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An Investigation of the Effects of a Driven Plasma Rotation on Fluctuation in a Magnetized Linear Plasma Source. Final Technical Report

Description: The rotation of a plasma is one of the most fundamental global modes of plasma behavior. It is the zeroth order plasma response to a transverse electric field. In its simplest kinetic form, the so-called E x B drift (here, E is the electric field vector and B is the magnetic field vector), both the ions and the electrons will undergo a drift in the same direction. This motion is considered a universal mode of a plasma since the mechanism of the E x B drift is, to zero-order, independent of both the mass and the charge of the particles.
Date: May 18, 2004
Creator: Thomas, E.
Partner: UNT Libraries Government Documents Department

Comment on "Paleoclassical Transport in Low-Collisionality Toroidal Plasmas"

Description: Paleoclassical transport [1] is a recently proposed fundamental process that is claimed to occur in resistive plasmas and to be missing in the collisional drift-kinetic equations (DKE) in standard use. In this Comment we raise three puzzles presented by paleoclassical transport as developed in [1], one to do with conservation and two concerning uniqueness.
Date: October 13, 2006
Creator: LoDestro, L L
Partner: UNT Libraries Government Documents Department

New perspectives on substorm injections

Description: There has been significant progress in understanding substorm injections since the Third International Conference on Substorms in 1996. Progress has come from a combination of new theories, quantitative modeling, and observations--particularly multi-satellite observations. There is now mounting evidence that fast convective flows are the mechanism that directly couples substorm processes in the mid tail, where reconnection occurs, with substorm processes the inner magnetosphere where Pi2 pulsations, auroral breakups, and substorm injections occur. This paper presents evidence that those flows combined with an earthward-propagating compressional wave are responsible for substorm injections and discusses how that model can account for various substorm injection signatures.
Date: December 1, 1998
Creator: Reeves, G.D.
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

Plasma gate switch experiment on Pegasus II

Description: The plasma gate switch is a novel technique for producing a long conduction time vacuum opening switch. The switch consists of an aluminum foil which connects the cathode to the anode in a coaxial geometry. The foil is designed so that the maximum axial acceleration is in the center of the foil and that at the appropriate time, the center opens up and magnetic flux is carried down the gun to the load region. The switch is designed to minimize the amount of mass transported into the load region. We have completed the first experimental test of this design and present results from the test. These results indicate there were some asymmetry problems in the construction of the switch but that otherwise the switch performed as expected.
Date: September 1995
Creator: Wysocki, F. J.; Benage, J. F., Jr. & Shlachter, J. S.
Partner: UNT Libraries Government Documents Department

Plasma flow in the DIII-D divertor

Description: Indications that flows in the divertor can exhibit complex behavior have been obtained from 2-D modeling but so far remain mostly unconfirmed by experiment. An important feature of flow physics is that of flow reversal. Flow reversal has been predicted analytically and it is expected when the ionization source arising from neutral or impurity ionization in the divertor region is large, creating a high pressure zone. Plasma flows arise to equilibrate the pressure. A radiative divertor regime has been proposed in order to reduce the heat and particle fluxes to the divertor target plates. In this regime, the energy and momentum of the plasma are dissipated into neutral gas introduced in the divertor region, cooling the plasma by collisional, radiative and other atomic processes so that the plasma becomes detached from the target plates. These regimes have been the subject of extensive studies in DIII-D to evaluate their energy and particle transport properties, but only recently it has been proposed that the energy transport over large regions of the divertor must be dominated by convection instead of conduction. It is therefore important to understand the role of the plasma conditions and geometry on determining the region of convection-dominated plasma in order to properly control the heat and particle fluxes to the target plates and hence, divertor performance. The authors have observed complex structures in the deuterium ion flows in the DIII-D divertor. Features observed include reverse flow, convective flow over a large volume of the divertor and stagnant flow. They have measured large gradients in the plasma potential across the separatrix in the divertor and determined that these gradients induce poloidal flows that can potentially affect the particle balance in the divertor.
Date: July 1998
Creator: Boedo, J. A.; Porter, G. D. & Schaffer, M. J.
Partner: UNT Libraries Government Documents Department

