100 Matching Results

Search Results

Advanced search parameters have been applied.

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

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

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

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

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

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 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 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

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

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

A theory of the high-mode phenomenon for stellarators

Description: It is shown that besides the ion orbit loss mechanism, which occurs in a region a {minus} {var_epsilon}{sub t}{rho}{sub p} < r < a, the collisionless drift-orbit transport flux can also drive the poloidal {rvec E} {times} {rvec B} velocity in a region r < a {minus} {var_epsilon}{sub t}{rho}{sub p} in stellarators. Here, r is the minor radius, a is the plasma radius, {var_epsilon}{sub t} is the toroidal amplitude of the magnetic field spectrum, {rvec E} is the electric field, {rvec B} is the magnetic field, and {rho}{sub p} is the poloidal ion gyroradius. The transport-fluxdriven {rvec E} {times} {rvec B} velocity can be triggered most efficiently by an increase of the ion temperature gradient. The theory is applied to the high-mode (H-mode) phenomenon observed in stellarators.
Date: September 1995
Creator: Shaing, K. C.
Partner: UNT Libraries Government Documents Department

Plasma pressure and flows during divertor detachment

Description: MHD theory applied to tokamak plasma scrape-off layer (SOL) equilibria requires Pfirsch-Schlueter current, which, because the magnetic lines are open, normally closes through electrically conducting divertor or limiter components. During detached divertor operation the Pfirsch-Schlueter current path to the divertor target is sometimes blocked, in which case theory predicts that the plasma develops a poloidal pressure gradient around the upstream SOL and a corresponding parallel flow, in order to satisfy all the conditions of MHD equilibrium. This paper reports the only known examples of detached diverted plasma in the DIII-D tokamak with blocked Pfirsch-Schlueter current, and they show no clear SOL poloidal pressure differences. However, the predicted pressure differences are small, near the limit of detectability with the available diagnostics. In the more usual DIII-D partially detached divertor operation mode, the Pfirsch-Schlueter current appears to never be blocked, and no unusual poloidal pressure differences are observed, as expected. Finally, a local overpressure is observed just inside the magnetic separatrix near the X-point in both attached and detached Ohmically heated plasmas.
Date: August 1, 1998
Creator: Schaffer, M.J.; Brooks, N.H.; Boedo, J.A.; Isler, R.C. & Moyer, R.A.
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

Radiative divertor plasmas with convection in DIII-D

Description: The radiation of divertor heat flux on DIII-D is shown to greatly exceed the limits imposed by assumptions of energy transport dominated by electron thermal conduction parallel to the magnetic field. Approximately 90% of the power flowing into the divertor is dissipated through low Z radiation and plasma recombination. The dissipation is made possible by an extended region of low electron temperature in the divertor. A one-dimensional analysis of the parallel heat flux finds that the electron temperature profile is incompatible with conduction dominated parallel transport. Plasma flow at up to the ion acoustic speed, produced by upstream ionization, can account for the parallel heat flux. Modeling with the two-dimensional fluid code UEDGE has reproduced many of the observed experimental features.
Date: January 1998
Creator: Leornard, A. W.; Porter, G. D. & Wood, R. D.
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

Impurity enrichment and radiative enhancement using induced SOL flow in DIII-D

Description: Experiments on DIII-D have demonstrated the efficacy of using induced scrap-off-layer (SOL) flow to preferentially enrich impurities in the divertor plasma. This SOL floe is produced through simultaneous deuterium gas injection at the midplane and divertor exhaust. Using this SOL flow, an improvement in enrichment (defined as the ratio of impurity fraction in the divertor to that in the plasma core) has been observed for all impurities in trace-level experiments (i.e., impurity level is non-perturbative), with the degree of improvement increasing with impurity atomic number. In the case of argon, exhaust gas enrichment using modest SOL flow is as high as 17. Using this induced SOL flow technique and argon injection, radiative plasmas have been produced that combine high radiation losses (P{sub rad}/P{sub input} > 70%), low core fuel dilution (Z{sub eff} < 1.9), and good core confinement ({tau}{sub E} > 1.0 {tau}{sub E,ITER93H}).
Date: July 1998
Creator: Wade, M.R.; West, W.P. & Wood, R.D.
Partner: UNT Libraries Government Documents Department

Electron heat transport in improved confinement discharges in DIII-D

Description: In DIII-D tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improved understanding for electron thermal transport within the plasma. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in the electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the T{sub e} profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm{sup {minus}1}), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wavenumber, but it is computed to be robustly stable near the axis. Consequently, in the plasmas the authors have examined, calculations of drift wave linear stability do not explain the observed transport near the axis in plasmas with or without additional electron heating, and there are probably other processes controlling transport in this region.
Date: January 1, 1999
Creator: Stallard, B.W.; Greenfield, C.M. & Staebler, G.M.
Partner: UNT Libraries Government Documents Department

