The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform
K Itoh - One of the best experts on this subject based on the ideXlab platform.
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quantification of turbulent driving forces for the geodesic Acoustic Mode in the jft 2m tokamak
Physical Review Letters, 2018Co-Authors: T Kobayashi, Yoshihiko Nagashima, Makoto Sasaki, K Itoh, A Fujisawa, K Kamiya, Y Miura, S Inagaki, S I ItohAbstract:: We investigate spatial structures of turbulence and turbulent transport modulated by the geodesic Acoustic Mode (GAM), from which the excitation mechanism of the GAM is discussed. The GAM is found to be predominantly excited through a localized Reynolds stress force, rather than the dynamic shearing force. The evaluated growth rate is larger than the linear damping coefficients and is on the same order of magnitude as the effective growth rate evaluated from time evolution in the GAM kinetic energy.
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multiple eigenModes of geodesic Acoustic Mode in collisionless plasmas
Physics of Plasmas, 2006Co-Authors: K Itoh, H Sanuki, J.q. DongAbstract:We report a series of eigenModes of the geodesic Acoustic Mode (GAM), which includes the standard GAM, a branch of low-frequency Mode, and a series of ion sound wave-like Modes. The case of Ti≫Te is investigated, and eigenfrequencies of these Modes are obtained analytically from a linear gyrokinetic Model in collisionless plasmas with a rigid constant electrostatic potential around a magnetic surface.
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geodesic Acoustic Mode in jft 2m tokamak plasmas
Plasma Physics and Controlled Fusion, 2006Co-Authors: Y Miura, K Itoh, Y Hamada, A Fujisawa, K Kamiya, S I Itoh, K Hoshino, A Nishizawa, H Ogawa, Y KusamaAbstract:The characteristics of geodesic–Acoustic-Mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the . The causality between the GAM and the modulation of the density fluctuation is revealed.
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extension of geodesic Acoustic Mode theory to helical systems
Physics of Plasmas, 2005Co-Authors: Taizan Watari, Y Hamada, A Fujisawa, K ItohAbstract:The present paper extends the theory of geodesic Acoustic Mode (GAM) oscillation, which so far has been applied to tokamaks, to helical systems. By using drift kinetic equations for three-dimensional equilibriums, a generalized dispersion relation is obtained including Landau damping. The oscillation frequency is obtained in terms of the squared sum of Fourier components of the magnetic field intensity expressed by means of magnetic flux coordinates. An analytic form of the collisionless damping rate of GAM is obtained by solving the dispersion relation perturbatively. It is found that the GAM frequency is higher in helical systems than in tokamaks and that damping rate is enhanced in multi-helicity magnetic configurations. However, damping rates are predicted to be small if the temperature of electrons is higher than that of ions.
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excitation of geodesic Acoustic Mode in toroidal plasmas
Plasma Physics and Controlled Fusion, 2005Co-Authors: K Itoh, K Hallatschek, Sanae-i. ItohAbstract:The instability of the geodesic Acoustic Mode (GAM) in tokamak turbulence is analysed. It can be caused by dynamic shearing of the ambient turbulence by GAMs combined with the poloidal inhomogeneity of the turbulent flux. The dispersion relation is derived. The competition between the drive mechanism and the damping by turbulence viscosity is discussed. GAMs are more unstable for high safety factors.
Ashwin Arunkumar Seshia - One of the best experts on this subject based on the ideXlab platform.
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Acoustic Mode confinement using coupled cavity structures in UHF unreleased MEMS resonators
Microsystem Technologies, 2019Co-Authors: Andreja Erbes, Dana Weinstein, Wentao Wang, Ashwin Arunkumar SeshiaAbstract:This papers investigates device approaches towards the confinement of Acoustic Modes in unreleased UHF MEMS resonators. Acoustic Mode confinement is achieved using specially designed mechanically coupled Acoustic cavities known as Acoustic Bragg Grating Coupler structures to spatially localize the vibration energy within the resonators and thereby improve the motional impedance ($$R_x$$Rx) and mechanical quality factor (Q). This enhancement in the mechanical response is demonstrated with numerical simulations using distinct unreleased resonator technologies involving dielectric transduction mechanisms. These initial investigations show improvements in the Q as well as enhanced vibrational amplitudes within the resonator domains (i.e. translating to improved $$R_x$$Rx values) in the case of coupled cavities as opposed to single cavity designs. An initial approach to fabricate the devices in a CMOS compatible dual-trench technology are presented.
