The Hamiltonian description of incompressible fluid ellipsoids

P. J. Morrison, Norman R. Lebovitz, Joseph A. Biello


We construct the noncanonical Poisson bracket associated with the phase space of first order moments of the velocity field and quadratic moments of the density of a fluid with a free-boundary, constrained by the condition of incompressibility. Two methods are used to obtain the bracket, both based on Dirac’s procedure for incorporating constraints. First, the Poisson bracket of moments of the unconstrained Euler equations is used to construct a Dirac bracket, with Casimir invariants corresponding to volume preservation and incompressibility. Second, the Dirac procedure is applied directly to the continuum, noncanonical Poisson bracket that describes the compressible Euler equations, and the moment reduction is applied to this bracket. When the Hamiltonian can be expressed exactly in terms of these moments, a closure is achieved and the resulting finite-dimensional Hamiltonian system provides exact solutions of Euler’s equations. This is shown to be the case for the classical, incompressible Riemann ellipsoids, which have velocities that vary linearly with position and have constant density within an ellipsoidal boundary. The incompressible, noncanonical Poisson bracket differs from its counterpart for the compressible –Poisson form. © 2009 Elsevier Inc.



Real-time predictions of geomagnetic storms and substorms: Use of the Solar Wind Magnetosphere-Ionosphere System model

M. L. Mays, W. Horton, E. Spencer, and J. Kozyra


[1] A low-dimensional, plasma physics-based, nonlinear dynamical model of the coupled magnetosphere-ionosphere system, called Real-Time Solar Wind Magnetosphere-Ionosphere System (WINDMI), is used to predict AL and Dst values approximately 1 h before geomagnetic substorm and storm event. Subsequently, every 10 min ground-based measurements compiled by World Data Center, Kyoto, are compared with model predictions (http://orion.ph.utexas.edu/ ~windmi/ realtime/). WINDMI model runs are also available at the Community Coordinated Modeling Center (http://ccmc.gsfc.nasa.gov/). The performance of the Real-Time WINDMI model is quantitatively evaluated for 22 storm/substorm event predictions from February 2006 to August 2008. Three possible input solar wind-magnetosphere coupling functions are considered: the standard rectified coupling function, a function due to Siscoe, and a recent function due to Newell. Model AL and Dst predictions are validated using the average relative variance (ARV), correlation coefficient (COR), and root mean squared error  (RMSE). The Newell input function yielded the best model AL predictions by all three measures (mean ARV, COR, and RMSE), followed by the rectified, then Siscoe input functions. Model AL predictions correlate at least 1 standard deviation better with the AL index data than a direct correlation between the input coupling functions and the AL index. The mean Dst ARV results show better prediction performance for the rectified input than the Siscoe and Newell inputs. The mean Dst COR and RMSE measures do not readily distinguish between the three input coupling functions. © 2009 by the American Geophysical Union



On generation of Alfvénic-like fluctuations by drift wave-zonal flow system in large plasma device experiments

E. Horton, C. Correa, G. D. Chagelishvili, V. S. Avsarkisov, J. G. Lominadze, J. C. Perez, J.-H. Kim, and T. A. Carter


According to recent experiments, magnetically confined fusion plasmas with "drift wave-zonal flow turbulence" (DW-ZF) give rise to broadband electromagnetic waves. Sharapov et al. [Europhysics Conference Abstracts, 35th EPS Conference on Plasma Physics, Hersonissos, 2008, edited by P. Lalousis and S. Moustaizis (European Physical Society, Switzerland, 2008), Vol. 32D, p. 4.071] reported an abrupt change in the magnetic turbulence during L-H transitions in Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] plasmas. A broad spectrum of Alfvénic-like (electromagnetic) fluctuations appears from E×B flow driven turbulence in experiments on the large plasma device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] facility at UCLA. Evidence of the existence of magnetic fluctuations in the shear flow region in the experiments is shown. We present one possible theoretical explanation of the generation of electromagnetic fluctuations in DW-ZF systems for an example of LAPD experiments. The method used is based on generalizing results on shear flow phenomena from the hydrodynamics community. In the 1990s, it was realized that fluctuation modes of spectrally stable nonuniform (sheared) flows are non-normal. That is, the linear operators of the flows modal analysis non-normal and the corresponding eigenmodes are not orthogonal. The non-normality results in linear transient growth with bursts of the perturbations and the mode coupling, which causes the generation of electromagnetic waves from the drift wave-shear flow system. We consider shear flow that mimics tokamak zonal flow. We show that the transient growth substantially exceeds the growth of the classical dissipative trapped-particle instability of the system. © 2009 American Institute of Physics