Turbulent fluctuations in the main core of TFTR plasmas with negative magnetic shear

Description: Turbulent fluctuations in plasmas with reversed magnetic shear have been investigated in TFTR. Under intense auxiliary heating, these plasmas are observed to bifurcate into two states with different transport properties. In the state with better confinement, it has been found that the level of fluctuations is very small throughout most of the region with negative shear. By contrast, the state with lower confinement is characterized by large bursts of fluctuations which suggest a competition between the driving and the suppression of turbulence. These results are consistent with the suppression of turbulence by the E x B velocity shear.
Date: September 1997
Creator: Mazzucato, E.; Beer, M. A.; Bell, M. G. & Batha, S. H.
Partner: UNT Libraries Government Documents Department

Theory, simulation, and experimental studies of zonal flows

Description: The authors report on current theoretical understanding of the characteristics of self-generated zonal flows as observed in nonlinear gyrokinetic simulations of toroidal ITG turbulence [Science 281, 1835 (1998)], and discuss various possibilities for experimental measurements of signature of zonal flows.
Date: July 13, 2000
Creator: Hahm, T. S.; K.H.Burrell; Z.Lin; Nazikian, R. & Synakowski, E.J.
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

The roles of shear and cross-correlations on the fluctuation levels in simple stochastic models. Revision

Description: Highly simplified models of random flows interacting with background microturbulence are analyzed. In the limit of very rapid velocity fluctuations, it is shown rigorously that the fluctuation level of a passively advected scalar is not controlled by the rms shear. In a model with random velocities dependent only on time, the level of cross-correlations between the flows and the background turbulence regulates the saturation level. This effect is illustrated by considering a simple stochastic-oscillator model, both exactly and with analysis and numerical solutions of the direct-interaction approximation. Implications for the understanding of self-consistent turbulence are discussed briefly.
Date: November 3, 1999
Creator: Krommes, J.A.
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

Measurement of the dynamo effect in a plasma

Description: A series of the detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the {alpha} effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E x B drift and electron diamagnetic drift. The three major findings are (1) the {alpha} effect accounts for the dynamo current generation, even in the time dependence through a ``sawtooth`` cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless Magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E x B drift; (3) at high collisionality, a new ``electron diamagnetic dynamo`` is observed, in which the fluctuating velocity is dominated by the diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying J.B. Taylor`s conjecture.
Date: November 1, 1995
Creator: Ji, H.; Prager, S.C.; Almagri, A.F.; Sarff, J.S.; Hirano, Y. & Toyama, H.
Partner: UNT Libraries Government Documents Department

Substorm effects in MHD and test particle simulations of magnetotail dynamics

Description: Recent magnetohydrodynamic simulations demonstrate that a global tail instability, initiated by localized breakdown of MHD, can cause plasmoid formation and ejection as well as dipolarization and the current diversion of the substorm current wedge. The connection between the reconnection process and the current wedge signatures is provided by earthward flow from the reconnection site. Its braking and diversion in the inner magnetosphere causes dipolarization and the magnetic field distortions of the current wedge. The authors demonstrate the characteristic properties of this process and the current systems involved. The strong localized electric field associated with the flow burst and the dipolarization is also the cause of particle acceleration and energetic particle injections. Test particle simulations of orbits in the MHD fields yield results that are quite consistent with observed injection signatures.
Date: December 31, 1998
Creator: Birn, J. & Hesse, M.
Partner: UNT Libraries Government Documents Department

Confinement in the RFP: Lundquist number scaling, plasma flow, and reduced transport