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Particulate mass sensing with piezoelectric bulk Acoustic Mode resonators
2016 IEEE International Frequency Control Symposium (IFCS), 2016Co-Authors: Arthur T. Zielinski, Markus Kalberer, Roderic L. Jones, Abhinav Prasad, Ashwin Arunkumar SeshiaAbstract:Current portable particle detection instruments typically rely on optical methods which are limited to 100 nm diameter particles. Microfabricated bulk Acoustic resonators, when used as mass balances, could take particle detection below this limit. This study examines the collection of particles onto piezoelectric bulk Acoustic Mode resonators from gaseous flow using classical impaction. Collection of both polystyrene latex particles and α-pinene secondary organic aerosol particles was examined in terms of frequency shift and collection efficiency. A new experimental setup was introduced which allows for adjusting major impactor, resonator, and aerosol properties. Preliminary results show the setup works for both particles while the saturation limit was not reached within an hour despite highly elevated particle concentrations.
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a bulk Acoustic Mode single crystal silicon microresonator with a high quality factor
Journal of Micromechanics and Microengineering, 2008Co-Authors: Ashwin Arunkumar SeshiaAbstract:This paper details a bulk Acoustic Mode resonator fabricated in single-crystal silicon with a quality factor of 15 000 in air, and over a million below 10 mTorr at a resonant frequency of 2.18 MHz. The resonator is a square plate that is excited in the square-extensional Mode and has been fabricated in a commercial foundry silicon-on-insulator (SOI) MEMS process through MEMSCAP. This paper also presents a simple method of extracting resonator parameters from raw measurements heavily buried in electrical feedthrough. Its accuracy has been demonstrated through a comparison between extracted motional resistance values measured at different voltage biases and those predicted from an analytical Model. Finally, a method of substantially cancelling electrical feedthrough through system-level electronic implementation is also introduced.
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A single-crystal-silicon bulk-Acoustic-Mode microresonator oscillator
IEEE Electron Device Letters, 2008Co-Authors: Joshua E.-y. Lee, Bahreyni Bahreyni, Yong Zhu, Ashwin Arunkumar SeshiaAbstract:A timing reference incorporating a single-crystal-silicon micromechanical resonator with a quality factor of larger than one million and a resonant frequency of 2.18 MHz is demonstrated. The resonator is excited in the square extensional bulk Acoustic Mode at 4 mtorr, and it has been fabricated in a foundry SOI MEMS process. The silicon microresonator is adapted as a timing element for a precision oscillator with a measured short-term Allan deviation of 0.6 ppb.
J.q. Dong - One of the best experts on this subject based on the ideXlab platform.
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Radial properties of the geodesic Acoustic Mode
Physics of Plasmas, 2017Co-Authors: Zhenqian Li, J.q. Dong, Zheng-mao Sheng, M. Y. Yu, W. X. WangAbstract:In magnetized toroidal plasmas, neoclassical effects and turbulent drift waves can induce the geodesic Acoustic Mode (GAM). We simulate the GAM using the gyro-kinetic code GTS for typical tokamak parameters and investigate its properties, especially its frequency continuum, evolution of its radial wave number, and propagation characteristics. The simulation results are compared with those of the relevant theory and experiment. It is found that the radial phase velocity of the GAM is roughly proportional to the ion thermal speed.
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Excitation of geodesic Acoustic Mode continuum by drift wave turbulence
Journal of Plasma Physics, 2012Co-Authors: Jun Yu, J.q. Dong, X. X. Li, D. Du, X. Y. GongAbstract:AbstractExcitation of the geodesic Acoustic Mode continuum by drift wave turbulence is studied using the wave kinetic approach. For a Model profile of weak non-uniform ion temperature, the forms of growth rate and radial structure of geodesic Acoustic Modes are obtained analytically. The growth rate is analyzed for several conditions for present-day tokamaks and compared with that for uniform ion temperature, as well as that given by the coherent Mode approach for non-uniform ion temperature.