Suppression of error-field-induced magnetic islands by Alfvén resonance effect in rotating plasmas

M. Furukawa and L.-J. Zheng


Error-field penetration is numerically studied in cylindrical tokamak geometry with plasma rotation. For a static error field, non-rotating magnetic islands are generated in the steady state. The penetrated perturbed magnetic flux is effectively reduced by the plasma rotation at small resistivity. Twin current sheets are formed at the Alfvén resonance positions when the plasma rotation is fast enough, and thereby the error-field penetration is significantly changed. The electromagnetic torque increases linearly in the plasma rotation velocity especially at high rotation velocity and low resistivity regime, which agrees with previous theoretical prediction, although the linear scaling can be easily affected if the Alfvén resonance is located close to the plasma edge. The electromagnetic torque in this regime does not depend on the resistivity. For high beta or small resistivity plasmas, the resultant volumeintegrated electromagnetic torque, which brakes the plasma rotation, becomes maximum at very small, almost zero experimentally, rotation velocity. © 2009 IOP Publishing Ltd



Effect of flow damping on drift-tearing magnetic islands in tokamak plasmas

R. Fitzpatrick and F. L. Waelbroeck


A systematic fluid theory of nonlinear drift-tearing magnetic island dynamics in a conventional large aspect-ratio low-β circular cross-section tokamak plasma is derived from a set of single-helicity reduced neoclassical-magnetohydrodynamical equations which incorporate electron and ion diamagnetic flows, ion gyroviscosity, poloidal and toroidal flow damping, cross flux-surface momentum and particle transport, the sound wave, and the drift wave. The equations neglect the compressible Alfvén wave, electron inertia, the electron viscosity tensor, magnetic field-line curvature, and finite ion orbit widths. A collisional closure is used for plasma dynamics parallel to the magnetic field. The influence of various different levels of flow damping on the phase velocity of an isolated island, as well as the ion polarization term appearing in its Rutherford equation, are investigated in detail. Furthermore, it is found that, under certain circumstances, a locked island is subject to destabilizing ion polarization term to which a comparable isolated (i.e., rotating) island is not. © 2009 American Institute of Physics



Evolution of the bounded magnetized jet and comparison with Helimak experiments

R. B. Dahlburg, W. Horton, W. L. Rowan, C. Correa, and J. C. Perez


Magnetized jets are important features of many systems of physical interest. To date, most interest has focused on solar and space physics and astrophysical applications, and hence the unbounded magnetized jet, and its cousin, the unbounded magnetized wake, have received the most attention. This work presents calculations of a bounded, magnetized jet for a laboratory experiments in the3 Helimak device [K. W. Gentle and H. He, Plasma Sci. Technol. 10, 284 (2008)] . The Helimak device has a toroidal magnetic field with a controlled velocity flow that represents jets in bounded systems. Experimental and theoretical features include three spatial dimensions, the inclusion of resistivity and viscosity, and the presence of no-slip walls. The results of the linearized model are computed with a Chebyshev-τ algorithm. The bounding walls stabilize the ideal varicose mode found in unbounded magnetized jets. The ideal sinuous mode persists in the bounded system. A comparison theorem is proved showing that two-dimensional modes are more unstable than the corresponding three-dimensional modes for any given set of system parameters. This result is a generalization of the hydrodynamic Squires theorem. An energy-stress theorem indicates that the Maxwell stress is crucial for the growth of the instability. The results of the analysis are consistent with the observed plasma fluctuations with in the limits of using a simple model for the more complex measured jet velocity flow profile. The working gas is singly ionized argon and the jet velocity profile is accurately measured with Doppler shift spectroscopy. © 2009 American Institute of Physics



Remote handling and plasma conditions to enable fusion nuclear science R&D using a component testing facility

