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W W Heidbrink - One of the best experts on this subject based on the ideXlab platform.
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convective beam ion losses due to alfven Eigenmodes in diii d reversed shear plasmas
Plasma Physics and Controlled Fusion, 2011Co-Authors: D C Pace, W W Heidbrink, R K Fisher, M Garciamunoz, M A Van ZeelandAbstract:Coherent losses of neutral beam ions are observed at frequencies corresponding to toroidal and reversed-shear Alfven Eigenmodes (RSAEs) in DIII-D. Reversed-shear profiles are created by injecting beam power during the plasma current ramp. Beam ion losses stemming from Alfven Eigenmode activity contribute to flattening of the energetic ion density profile in such discharges. This is the first observation of convective beam ion losses due to RSAEs. The energies and pitch angles of lost ions are measured and found to exist within a well-defined region of phase space. Loss flux signals decrease in time as current penetrates and Alfven Eigenmode activity becomes more core localized. Preliminary Monte Carlo simulations of energetic ion interactions with measured mode structures show the dominant loss mechanism is a transition from a counter-passing orbit to a trapped orbit that is lost to the wall.
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measurements modelling and electron cyclotron heating modification of alfven Eigenmode activity in diii d
Nuclear Fusion, 2009Co-Authors: M A Van Zeeland, W W Heidbrink, N N Gorelenkov, G.j. Kramer, R. Nazikian, M. E. Austin, C Z Cheng, C T Holcomb, A W Hyatt, J LohrAbstract:Neutral beam injection into reversed magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including toroidicity and ellipticity induced Alfven Eigenmodes (TAE/EAE, respectively) and reversed shear Alfven Eigenmodes (RSAE) as well as their spatial coupling. These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and strong drive due to multiple higher order resonances. It is found that ideal MHD modelling of Eigenmode spectral evolution, coupling and structure are in excellent agreement with experimental measurements. It is also found that higher radial envelope harmonic RSAEs are clearly observed and agree with modelling. Some discrepancies with modelling such as that due to up/down Eigenmode asymmetries are also pointed out. Concomitant with the Alfvenic activity, fast ion (FIDA) spectroscopy shows large reductions in the central fast ion profile, the degree of which depends on the Alfven Eigenmode amplitude. Interestingly, localized electron cyclotron heating (ECH) near the mode location stabilizes RSAE activity and results in significantly improved fast ion confinement relative to discharges with ECH deposition on axis. In these discharges, RSAE activity is suppressed when ECH is deposited near the radius of the shear reversal point and enhanced with deposition near the axis. The sensitivity of this effect to deposition power and current drive phasing as well as ECH modulation are presented.
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Coupling of global toroidal Alfvén Eigenmodes and reversed shear Alfvén Eigenmodes in DIII-D
Physics of Plasmas, 2007Co-Authors: M. A. Van Zeeland, M. A. Makowski, G.r. Mckee, W W Heidbrink, E Ruskov, N N Gorelenkov, G.j. Kramer, R. Nazikian, M. E. Austin, A D TurnbullAbstract:Reversed shear Alfvén Eigenmodes (RSAEs) are typically thought of as being localized near the minima in the magnetic safety factor profile, however, their spatial coupling to global toroidal Alfvén Eigenmodes(TAEs) has been observed in DIII-D discharges. For a decreasing minimum magnetic safety factor, the RSAE frequency chirps up through that of stable and unstable TAEs. Coupling creates a small gap at the frequency degeneracy point forming two distinct global modes. The core-localized RSAE mode structure changes and becomes temporarily global. Similarly, near the mode frequency crossing point, the global TAE extends deeper into the plasma core. The frequency splitting and spatial structure of the two modes throughout the various coupling stages, as measured by an array of internal fluctuation diagnostics, are in close agreement with linear ideal MHD calculations using the NOVA code. The implications of this coupling for Eigenmode stability is also investigated and marked changes are noted throughout the coupling process.