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spectral features of the geodesic Acoustic Mode and its interaction with turbulence in a tokamak plasma
Physics of Plasmas, 2008Co-Authors: C X Yu, J.q. Dong, K J Zhao, W Y Hong, J Cheng, D L Yu, J P Qian, Q W YangAbstract:The three-dimensional wavenumber and frequency spectrum for the geodesic Acoustic Mode (GAM) has been measured in the HuanLiuqi-2A tokamak for the first time. The spectrum provides definite evidence for the GAM, which is characterized by kθ=kϕ=0 and krρi≈0.04−0.09 with the full width at half-maximum Δkrρi≈0.03−0.07. The localized GAM packet is observed to propagate outward in the radial direction with nearly the same phase and group velocity. The envelopes of the radial electric field and density fluctuations are observed to be modulated by the GAM. By comparing the experimental result with that of the envelope analysis using Model signals, the mechanism of the envelope modulation has been identified. The results strongly suggest that the envelope modulation of the Er fluctuations is dominantly caused by the direct regulation of the GAM during the GAM generation in the energy-conserving triad interaction, and the envelope modulation of the density fluctuations is induced by the GAM shearing effect, which...
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multiple eigenModes of geodesic Acoustic Mode in collisionless plasmas
Physics of Plasmas, 2006Co-Authors: K Itoh, H Sanuki, J.q. DongAbstract:We report a series of eigenModes of the geodesic Acoustic Mode (GAM), which includes the standard GAM, a branch of low-frequency Mode, and a series of ion sound wave-like Modes. The case of Ti≫Te is investigated, and eigenfrequencies of these Modes are obtained analytically from a linear gyrokinetic Model in collisionless plasmas with a rigid constant electrostatic potential around a magnetic surface.
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toroidal symmetry of the geodesic Acoustic Mode zonal flow in a tokamak plasma
Physical Review Letters, 2006Co-Authors: K J Zhao, J.q. Dong, W Y Hong, C X Yu, J Qian, J Cheng, D L Yu, Q W Yang, X T DingAbstract:The toroidal symmetry of the geodesic Acoustic Mode (GAM) zonal flows is identified with toroidally distributed three step Langmuir probes at the edge of the HuanLiuqi-2A (commonly referred to as HL-2A) tokamak plasmas for the first time. High coherence of both the GAM and the ambient turbulence for the toroidally displaced measurements along a magnetic field line is observed, in contrast with the high coherence of the GAM but low coherence of the ambient turbulence when the toroidally displaced measurements are not along the same field line. The radial and poloidal features of the flows are also simultaneously determined. The nonlinear three wave coupling between the high frequency turbulent fluctuations and the flows is demonstrated to be a plausible formation mechanism of the flows.
Y Hamada - One of the best experts on this subject based on the ideXlab platform.
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Geodesic Acoustic Mode oscillation in the low frequency range
Physics of Plasmas, 2006Co-Authors: T Watari, Y Hamada, Takashi Notake, N. Takeuchi, Kimitaka ItohAbstract:In order to understand the various appearances of geodesic Acoustic Modes (GAM) in experiments, the following specific problems are theoretically addressed: (1) The asymmetry of the potential field of GAMs, which is enhanced by the coupling with ion Acoustic Modes. It may affect GAMs in plasmas with electron temperatures higher than those of the ions. (2) The possible existence of GAMs in the lower frequency range: This is discussed in connection with the uniqueness of the kinetic response of the plasma to an external field associated with the geodesic curvature of the magnetic lines of force. (3) The extension of the theory to cover both tokamaks and helical systems: Differences between the helical-type and the tokamak-type GAMs are discussed in terms of their differences in connection length. In a device of mixed helicity, helical natured GAMs are predicted to appear depending on the intensity of the corresponding geodesic curvature and electron temperature.