Y. K. M. Peng, T. W. Burgess, A. J. Carroll, C. L. Neumeyer, J. M. Canik, M. J. Cole, W. D. Dorland, P. J. Fogarty, L. Grisham, D. L. Hillis, Y. Katoh, K. Korsah, M. Kotschenreuther, R. LaHaye, S. Mahajan, R. Majeski, B. E. Nelson, B. D. Patton, D. A. Rasmussen, S. A. Sabbagh, A. C. Sontag, R. E. Stoller, C.-C. Tsai, P. Valanju, J. C. Wagner, G. L. Yoder


The use of a fusion component testing facility to study and establish, during the ITER era, the remaining scientific and technical knowledge needed by fusion Demo is considered and described in this paper. This use aims to test components in an integrated fusion nuclear environment, for the first time, to discover and understand the underpinning of physical properties, and to develop improved components for further testing, in a time-efficient manner. It requires a design with extensive modularization and remote handling of activated components, and flexible hot-cell laboratories. It further requires reliable plasma conditions to avoid disruptions and minimize their impact, and designes to reduce the divertor heat flux to the level of ITER design. As the plasma duration is extended through the planned ITER level (~10,3s) and beyond, physical properties with increasing time constants, progressively for ~104s, ~105s, and ~106s, would become accessible for testing and R&D. The longest time constants of these are likely to be of the order of a week (~106s). Progressive stages of research operation are envisioned in deuterium, deuterium-tritium for the ITER duration, and deuterium-tritium with increasingly longer plasma durations. The fusion neutron fluence and operational duty factor anticipated for this "scientific exploration" phase of a component test facility are estimated to be up to 1 MW-rt/m2 and up to 10%, respectively. © 2009 Fusion Science and Technology.



2D divertor design calculations for the national high-power advanced torus experiment

J. M. Canik, R. Maingi, L. Owen, J. Menard, R. Goldston, M. Kotschenreuther, P. Valanju, S. Mahajan


The national high-power advanced torus experiment is a concept for a new facility to address the FESAC theme of ‘taming the plasma-material interface’. This concept exploits the compactness and excellent access provided by low aspect ratio to achieve a high ratio of exhaust power to major radius in order to study the integration of high-performance, long-pulse plasmas with a reactor-relevant high heat flux plasma boundary. Predictions of the scrape-off-layer plasma characteristics are presented, as calculated with the 2D edge modeling code SOLPS. Calculations in a variety of magnetic geometries indicate that very high levels of divertor heat flux can be expected, with peak values far in excess of the power handling capabilities of presently-used materials. Possible methods to reduce the heat flux to acceptable levels are discussed. © 2009 Elsevier B.V.



Weak and strong regimes of incompressible magnetohydrodynamic turbulence

G. Gogoberidze, S. M. Mahajan, and S. Poedts


It is shown that in the framework of the weak turbulence theory, the autocorrelation and cascade time scales are always of the same order of magnitude. This means that, contrary to the general belief, any model of turbulence that implies a large number of collisions among wave packets for an efficient energy cascade (such as the Iroshnikov–Kraichnan model) is not compatible with the weak turbulence theory. © 2009 American Institute of Physics

DOI: 10.1063/1.3177455


Laboratory experiments simulating solar wind driven magnetospheres

P. Brady, T. Ditmire, W. Horton, M. L. Mays, and Y. Zakharov


Magnetosphere-solar wind interactions are simulated in a laboratory setting with a small permanent magnet driven by two types of supersonic plasma wind sources. The first higher speed, shorter duration plasma wind is from a laser blow-off plasma while the second longer duration, lower speed plasma wind is produced with a capacitor discharge driven coaxial electrode creating plasma jets. The stand off distance of the solar wind from the magnetosphere was measured to be 1.7 ± 0.3 cm for the laser-produced plasma experiment and 0.87 ± 0.03 cm for the coaxial electrode plasma experiment. The stand off distance of the plasma was calculated using data from HYADES [J. T. Larsen and S. M. Lane, J. Quant. Spectrosc. Radiat. Transf. 51, 179 (1994)] as 1.46 ± 0.02 cm for the laser-produced plasma, and estimated for the coaxial plasma jet as rmp= 0.72 ± 0.07 cm. Plasma build up on the poles of the magnets, consistent with magnetosphere systems, was also observed. © 2009 American Institute of Physics



