Energetic-particle-induced geodesic acoustic mode

A new energetic particle-induced geodesic acoustic mode (EGAM) is shown to exist. The mode frequency and mode structure are determined nonperturbatively by energetic particle kinetic effects. In particular the EGAM frequency is found to be substantially lower than the standard GAM frequency. The radial mode width is determined by the energetic particle drift orbit width and can be fairly large for high energetic particle pressure and large safety factor. These results are consistent with the recent experimental observation of the beam-driven n=0 mode in DIII-D.     

Cooling and transport of energetic ions due to the global geodesic acoustic mode

It is shown that (i) the destabilization of global Geodesic Acoustic modes (GAM or E-GAM) by passing energetic ions in tokamaks can be accompanied with a considerable energy transfer from these ions to the mode; (ii) the mode-induced slowing down of the energetic ions leads to a radial shift outwards/inwards of the ions moving in the direction counter to/of the plasma current, in spite of the fact that the canonical angular momentum of the particles is conserved during GAMs. Some practical consequences of these phenomena are discussed.     

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在文献[1]中,通过分析实验上观察到的测地声模的小密度-电位比与测地声模谐波定级的自洽性,论证了测地声模是在转向点附近一种“蝌蚪状”的定域结构,但没有给出导致这种结构的物理机制.从随机媒质对波传播产生定域化的角度进行了探讨,采用测地声模由漂移波湍流参量激发的产生模型,计算了随机定域化导致的也是文献[1]所关注的相干长度.讨论了与实验观测比较的有关问题.     

托卡马克等离子体环向转动对测地声模的影响

采用回旋动理学方程推导得到了环向转动托卡马克等离子体中测地声模的色散关系,分析了环向转动对测地声模、低频模和声波分支的频率以及无碰撞阻尼率的影响。结果表明,测地声模的频率会随着环向转动马赫数而逐渐增大,而其无碰撞阻尼率则会随着环向转动马赫数而迅速减小。此外,低频模和声波分支的频率以及无碰撞阻尼率都会随着环向转动马赫数而逐渐减小,其中环向转动对声波分支的频率以及无碰撞阻尼率的影响非常小,基本上可以忽略。     

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采用回旋动理学方程推导得到了环向转动托卡马克等离子体中测地声模的色散关系,分析了环向转动对测地声模、低频模和声波分支的频率以及无碰撞阻尼率的影响.结果表明,测地声模的频率会随着环向转动马赫数而逐渐增大,而其无碰撞阻尼率则会随着环向转动马赫数而迅速减小.此外,低频模和声波分支的频率以及无碰撞阻尼率都会随着环向转动马赫数而逐渐减小,其中环向转动对声波分支的频率以及无碰撞阻尼率的影响非常小,基本上可以忽略.     

Quantum critical transport near the Mott transition

We perform a systematic study of incoherent transport in the high temperature crossover region of the half filled one-band Hubbard model. We demonstrate that the family of resistivity curves displays characteristic quantum critical scaling of the form ρ(T, δU) = ρ(c)(T)f(T/T?(δU)), with T?(δU) ~ |δU|(zν), and ρ(c)(T) ~ T. The corresponding β function displays a "strong coupling" form β ~ ln(ρ(c)/ρ), reflecting the peculiar mirror symmetry of the scaling curves. This behavior, which is surprisingly similar to some experimental findings, indicates that Mott quantum criticality may be acting as the fundamental mechanism behind the unusual transport phenomena in many systems near the metal-insulator transition.     

Observation of the critical regime near Anderson localization of light

The transition from diffusive transport to localization of waves should occur for any type of classical or quantum wave in any media as long as the wavelength becomes comparable to the transport mean free path l*. The signatures of localization and those of absorption, or bound states, can, however, be similar, such that an unequivocal proof of the existence of wave localization in disordered bulk materials is still lacking. Here we present time resolved measurements of light transport through strongly scattering samples with kl* values as low as 2.5. In transmission, we observe deviations from diffusion which cannot be explained by absorption, sample geometry, or reduction in transport velocity. Furthermore, the deviations from classical diffusion increase strongly with decreasing l* as expected for a phase transition. This constitutes an experimental realization of the critical regime in the approach to Anderson localization.     

Toroidal symmetry of the geodesic acoustic mode zonal flow in a tokamak plasma

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.     

Sigma mesonic mode near the QCD critical point

Properties of the QCD critical point, an endpoint of the first order boundary line in the temperature T–baryochemical potential μ _B plane, are discussed with the Nambu–Jona-Lasinio model. It is demonstrated that the softening of the sigma mesonic mode is inherent to the crossover transition of the chiral symmetry while the spectral enhancement of the particle–hole mode near zero frequency is associated with the QCD critical point. Phenomenological implications in heavy-ion experiments are mentioned briefly.     