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observation of the interaction between the geodesic Acoustic Mode and ambient fluctuation in the jft 2m tokamak
Nuclear Fusion, 2006Co-Authors: Y Miura, Yoshihiko Nagashima, Y Hamada, K Kamiya, K Hoshino, A Nishizawa, H Ogawa, Y Kawasumi, K Shinohara, Y KusamaAbstract:The electrostatic and density fluctuation are measured simultaneously with a heavy ion beam probe. The electrostatic fluctuation with the geodesic Acoustic Mode (GAM) frequency is observed in L-Mode plasmas and not in H-Mode plasmas. The poloidal and radial structure is consistent with the GAM. So the fluctuation is concluded to be the GAM.The amplitude of the GAM changes in the radial direction; it is small near the separatrix, has a maximum at 3 cm inside the separatrix and decreases again to 5 cm inside the separatrix.The GAM and the temporal behaviour of the ambient density fluctuation show a significant coherence, and the phase of modulation of the ambient density fluctuation tends to delay the potential oscillation of the GAM. It is clearly verified that the GAM affects ambient fluctuation and also the local particle transport through modulation of the amplitude of the ambient fluctuation.
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geodesic Acoustic Mode in jft 2m tokamak plasmas
Plasma Physics and Controlled Fusion, 2006Co-Authors: Y Miura, K Itoh, Y Hamada, A Fujisawa, K Kamiya, S I Itoh, K Hoshino, A Nishizawa, H Ogawa, Y KusamaAbstract:The characteristics of geodesic–Acoustic-Mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the . The causality between the GAM and the modulation of the density fluctuation is revealed.
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extension of geodesic Acoustic Mode theory to helical systems
Physics of Plasmas, 2005Co-Authors: Taizan Watari, Y Hamada, A Fujisawa, K ItohAbstract:The present paper extends the theory of geodesic Acoustic Mode (GAM) oscillation, which so far has been applied to tokamaks, to helical systems. By using drift kinetic equations for three-dimensional equilibriums, a generalized dispersion relation is obtained including Landau damping. The oscillation frequency is obtained in terms of the squared sum of Fourier components of the magnetic field intensity expressed by means of magnetic flux coordinates. An analytic form of the collisionless damping rate of GAM is obtained by solving the dispersion relation perturbatively. It is found that the GAM frequency is higher in helical systems than in tokamaks and that damping rate is enhanced in multi-helicity magnetic configurations. However, damping rates are predicted to be small if the temperature of electrons is higher than that of ions.
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zonal flows in the geodesic Acoustic Mode frequency range in the jipp t iiu tokamak plasmas
Nuclear Fusion, 2005Co-Authors: Y Hamada, A Nishizawa, Y Kawasumi, T Watari, M Kojima, K NariharaAbstract:Large potential oscillations were detected in JIPPT-IIU tokamak plasmas in a wide range of plasma cross-sections in measurements using a multi-sample-volume heavy ion beam probe. These oscillations have large amplitudes reaching a few hundreds of volts and their frequencies are in the range of the geodesic Acoustic Mode (GAM). They are found over a wide range of plasma cross-sections and commonly have m = 0 structures. As they were Fourier analysed, it was found that the central frequency is higher in the core of the plasma and lower in the edge of the plasma. These observations agree with the properties of theoretically predicted GAM oscillations. It was also found that the frequency spectrum is peaked in the core and broad in the edge, which may have something to do with damping mechanisms of the GAM. The phase relation between the density and the electric field fluctuations was studied extensively in terms of the cross-correlation function. The level of the density fluctuation was low as it should be, and the expected 90° phase difference was found in a limited radial domain.
Joshua E.-y. Lee - One of the best experts on this subject based on the ideXlab platform.
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A single-crystal-silicon bulk-Acoustic-Mode microresonator oscillator
IEEE Electron Device Letters, 2008Co-Authors: Joshua E.-y. Lee, Bahreyni Bahreyni, Yong Zhu, Ashwin Arunkumar SeshiaAbstract:A timing reference incorporating a single-crystal-silicon micromechanical resonator with a quality factor of larger than one million and a resonant frequency of 2.18 MHz is demonstrated. The resonator is excited in the square extensional bulk Acoustic Mode at 4 mtorr, and it has been fabricated in a foundry SOI MEMS process. The silicon microresonator is adapted as a timing element for a precision oscillator with a measured short-term Allan deviation of 0.6 ppb.