Asymmetry-driven structure formation in pair plasmas

S. M. Mahajan, N. L. Shatashvili, and V. I. Brerezhiani


The nonlinear propagation of electromagnetic waves in pair plasmas, in which the electrostatic potential plays a very important but subdominant role of a “binding glue” is investigated. Several mechanisms for structure formation are investigated, in particular, the “asymmetry” in the initial temperatures of the constituent species. It is shown that the temperature asymmetry leads to a (localizing) nonlinearity that is qualitatively different from the ones originating in ambient mass or density difference. The temperature-asymmetry-driven focusing-defocusing nonlinearity supports stable localized wave structures in 1–3 dimensions, which, for certain parameters, may have flat-top shapes. © 2009 The American Physical Society

DOI: 10.1103/PhysRevE.80.066404


Size distribution and mass fraction of microclusters in laser-irradiated plasmas

A.V. Arefiev, X. Gao, M.R. Tushentsov, X. Wang, B. Shim, B.N. Breizman, and M.C. Downer


Laser interactions with a mixture of a gaseous plasma and microclusters depend strongly on the cluster-size distribution, which is usually difficult to measure directly. We present a new method for recovering the cluster-size distribution and cluster mass fraction from measurements of a time-dependent refractive index of the medium in a two-pulse pump-probe experiment. The refractive index is determined from power absorption and phase shift of the probe measured for various delays between the two pulses. The primary cause of absorption is plasma resonances in clusters with peak density above the critical density, which makes the approach especially suitable for determining the tail of the cluster-size distribution. We demonstrate the feasibility of the method by analyzing the data from recent pump-probe experiments at the University of Texas. We have determined that the distribution of clusters over the initial cluster radii for these experiments is well approximated by a lognormal distribution shifted with respect to zero radius. © 2010 Elsevier B.V.

DOI: 10.1016/j.hedp.2009.12.010


An intrinsic source of radial electric field and edge flows in tokamaks

A. Y. Aydemir


We propose a new mechanism for radial electric fields and edge flows in tokamaks that will also serve as an intrinsic momentum source in systems without an up–down symmetry. An essential feature of toroidal plasmas is that charge-dependent ∇B and curvature drifts would lead to a vertical polarization of the discharge if it were not for the Pfirsch–Schlüter currents that neutralize the resulting charge separation. However, in the presence of collisions, there is a residual vertical electric field that drives an E×B flowin the direction of increasing major radius, regardless of the orientation of the fields and currents. This flow is excluded from the hot core and is localized to the more collisional edge plasma. It has many features in common with the edge flows observed in tokamaks such as C-Mod. In an up–down symmetric geometry it carries no net toroidal angular momentum; however, its viscous interaction with asymmetric boundaries leads to a net momentum input to the plasma. Both this momentum input, and the residual vertical electric field, the source of these flows, may play an important role in the∇B direction-dependence of the power threshold for the L–H transition. © 2009 IOP Publishing Ltd, IAEA, Vienna



Resonant excitation of shear Alfvén perturbations by trapped energetic ions in a tokamak

I. G. Abel, B. N. Breizman, S. E. Sharapov, and JET EFDA Contributors


A new analytic expression is derived for the resonant drive of high n Alfvénic modes by particles accelerated to high energy by ion cyclotron resonance heating. This derivation includes finite orbit effects, and the formalism is completely nonperturbative. The high-n limit is used to calculate the complex particle response integrals along the orbits explicitly. This new theory is applied to downward sweeping Alfvén cascade quasimodes completing the theory of these modes and making testable predictions. These predictions are found to be consistent with experiments carried out on the Joint Eurpoean Torus [P.H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)]. © 2009 The American Institute of Physics.

DOI: 10.1063/1.3237026


Electron Self-Injection and Trapping into an Evolving Plasma Bubble

S. Kalmykov, S. A. Yi, V. Khudik, and G. Shvets


The blowout (or bubble) regime of laser wakefield acceleration is promising for generating monochromatic high-energy electron beams out of low-density plasmas. It is shown analytically and by particle-in-cell simulations that self-injection of the background plasma electrons into the quasistatic plasma bubble can be caused by slow temporal expansion of the bubble. Sufficient criteria for the electron trapping and bubble's expansion rate are derived using a semianalytic nonstationary Hamiltonian theory. It is further shown that the combination of bubble's expansion and contraction results in monoenergetic electron beams. © 2009 The American Physical Society