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Comparison between measurements of the poloidal distribution of magnetic fluctuations and predictions of theoretical models during TAE activity
Nuclear Fusion, 1997Co-Authors: W W Heidbrink, A. Jaun, H. A. HoltiesAbstract:Fluctuations produced by beam-driven toroidicity-induced Alfven Eigenmode (TAE) activity in the DIII-D tokamak are measured by a poloidal array of magnetic probes and compared with the wavefields computed by two theoretical models. Fluid resistive models compute continuum damped TAEs. A kinetic plasma model that retains Landau damping and finite Larmor radius effects computes global drift-kinetic Alfven Eigenmodes. The phases of the probes disagree with both theoretical predictions, while the amplitudes agree best with the kinetic model.
M A Van Zeeland - One of the best experts on this subject based on the ideXlab platform.
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convective beam ion losses due to alfven Eigenmodes in diii d reversed shear plasmas
Plasma Physics and Controlled Fusion, 2011Co-Authors: D C Pace, W W Heidbrink, R K Fisher, M Garciamunoz, M A Van ZeelandAbstract:Coherent losses of neutral beam ions are observed at frequencies corresponding to toroidal and reversed-shear Alfven Eigenmodes (RSAEs) in DIII-D. Reversed-shear profiles are created by injecting beam power during the plasma current ramp. Beam ion losses stemming from Alfven Eigenmode activity contribute to flattening of the energetic ion density profile in such discharges. This is the first observation of convective beam ion losses due to RSAEs. The energies and pitch angles of lost ions are measured and found to exist within a well-defined region of phase space. Loss flux signals decrease in time as current penetrates and Alfven Eigenmode activity becomes more core localized. Preliminary Monte Carlo simulations of energetic ion interactions with measured mode structures show the dominant loss mechanism is a transition from a counter-passing orbit to a trapped orbit that is lost to the wall.
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measurements modelling and electron cyclotron heating modification of alfven Eigenmode activity in diii d
Nuclear Fusion, 2009Co-Authors: M A Van Zeeland, W W Heidbrink, N N Gorelenkov, G.j. Kramer, R. Nazikian, M. E. Austin, C Z Cheng, C T Holcomb, A W Hyatt, J LohrAbstract:Neutral beam injection into reversed magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including toroidicity and ellipticity induced Alfven Eigenmodes (TAE/EAE, respectively) and reversed shear Alfven Eigenmodes (RSAE) as well as their spatial coupling. These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and strong drive due to multiple higher order resonances. It is found that ideal MHD modelling of Eigenmode spectral evolution, coupling and structure are in excellent agreement with experimental measurements. It is also found that higher radial envelope harmonic RSAEs are clearly observed and agree with modelling. Some discrepancies with modelling such as that due to up/down Eigenmode asymmetries are also pointed out. Concomitant with the Alfvenic activity, fast ion (FIDA) spectroscopy shows large reductions in the central fast ion profile, the degree of which depends on the Alfven Eigenmode amplitude. Interestingly, localized electron cyclotron heating (ECH) near the mode location stabilizes RSAE activity and results in significantly improved fast ion confinement relative to discharges with ECH deposition on axis. In these discharges, RSAE activity is suppressed when ECH is deposited near the radius of the shear reversal point and enhanced with deposition near the axis. The sensitivity of this effect to deposition power and current drive phasing as well as ECH modulation are presented.
Nicolas Triantafyllidis - One of the best experts on this subject based on the ideXlab platform.
