Department
open access

Linear and nonlinear theory of trapped-particle instabilities

Description: This paper analyzes several important features of trapped-particle instabilities. For trapped-electron modes, the complete two-dimensional (2D) spatial structure, including the effects of magnetic shear, is numerically calculated within the framework of a differential formulation for long radial wavelength modes. Growth rates obtained for representative cases correlate reasonably well with the usual one-dimensional (1D) estimates of shear stabilization. However, the spatial structure of the mod… more
Date: September 1, 1976
Creator: Tang, W. M.; Adam, J. C. & Cohen, B. I.
Partner: UNT Libraries Government Documents Department
open access

Magnetic Reconnection in the Spheromak: Physics and Consequences

Description: Magnetic reconnection in the spheromak changes magnetic topology by conversion of injected toroidal flux into poloidal flux and by magnetic surface closure (or opening) in a slowly decaying spheromak. Results from the Sustained Spheromak Physics Experiment, SSPX, are compared with resistive MHD simulations using the NIMROD code. Voltage spikes on the SSPX gun during spheromak formation are interpreted as reconnection across a negative-current layer close to the mean-field x-point. Field lines a… more
Date: February 28, 2006
Creator: Hooper, E. B.; Cohen, B. I.; Hill, D. N.; LoDestro, L. L.; McLean, H. S.; Romero-Talamas, C. A. et al.
Partner: UNT Libraries Government Documents Department
open access

Effects of Ion-Ion Collisions and Inhomogeneity in Two-Dimensional Kinetic Ion Simulations of Stimulated Brillouin Backscattering

Description: Two-dimensional simulations with the BZOHAR [B.I. Cohen, B.F. Lasinski, A.B. Langdon, and E.A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability including the effects of ion-ion collisions and inhomogeneity. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and… more
Date: October 17, 2005
Creator: Cohen, B. I.; Divol, L.; Langdon, A. B. & Williams, E. A.
Partner: UNT Libraries Government Documents Department
open access

Time-step Considerations in Particle Simulation Algorithms for Coulomb Collisions in Plasmas

Description: The accuracy of first-order Euler and higher-order time-integration algorithms for grid-based Langevin equations collision models in a specific relaxation test problem is assessed. We show that statistical noise errors can overshadow time-step errors and argue that statistical noise errors can be conflated with time-step effects. Using a higher-order integration scheme may not achieve any benefit in accuracy for examples of practical interest. We also investigate the collisional relaxation of a… more
Date: October 29, 2009
Creator: Cohen, B. I.; Dimits, A.; Friedman, A. & Caflisch, R.
Partner: UNT Libraries Government Documents Department
open access

Plasma heating and current drive using intense, pulsed microwaves

Description: The use of powerful new microwave sources, e.g., free-electron lasers and relativistic gyrotrons, provide unique opportunities for novel heating and current-drive schemes in the electron-cyclotron and lower-hybrid ranges of frequencies. These high-power, pulsed sources have a number of technical advantages over conventional, low-intensity sources; and their use can lead to improved current-drive efficiencies and better penetration into a reactor-grade plasma in specific cases. The Microwave Tok… more
Date: January 1, 1988
Creator: Cohen, B. I.; Cohen, R. H.; Nevins, W. M.; Rognlien, T. D.; Bonoli, P. T. & Porkolab, M.
Partner: UNT Libraries Government Documents Department
open access

Current drive by intense microwave pulses

Description: The advent of high-peak-power pulsed microwave sources opens possibilities for novel current-drive schemes in tokamaks. Four such schemes are considered: accelerating phase-space buckets, overlap of relativistic electron-cyclotron harmonics, beat-wave generation and burnthrough of intense lower-hybrid waves. These schemes are found to offer one or more of the following advantages over their continuous-power counterparts: improved efficiency, improved access to the core plasma, and precise contr… more
Date: September 14, 1988
Creator: Cohen, R. H.; Cohen, B. I.; Nevins, W. M.; Rognlien, T. D.; Bonoli, P. T. & Porkolab, M.
Partner: UNT Libraries Government Documents Department
open access

The Microwave Tokamak Experiment (MTX)

Description: A new experimental facility is being assembled at the Lawrence Livermore National Laboratory (LLNL) for studying microwave propagation and absorption in high density plasmas. A unique feature of the facility is the free electron laser (FEL) used to generate high peak power microwaves at 250 GHz, at a repetition rate so as to produce up to 2 MW of average power for up to 30 s. Called the Microwave Tokamak Experiment (MTX), the facility will be used for studies of plasma heating, current drive, a… more
Date: October 2, 1987
Creator: Thomassen, K. I.; Cohen, B. I.; Hooper, E. B.; Lang, D. D. & Nevins, W. M.
Partner: UNT Libraries Government Documents Department
open access

A numerical study of ultra-short-pulse reflectometry

Description: Ultra-short-pulse reflectometry is studied by means of the numerical integration of a one-dimensional full-wave equation for ordinary modes propagating in a plasma. The numerical calculations illustrate the potential of using the reflection of ultra-short-pulse, microwaves as an effective probe of the density profile even in the presence of significant density fluctuations. The difference in time delays of differing frequency components of the microwaves can be used to deduce the density profil… more
Date: May 1, 1994
Creator: Cohen, B. I.; Afeyan, B. B.; Chou, A. E. & Luhmann, N. C. Jr.
Partner: UNT Libraries Government Documents Department
open access

Improved Operation and Modeling of the SSPX Spheromak

Description: No Description Available.
Date: October 15, 2004
Creator: Wood, R. D.; Cohen, B. I.; Hill, D. N.; Cohen, R. H.; Woodruff, S.; McLean, H. S. et al.
Partner: UNT Libraries Government Documents Department
open access

Progress in the SSPX Spheromak

Description: The spheromak, with its simply connected geometry, holds promise as a less expensive fusion reactor. It has reasonably good plasma beta and can be formed and sustained in steady state with a magnetized coaxial plasma gun. The Sustained Spheromak Physics Experiment (SSPX) shown in Fig. 1 was constructed to investigate the key issues of magnetic field generation and energy confinement. In addition to the coaxial gun, nine magnetic field coils are utilized to shape the vacuum magnetic flux. This f… more
Date: July 7, 2003
Creator: McLean, H. S.; Woodruff, S.; Hill, D. N.; Bulmer, R. H.; Cohen, B. I.; Hooper, E. B. et al.
Partner: UNT Libraries Government Documents Department
Back to Top of Screen