DOI: 10.1103/PhysRevLett.103.135004


Critically coupled surface phonon-polariton excitation in silicon carbide

B. Neuner III, D. Korobkin, C. Fietz, D. Varole, G. Ferro, and G. Shvets


We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of mid-IR radiation. Reflectance measurements demonstrate critical coupling by a double scan of wavelength and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and the substrate, resulting in maximal electric field enhancement. © 2009 Optical Society of America



Erratum: "The effect of parallel electric field in shock waves on the acceleration of relativistic ions, electrons, and positrons" [Phys. Plasmas 16, 112308 (2009)]

S. Takahashi, H. Kawai, Y. Ohsawa, S. Usami, C. Chiu, and W. Horton


Figure 4 in Ref. 1 should be replaced by Fig. 1 presented in this Erratum. Below Fig. 3 in Ref. 1, which displays the time variations of position and energy of a particle with its initial energy γ0=40, we should have shown in Fig. 4 the time variations of the fields that this particle with γ0=40 felt. However, we mistakenly used the figure for the particle with γ0=5 discussed in Fig. 5. Since Fig. 4 and the figure presented in this Erratum are so similar, we do not have to modify the sentences in the paper.© 2009 American Institute of Physics



The effect of parallel electric field in shock waves on the acceleration of relativistic ions, electrons, and positrons

S. Takahashi, H. Kawai, Y. Ohsawa, S. Usami, C. Chiu, and W. Horton


The effect of an electric field E parallel to the magnetic field B on particle acceleration in shock waves is studied. With test particle calculations, for which the electromagnetic fields of shock waves are obtained from one-dimensional, fully kinetic, electromagnetic, particle simulations, the motions of relativistic ions, electrons, and positrons are analyzed. In these simulations, the shock speed vsh is taken to be close to c cos θ, where θ is the angle between the external magnetic field and wave normal, and thus strong particle acceleration takes place. Test particle motions calculated in two different methods are compared: In the first method the total electric field E is used in the equation of motion, while in the second method E is omitted. The comparison confirms that in the acceleration of relativistic ions E is unimportant for high-energy particles. For the acceleration of electrons and positrons, however, E is essential. © 2009 American Institute of Physics



Energetic-Electron-Driven Instability in the Helically Symmetric Experiment

C. B. Deng, D. L. Brower, B. N. Breizman, D. A. Spong, A. F. Almagri, D. T. Anderson, F. S. B. Anderson, W. X. Ding, W. Guttenfelder, K. M. Likin, and J. N. Talmadge


Energetic electrons generated by electron cyclotron resonance heating are observed to drive instabilities in the quasihelically symmetric stellarator device. The coherent, global fluctuations peak in the plasma core and are measured in the frequency range of 20-120 kHz. Mode propagation is in the diamagnetic drift direction of the driving species. When quasihelical symmetry is broken, the mode is no longer observed. Experimental observations indicate that the unstable mode is acoustic rather than Alfvénic. © 2009 The American Physical Society



Theory and observations of magnetic islands

F.L. Waelbroeck


Magnetic islands are a ubiquitous feature of magnetically confined plasmas. They arise as the result of plasma instabilities as well as externally imposed symmetry-breaking perturbations. In the core, effective suppression techniques have been developed. Even thin islands, however, are observed to have nonlocal effects on the profiles of rotation and current. This has stimulated interest in using magnetic islands to control plasma transport, particularly in the edge. They are also of interest as a tool to improve our understanding of microscopic plasma dynamics. © 2009 IAEA, Vienna



Error field penetration in the presence of diamagnetic effects

F. Militello and F.L. Waelbroeck


The penetration of the magnetic field in a rotating inhomogeneous plasma is investigated with direct numerical simulations. The main focus of this work is to test the linear, singular-layer models when diamagnetic and finite Larmor radius effects are included. Our results confirm the existing analytical prediction when the plasma velocity at the resonant surface is outside the drift band, which is the band bounded by the electric drift velocity and the electron diamagnetic velocity. In the drift band, however, a revision of the theory is required. In this regime of velocity, the magnetic island radiates drift waves which can affect the dynamics of the system. Our results show that the penetration of the magnetic field occurs more easily than predicted by the theoretical models, which commonly neglect drift wave radiation effects. © 2009 IAEA, Vienna



Effect of local E×B flow shear on the stability of magnetic islands in tokamak plasmas