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从Braginskii冷离子流体模型出发,利用准环坐标系,导出了轴对称(环向模数为零的)扰动密度和扰动电位所服从的两个二维线性偏微分本征波方程。采用WKB法及Langer变换,解出了测地声模过转向点一致有效的整体模结构和分立本征频率谱;论述了不存在连续谱解的原因;指出Schrödinger意义下的径向高激发态对应了近零频模;论证了与测地声模定级分析的自洽性将导致“蝌蚪”定域化;并讨论了“蝌蚪”定域化理论与实验上所观察到的“多个测地声模共存”现象之间的关係。     

The intermittent excitation of geodesic acoustic mode by resonant Instanton of electron drift wave envelope in L-mode discharge near tokamak edge

There are two distinct phases in the evolution of drift wave envelope in the presence of zonal flow. A long-lived standing wave phase, which we call the Caviton, and a short-lived traveling wave phase (in radial direction) we call the Instanton. Several abrupt phenomena observed in tokamaks, such as intermittent excitation of geodesic acoustic mode (GAM) shown in this paper, could be attributed to the sudden and fast radial motion of Instanton. The composite drift wave—zonal flow system evolves at the two well-separate scales:the micro-scale and the meso-scale. The eigenmode equation of the model defines the zero-order (micro-scale) variation; it is solved by making use of the two-dimensional (2D) weakly asymmetric ballooning theory (WABT), a theory suitable for modes localized to rational surface like drift waves, and then refined by shifted inverse power method, an iterative finite difference method. The next order is the equation of electron drift wave (EDW) envelope (containing group velocity of EDW) which is modulated by the zonal flow generated by Reynolds stress of EDW. This equation is coupled to the zonal flow equation, and numerically solved in spatiotemporal representation; the results are displayed in self-explanatory graphs. One observes a strong correlation between the Caviton-Instanton transition and the zero-crossing of radial group velocity of EDW. The calculation brings out the defining characteristics of the Instanton:it begins as a linear traveling wave right after the transition. Then, it evolves to a nonlinear stage with increasing frequency all the way to 20 kHz. The modulation to Reynolds stress in zonal flow equation brought in by the nonlinear Instanton will cause resonant excitation to GAM. The intermittency is shown due to the random phase mixing between multiple central rational surfaces in the reaction region.     

The acoustic emission of a distributed mode loudspeaker near a porous layer

Experimental and theoretical modeling of the vibro-acoustic performance of a distributed mode loudspeaker (DML) suggest that their acoustic emission can be significantly affected by the presence of a porous layer. The amplitude of the surface velocity of the panel and the acoustic pressure on the porous surface are reduced largely in the vicinity of structural resonances due to the additional radiation damping and visco-thermal absorption phenomenon in the porous layer. The experimental results suggest that a porous layer between a rigid base and a DML panel can considerably alter its acoustic emission in the near field and in the far field. This is illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with the interference between the sound emitted by the front surface of the speaker and that emitted from the back. Another contribution comes from the pronounced structural resonances in the surface velocity spectrum. The results of this work suggest that the acoustic boundary conditions near a DML can be modified by the porous layer so that a desired acoustic output can be attained.     

Investigation of the plasma potential oscillations in the range of geodesic acoustic mode frequencies by heavy ion beam probing in tokamaks

Specific oscillations within a range of 20 kHz (20 kHz-mode) were investigated on the T-10 and TEXT tokamaks using Heavy Ion Beam Probe (HIBP) diagnostic. Regimes with ohmic heating on both machines, and with off-axis ECRH in T-10 were studied. It was shown that 20 kHz-modes are mainly the potential oscillations. The power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with a contrast range of (3–5). They are the manifestation of torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. It was shown that in TEXT the radial electric field oscillations exist in a limited radial range of 0.65 > < 0.95. The frequency of 20 kHz-mode is varied in the region of observation; it diminishes to the plasma edge. In T-10, after ECRH switch-on, the frequency increases, correlating with the growth of the electron temperature T _e. In both machines the frequency of the 20 kHz-mode varies with local T _e: f T _e ^1/2 , which is consistent with theoretical scaling for geodesic acoustic modes (GAM): f _GAM c _s/R T _e ^1/2 , where c _s is the speed of sound. The absolute frequencies are close to GAM values within a factor of unity.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Intensification of acoustic beams in crystals at mode conversion near total internal reflection

Geometrical conditions under which mode conversion occurs in a crystal near total internal reflection of an acoustic beam are found. In this case, the entire energy of an incident pump wave is spent on the excitation of a narrow intense reflected beam close in structure to an eigenmode. A consistent choice of orientations of the sagittal plane and surface that excludes the reflection of a parasitic wave of leakage is found.     

Properties of geodesic acoustic modes and the relation to density fluctuations

The geodesic acoustic mode (GAM) is a high frequency branch of zonal flows, which is observed in toroidal plasmas. Because of toroidal curvature effects, density fluctuations are excited, which are investigated with the O-mode correlation reflectometer at TEXTOR. This Letter reports on the poloidal distribution of GAM induced density fluctuation and compares them with theoretical predictions. The influence of the GAM flows on the ambient turbulence is studied, too.     

Nano-plasmonic antennas in the near infrared regime

Plasmonic nano-antennas constitute a central research topic in current science and engineering with an enormous variety of potential applications. Here we review the recent progress in the niche of plasmonic nano-antennas operating in the near infrared part of the spectrum which is important for a variety of applications. Tuning of the resonance into the near infrared regime is emphasized in the perspectives of fabrication, measurement, modeling, and analytical treatments, concentrating on the vast recent achievements in these areas.     

Heat transport and critical slowing down near the Rayleigh-Bénard instability in cylindrical containers

Measurements of heat transport across layers of liquid He I heated from below show a sharp transition to fluid flow at the gravitationally driven Rayleigh-Bénard instability. The effective thermal conductivity of the convecting fluid is found to be a linear function of the reduced Rayleigh number (R - R_c)/R_c and to have a slope which agrees with calculations. Critical slowing down near R_c is measured and found to be in excellent agreement with theory.