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buckling of rectangular and hexagonal honeycomb under combined axial compression and transverse shear
International Journal of Solids and Structures, 2013Co-Authors: Lopez F Jimenez, Nicolas TriantafyllidisAbstract:Abstract One of the many uses of honeycomb is as core in sandwich plates, producing very high stiffness-to-weight ratio structures. The macroscopically observed crushing mechanism of these structures has its origin in instabilities at the local scale. Of particular interest here are the critical (i.e., onset of a buckling-type instability) loads and corresponding Eigenmodes of honeycomb under general 3D loading involving simultaneous axial compression and transverse shear. Since the critical Eigenmodes in honeycomb often involve more than one unit cell, numerical studies are limited by the size of the domain considered for their analyses. We propose a new theoretical approach to determine the critical loads and Eigenmodes of perfect honeycomb of infinite extent under general loading conditions based entirely on unit-cell calculations. It combines Bloch wave representation theorem for the Eigenmode with the analytical solution of the linearized von Karman plate equations for the walls. The proposed approach uses the fact that the honeycomb walls remain flat in the principal solution prior to the onset of the first instability and solves analytically the corresponding eigenvalue problem. Three different geometries are considered: rectangular honeycomb with varying in-plane aspect ratios, an isotropic-section hexagonal honeycomb, and an anisotropic-section hexagonal honeycomb (resulting from its manufacturing process). Several different loading cases are investigated: axial compression under free or fully constrained lateral expansion, transverse shear and combined axial compression and transverse shear. The results show that the buckling mode is highly dependent on the type of loading: e.g., laterally unconstrained axial compression results in local critical Eigenmodes, while constraining the lateral expansion leads to global ones. The addition of transverse shear not only reduces the critical axial strain, but also affects the wavelengths of the critical Eigenmode.
Victor V. Moshchalkov - One of the best experts on this subject based on the ideXlab platform.
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Eigenmode analysis of plasmonic scatterers by volumetric integral equation technique
2013 7th European Conference on Antennas and Propagation (EuCAP), 2013Co-Authors: Xuezhi Zheng, Guy A. E. Vandenbosch, Vladimir Volski, Victor V. MoshchalkovAbstract:A Volumetric Method of Moments (V-MoM) based full wave Eigenmode analysis technique is introduced to the nanoscale and applied to analyze plasmonic structures (monomer, dimer, and the associated composite dolmen structure). It is shown that the eigenfrequencies and Eigenmodes determine the resonant frequencies and quality factors of the plasmonic response. Further, the physical origin of the Fano resonance in the dolmen structure is revealed from both individual component and complete structural behavior.
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Line Position and Quality Factor of Plasmonic Resonances Beyond the Quasi-Static Limit: A Full-Wave Eigenmode Analysis Route
IEEE Journal of Selected Topics in Quantum Electronics, 2013Co-Authors: Xuezhi Zheng, Vladimir Volskiy, Ventsislav K. Valev, Guy A. E. Vandenbosch, Victor V. MoshchalkovAbstract:In this study, we introduce a rigorous full-wave Eigenmode analysis technique based on a volumetric method of moments to the optical spectrum. We first apply this technique to a nanorod as an example to illustrate how the real part of the eigenfrequency and the modal quality factor (defined as the ratio of the real part of the eigenfrequency to the imaginary part) together with the Eigenmode determine the line position and quality factor of a resonance and the corresponding resonant mode. Then, the eigenfrequencies and Eigenmodes of a composite plasmonic nanostructure, a Dolmen, and its two individual constituents, a dimer and a monomer, are extracted. The line position of the Fano dip in Dolmen's spectrum is discussed by examining the relative positions of the eigenfrequencies of the dimer and the monomer in the complex plane. Further, the formation of the Fano dip is reinterpreted as the destructive interference between the nonorthogonal Eigenmodes of the whole Dolmen structure. The proposed full-wave modal analysis brings a new perspective on understanding and designing the plasmonic response of nanoantennae beyond the quasi-static limit.
Kinglap Wong - One of the best experts on this subject based on the ideXlab platform.
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a review of alfven Eigenmode observations in toroidal plasmas
Plasma Physics and Controlled Fusion, 1999Co-Authors: Kinglap WongAbstract:In toroidal magnetically confined plasmas, Eigenmodes of Alfven waves can be destablized by energetic ions with velocities comparable to the Alfven velocity. With the advent of tokamak experiments in which populations of energetic ions can be introduced by neutral beam injection, radio frequency wave heating or by fusion reactions, major advances have been made in Alfven Eigenmode research in the past 10 years. After introducing the basic concepts on the Alfven Eigenmode instability, data on this subject from various toroidal devices are described, emphasizing the interplay between experiment and theory. Experimental results on mode identification, instability drive, mode damping and saturation, and energetic ion redistribution are compared with theory.