R. Fitzpatrick and F.L. Waelbroeck


The influence of local E×B flow shear on a relatively wide, constant-ψ, magnetic island embedded in a large-aspect-ratio, low-β, circular cross-section tokamak plasma is examined, using a slab approximation to model the magnetic geometry. It is found that there are three separate solution branches characterized by low, intermediate, and high values of the shear. Flow shear is found to have a stabilizing effect on island solutions lying on the low and high shear branches, via a nonlinear modification of the ion polarization term in the Rutherford island width evolution equation, but to have a destabilizing effect on solutions lying on the intermediate shear branch. Moreover, the effect is independent of the sign of the shear. The modification of island stability by local E×B flow shear is found to peak when the magnitude of the shear is approximately vi /Ls, where vi is the ion thermal velocity, and Ls the magnetic shear length. © 2009 American Institute of Physics



A Hamiltonian electronmagnetic gyrofluid model

F.L. Waelbroeck, R.D. Hazeltine, and P.J. Morrison


An isothermal truncation of the electromagnetic gyrofluid model of Snyder and Hammett [Phys. Plasmas 8, 3199 (2001)] is shown to be Hamiltonian. The corresponding noncanonical Lie–Poisson bracket and its Casimir invariants are presented. The invariants are used to obtain a set of coupled Grad–Shafranov equations describing equilibria and propagating coherent structures. © 2009 American Institute of Physics



Super-X divertors for solving heat and neutron flux problems of fusion devices

P. M. Valanju, M. Kotschenreuther, and S. M. Mahajan


We present a new magnetic geometry, called the Super X divertor (SXD), that could potentially solve the enormous heat exhaust problem of next-generation high power-density experiments and fusion reactors. With only small changes in net coil currents, the axisymmetric SXD modification of the standard divertor (SD) coils greatly increases the divertor radius, the line length, and the plasma-wetted area. The lower B at large R decreases parallel heat flux and hence lowers the plasma temperature at SXD plates to below 10 eV, allowing higher divertor radiation fractions. The SXD could safely exhaust five times more heat than an SD, is unique in allowing adequate shielding of divertor target from neutron damage, and can enable much improved, reactor-relevant core plasma performance. © 2009 Elsevier B.V. All rights reserved.



Super-X divertors and high power density fusion devices

P. M. Valanju, M. Kotschenreuther, S. M. Mahajan, and J. Canik


The Super-X Divertor (SXD), a robust axisymmetric redesign of the divertor magnetic geometry that can allow a fivefold increase in the core power density of toroidal fusion devices, is presented. With small changes in poloidal coils and currents for standard divertors, the SXD allows the largest divertor plate radius inside toroidal field coils. This increases the plasma-wetted area by 2–3 times over all flux-expansion-only methods (e.g., plate near main X point, plate tilting, X divertor, and snowflake), decreases parallel heat flux and hence plasma temperature at plate, and increases connection length by 2–5 times. Examples of high-power-density fusion devices enabled by SXD are discussed; the most promising near-term device is a 100 MW modular compact fusion neutron source “battery” small enough to fit inside a conventional fission blanket. © 2009 American Institute of Physics



Evaluation of solar wind-magnetosphere coupling functions during geomagnetic storms with the WINDMI model

E. Spencer, A. Rao, W. Horton, M. L. Mays


We evaluate the performance of three solar wind-magnetosphere coupling functions in training the physics-based WINDMI model on the 3–7 October 2000 geomagnetic storm and predicting the geomagnetic Dst and AL indices during the 15–24 April 2002 geomagnetic storm. The rectified solar wind electric field, a coupling function by Siscoe, and a recent formula proposed by Newell are evaluated. The Newell coupling function performed best in both the training and prediction phases for Dst prediction. The Siscoe formula performed best during the training phase in reproducing the AL faithfully and capturing storm time events. The rectified driver was discovered to be the best in overall performance during both training as well as prediction phases, even though the other two coupling functions outperform it in the training phase. The results indicate that multiple drivers need to be concurrently employed in space weather models to yield different possible levels of geomagnetic activity. © 2009 American Geophysical Union



Destabilizing effect of dynamical friction on fast-particle-driven waves in a near-threshold nonlinear regime