Description: Global heat and particle transport in the reversed field pinch (RFP) result primarily from large-scale, resistive MHD fluctuations which cause the magnetic field in the core of the plasma to become stochastic. Achieving a better understanding of t his turbulent transport and identifying ways to reduce it are critical RFP development issues. The authors report measurements of the Lundquist number (S-scaling) of magnetic and ion flow velocity fluctuations in the Madison Symmetric Torus (MST) RFP. The S-scaling of magnetic fluctuations in MST is weaker than previous measurements {tilde b}/B {approximately} S{sup {minus}1/2} in smaller (lower S) RFP plasmas. Impurity ion flow velocity fluctuations (measured with fast Doppler spectroscopy) have a scaling similar to the magnetic fluctuations, falling in the range {tilde V}/V{sub A} {approximately} S{sup {minus}[0.08-0.10]}. The MHD dynamo ({tilde V} x {tilde b}) up to 15 V/cm was measured in the plasma core. Interestingly, the scaling of the MHD dynamo ({tilde V} x {tilde b}) {approximately} S{sup {minus}[0.64-0.88]} is stronger than for its constituents, a result of decreased coherency between {tilde V} and {tilde b} with increasing S. A weak S-scaling of magnetic fluctuations implies fluctuation suppression measures (e.g., current profile control) may be required in higher-S RFP plasmas. Two types of current profile modifications have been examined--inductive and electrostatic. The inductive control halves the amplitude of global magnetic fluctuations and improves the confinement by a factor of 5. The electrostatic current injection, localized in the edge plasma, reduces edge resonant fluctuations and improves the energy confinement. In addition, regimes with confinement improvement associated with the plasma flow profile are attained.
Date: October 1, 1998
Creator: Fiksel, G.; Almagri, A.F. & Anderson, J.K.
Partner: UNT Libraries Government Documents Department

Global structure of mirror modes in the magnetosheath

Description: A global stability analysis of mirror modes in the magnetosheath is presented. The analysis is based upon the kinetic-MHD formulation which includes relevant kinetic effects such as Landau resonance and gradient drift effects related to inhomogeneities in the background density, temperature, pressure and its anisotropy, magnetic field, and plasma flow velocity. Pressure anisotropy provides the free energy for the global mirror mode. The local theory of mirror modes predicts purely growing modes confined in the unstable magnetosheath region; however, the nonlocal theory that includes the effects of gradients and plasma flow predicts modes with real frequencies which propagate with the flow from the magnetosheath toward the magnetopause boundary. The real frequency is on the order of a combination of the diamagnetic drift frequency and the Doppler shift frequency associated with plasma flow. The diamagnetic drift frequency provides a wave phase velocity in the direction of the magnetopause so that wave energy accumulates against the magnetopause boundary, and the amplitude is skewed in that direction. On the other hand, plasma flow also gives rise to a real phase velocity, but the phase velocity is smaller than the flow velocity. As a result, the wave amplitude is increased in the wake of the plasma flow and piles up against the bow shock boundary.
Date: November 1, 1996
Creator: Johnson, J.R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Physics of turbulence control and transport barrier formation in DIII-D

Description: This paper describes the physical mechanisms responsible for turbulence control and transport barrier formation on DIII-D as determined from a synthesis of results from different enhanced confinement regimes, including quantitative and qualitative comparisons to theory. A wide range of DIII-D data support the hypothesis that a single underlying physical mechanism, turbulence suppression via E x B shear flow is playing an essential, though not necessarily unique, role in reducing turbulence and transport in all of the following improved confinement regimes: H-mode, VH-mode, high-{ell}{sub i} modes, improved performance counter-injection L-mode discharges and high performance negative central shear (NCS) discharges. DIII-D data also indicate that synergistic effects are important in some cases, as in NCS discharges where negative magnetic shear also plays a role in transport barrier formation. This work indicates that in order to control turbulence and transport it is important to focus on understanding physical mechanisms, such as E x B shear, which can regulate and control entire classes of turbulent modes, and thus control transport. In the highest performance DIII-D discharges, NCS plasmas with a VH-mode like edge, turbulence is suppressed at all radii, resulting in neoclassical levels of ion transport over most of the plasma volume.
Date: October 1996
Creator: Doyle, E.J.; Burrell, K.H. & Carlstrom, T.N.
Partner: UNT Libraries Government Documents Department