Studies of instability and transport in tokamak plasmas with very weak magnetic shear

Description: Ion temperature gradient (ITG or {eta}{sub i}) driven microinstabilities are studied, using kinetic theory, for tokamak plasmas with very weak (positive or negative) magnetic shear (VWS). The gradient of magnetic shear as well as the effects of parallel and perpendicular velocity shear (v{prime}{sub {parallel}} and v{prime}{sub E}) are included in the defining equations. Two eigenmodes: the double (D) and the global (G) are found to coexist. Parametric dependence of these instabilities, and of the corresponding quasilinear transport is systematically analyzed. It is shown that, in VWS plasmas, a parallel velocity shear (PVS) may stabilize or destabilize the modes, depending on the individual as well as the relative signs of PVS and of the gradient of magnetic shear. The quasilinear transport induced by the instabilities may be significantly reduced with PVS in VWS plasmas. The v{prime}{sub E} values required to completely suppress the instabilities are much lower in VWS plasmas than they are in normal plasmas. Possible correlations with tokamak experiments are discussed.
Date: April 1, 1997
Creator: Dong, J.Q.; Zhang, Y.Z. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Effects of orbit squeezing on ion transport processes close to magnetic axis

Description: It is shown that ion thermal conductivity close to the magnetic axis in tokamaks is reduced by a factor of {vert_bar}S{vert_bar}{sup 5/3} if (M{sub i}/M{sub e}){sup 2/3}(T{sub e}/T{sub i}){sup 4/3}/{vert_bar}S{vert_bar}{sup 5/3} {much_gt} 1. Here, S is the orbit squeezing factor, M{sub i}(M{sub e}) is the ion (electron) mass, and T{sub i}(Te{sub e}) is the ion (electron) temperature. The reduction reflects both the increase of the fraction of trapped particles by a factor of {vert_bar}S{vert_bar}{sup 1/3}, and the decrease of the orbit size in units of the poloidal flux {psi} by a factor of {vert_bar}S{vert_bar}{sup 2/3}.
Date: January 1997
Creator: Shaing, K. C.; Hazeltine, R. D. & Zarnstorff, M. C.
Partner: UNT Libraries Government Documents Department

Study of driven magnetic reconnection in a laboratory plasma

Description: The Magnetic Reconnection Experiment (MRX) has been constructed to investigate the fundamental physics of magnetic reconnection in a well controlled laboratory setting. This device creates an environment satisfying the criteria for a magnetohydrodynamic (MHD) plasma (S {much_gt} 1, {rho}{sub i} {much_lt} L). The boundary conditions can be controlled externally, and experiments with fully three-dimensional reconnection are now possible. In the initial experiments, the effects of the third vector component of reconnecting fields have been studied. Two distinctively different shapes of neutral sheet current layers, depending on the third component, are identified during driven magnetic reconnection. Without the third component (anti-parallel or null-helicity reconnection), a thin double-Y shaped diffusion region is identified. A neutral sheet current profile is measured accurately to be as narrow as order ion gyro-radius. In the presence of an appreciable third component (co-helicity reconnection), an O-shaped diffusion region appears and grows into a spheromak configuration.
Date: December 31, 1998
Creator: Yamada, Masaaki; Ji, H.; Hsu, S.; Carter, T.; Kulsrud, R.; Bretz, N. et al.
Partner: UNT Libraries Government Documents Department

Experimental constraints on transport from dimensionless parameter scaling studies

Description: The scalings of heat transport with safety factor (q), normalized collisionality (v), plasma beta ({beta}), and relative gyroradius ({rho}*) have been measured on the DIII-D tokamak. The measured {rho}* {beta} and v scalings of heat transport indicate that E x B transport from drive wave turbulence is a plausible basis for anomalous transport. For high confinement (H) mode plasmas where the safety factor was varied at fixed magnetic shear, the effective (or one-fluid) thermal diffusivity was found to scale like {chi}{sub eff} {proportional_to} q{sup 2.3{+-}0.64}, with the ion and electron fluids having the same q scaling to within the experimental errors except near the plasma edge. The scaling of the thermal confinement time with safety factor was in good agreement with this local transport dependence, {tau}{sub th} {proportional_to} q{sup {minus}2.42{+-}0.31}; however, when the magnetic shear was allowed to vary to keep q{sub 0} fixed during the (edge) safety factor scan, a weaker global dependence was observed, {tau}{sub th} {proportional_to} q{sub 95}{sup {minus}1.43{+-}0.23}. This weaker dependence was mainly due to the change in the local value of q between the two types of scans. The combined {rho}*, {beta}, v and q scalings of heat transport for H-mode plasmas on DIII-D reproduce the empirical confinement scaling using physical (dimensionless) parameters with the exception of weaker power degradation.
Date: February 1998
Creator: Petty, C. C.; Luce, T. C. & Baker, D. R.
Partner: UNT Libraries Government Documents Department