M.K. Lilley, B.N. Breizman, and S.E. Sharapov


The nonlinear evolution of waves excited by the resonant interaction with energetic particles, just above the instability threshold, is shown to depend on the type of relaxation process that restores the unstable distribution function. When dynamical friction dominates over diffusion in the phase space region surrounding the wave-particle resonance, an explosive evolution of the wave is found to be the only solution. This is in contrast with the case of dominant diffusion when the wave may exhibit steady-state, amplitude modulation, chaotic and explosive regimes near marginal stability. The experimentally observed differences between Alfvénic instabilities driven by neutral beam injection and those driven by ion-cyclotron resonance heating are interpreted. © 2009 The American Physical Society

DOI: 10.1103/PhysRevLett.102.195003


Effect of electrostatic turbulence on magnetic islands

F. L. Waelbroeck, F. Militello, R. Fitzpatrick, W. Horton


A numerical analysis of the interaction of resistive drift wave and interchange turbulence with a magnetic island in a two-dimensional slab is presented. The time-scale for the evolution of the island is assumed to be much longer than that for the turbulence, allowing the use of an electrostatic model. The effects of the turbulence are isolated by choosing the parameters such that only even modes are unstable. This makes it possible to compare turbulent states with quiescent states in which turbulence is suppressed by enforcing odd parity. The turbulence is found to reduce the propagation velocity of the island. Its effect is destabilizing for thin islands but becomes stabilizing for islands greater than a few times the Larmor radius. Analysis of the quiescent solutions reveals the possibility of oscillations of the island amplitude and frequency through hysteretic transitions between bistable states. © 2009 IOP Publishing Ltd

DOI: 10.1088/0741-3335/51/1/015015


A simple ideal magnetohydrodynamical model of vertical disruption events in tokamaks

R. Fitzpatrick


A simple model of axisymmetric vertical disruption events (VDEs) in tokamaks is presented in which the halo current force exerted on the vacuum vessel is calculated directly from linear, marginally stable, ideal-magnetohydrodynamical (MHD) stability analysis. The basic premise of the model is that the halo current force modifies pressure balance at the edge of the plasma, and therefore also modifies ideal-MHD plasma stability. In order to prevent the ideal vertical instability, responsible for the VDE, from growing on the very short Alfvén time scale, the halo current force must adjust itself such that the instability is rendered marginally stable. The model predicts halo currents which are similar in magnitude to those observed experimentally. An approximate nonaxisymmetric version of the model is developed in order to calculate the toroidal peaking factor for the halo current force. © 2009 American Institute of Physics

DOI: 10.1063/1.3068467


Plume detachment from a magnetic nozzle

C. Deline, R. Bengtson, B. Breizman, M. Tushentsov, J. Jones, D. Chavers, C. Dobson, B. Schuettpelz


High-powered electric propulsion thrusters utilizing a magnetized plasma require that plasma exhaust detach from the applied magnetic field in order to produce thrust. This paper presents experimental results demonstrating that a sufficiently energetic and flowing plasma can indeed detach from a magnetic nozzle. Microwave interferometer and probe measurements provide plume density, electron temperature, and ion flux measurements in the nozzle region. Measurements of ion flux show a low-beta plasma plume which follows applied magnetic field lines until the plasma kinetic pressure reaches the magnetic pressure and a high-beta plume expanding ballistically afterward. Several magnetic configurations were tested including a reversed field nozzle configuration. Despite the dramatic change in magnetic field profile, the reversed field configuration yielded little measurable change in plume trajectory, demonstrating the plume is detached. Numerical simulations yield density profiles in agreement with the experimental results. © 2009 American Institute of Physics

DOI: 10.1063/1.3080206


Wide-angle infrared absorber based on a negative-index plasmonic metamaterial

Y. Avitzour, Y. Urzhumov, G. Shvets


A metamaterial-based approach in making a wide-angle absorber of infrared radiation is described. The technique is based on an anisotropic perfectly impedance-matched negative-index material (PIMNIM). It is shown analytically that a PIMNIM that is subwavelength in all three dimensions enables absorption close to 100% for incidence angles up to 45° to the normal. A specific implementation of such frequency-tunable PIMNIM based on plasmonic metamaterials is presented. Applications to infrared imaging and coherent thermal sources are described. © 2009 The American Physical Society

DOI: 10.1103/PhysRevB.79.045131


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