Nonlinear Energy Cascading in Turbulence during the Internal Reconnection Event at the Sino-United Spherical Tokamak

The characteristics of the energy transfer and nonlinear coupling among edge electromagnetic turbulence in thermal quench sub-period of the internal reconnection event(IRE) are studied at the sino-united spherical tokamak device using multiple Langmuir and magnetic probe arrays. The wavelet bispectral analysis and the modified Kim method are applied to investigate linear growth/damping and nonlinear energy transfer rates, along with multi-field turbulence interactions. The results show a multi-field nonlinear energy transfer from electrostatic to magnetic turbulence that results in two-mode coupling in magnetic turbulence, which may play a crucial role to trigger the IRE.     

Observation of all-optical bump-on-tail instability

We demonstrate an all-optical bump-on-tail instability by considering the nonlinear interaction of two partially coherent spatial beams. For weak wave coupling, we observe momentum transfer with no variation in intensity. For strong wave coupling, modulations appear in intensity and evidence appears for wave (Langmuir) collapse at large scales. Borrowing plasma language, these limits represent regimes of weak and strong spatial optical turbulence. In both limits, the internal spectral energy redistribution is observed by recording and reconstructing a hologram of the evolving dynamics. The results are universal and can appear in any wave-kinetic system with short-wave-long-wave coupling.     

多光子非线性Compton散射对激光等离子体中强朗缪尔湍动谱的影响

 基于电子与多光子集团非线性Compton散射模型,研究了多光子非线性Compton散射对激光等离子体中强缪尔湍动谱的影响,提出了将入射光和散射光作为形成强缪尔湍动的新机制,给出了横等离激元、强朗缪尔激元和离声激元之间相互作用满足的修正方程,并进行了数值模拟.结果表明:Compton散射使横等离激元和朗缪尔激元间的碰撞频率大大增加,随着时间的演化,横等离激元和朗缪尔激元的能量由小波数区向大波数区的转移比散射前要快得多,同时产生剧烈的坍塌.坍塌后期,等离激元的强非线性作用激发出高次共振谐波,使能量从一个谐波转移到另一个谐波,形成无限高次谐波,引起波的破碎,出现由调制不稳定性控制的强朗缪尔湍动、较强的激光成丝和能量均分现象.研究结果为进一步研究强朗缪尔湍动的加速机制、反常碰撞、激光加热实验及快点火实验提供了理论支持.     

Gyrokinetic simulations of solar wind turbulence from ion to electron scales

A three-dimensional, nonlinear gyrokinetic simulation of plasma turbulence resolving scales from the ion to electron gyroradius with a realistic mass ratio is presented, where all damping is provided by resolved physical mechanisms. The resulting energy spectra are quantitatively consistent with a magnetic power spectrum scaling of k(-2.8) as observed in in situ spacecraft measurements of the "dissipation range" of solar wind turbulence. Despite the strongly nonlinear nature of the turbulence, the linear kinetic Alfvén wave mode quantitatively describes the polarization of the turbulent fluctuations. The collisional ion heating is measured at subion-Larmor radius scales, which provides evidence of the ion entropy cascade in an electromagnetic turbulence simulation.     

一种等离子体湍流实验数据分析程序的设计与应用

针对磁约束聚变等离子体边缘湍流中非线性能量传递过程的问题,开发了数据分析程序。该程序基于数字谱分析技术来求解波耦合方程,进而计算与三波相互作用相关联的线性和非线性耦合系数以及能量转移。介绍了算法设计和程序开发的主要思想,并对程序进行了仿真测试。然后应用该程序分析了反场箍缩装置实验的一次放电实验数据,观察到湍流之间的波耦合和能量级联现象,发现或证实了聚变等离子体湍流激发与增长的一种内在机制。     

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针对磁约束聚变等离子体边缘湍流中非线性能量传递过程的问题,开发了数据分析程序.该程序基于数字谱分析技术来求解波耦合方程,进而计算与三波相互作用相关联的线性和非线性耦合系数以及能量转移.介绍了算法设计和程序开发的主要思想,并对程序进行了仿真测试.然后应用该程序分析了反场箍缩装置实验的一次放电实验数据,观察到湍流之间的波耦合和能量级联现象,发现或证实了聚变等离子体湍流激发与增长的一种内在机制.     

Weak magnetohydrodynamic turbulence of a magnetized plasma

A weak turbulence of the magnetohydrodynamic waves in a strongly magnetized plasma was studied in the case when the plasma pressure is small as compared to the magnetic field pressure. In this case, the principal nonlinear mechanism is the resonance scattering of fast magnetoacoustic and Alfvén waves on slow magnetoacoustic waves. Since the former waves are high-frequency (HF) with respect to the latter, the total number of HF waves in the system is conserved (adiabatic invariant). In the weak turbulence regime, this integral of motion generates a Kolmogorov spectrum with a constant flux of the number of HF waves toward the longwave region. The shortwave region features a Kolmogorov spectrum with a constant energy flux. An exact angular dependence of the turbulence spectra is determined for the wave propagation angles close to the average magnetic field direction.     

Estimation of nonlinear transfer functions for fully developed turbulence

A statistical method for modeling the linear and quadratically nonlinear relationship between fluctuations monitored at two points in space or time in a turbulent medium is presented. This relationship is described with the aid of linear and quadratic transfer functions and the concept of coherency is extended to quantify the goodness of the quadratic model. A unique feature of the approach described in this paper is that it is valid for non-Gaussian “input” and “output” signals. The validity of the approach is demonstrated with simulation data. The method is applied to experimental data taken in the turbulent edge plasma of the TEXT tokamak. The results indicate a three wave process with energy transfer to large scale fluctuations. The estimation of transfer functions is a first step in quantitatively measuring coupling coefficients and the energy transfer.     

The energy spectrum of fully developed turbulence generated by random stirring

For fully developed turbulence in an incompressible fluid described by the Navier-Stokes equations with Gaussian random forces the relation between the energy spectrum and the stirring mechanism is investigated within a variational approach. Therein, the effect of nonlinear mode coupling is approximated by a wave number dependent eddy viscosity determined via a nonlinear integral equation for the energy spectrum. For various stirring spectra analytic approximations are compared with the solution obtained numerically with a cutoff in the integral kernel which ensures in eddy relaxing processes that the stirring forces exert strain only on scales larger than the eddy size. The results are compared with renormalization group calculations and closure approximations. Random forces injecting energy at a ratek ^–1 into the wave number banddk aroundk lead to a Kolmogorov distribution of energy. The spectrum of small-scale velocity fluctuations is shown to be universal in the sense that it remains unchanged under variations of the long wavelength stirring spectra.     

Generation of a sheared plasma rotation by emission, propagation, and absorption of drift wave packets

Collisional electron drift wave turbulence generates drift wave packet structures with density and vorticity fluctuations in the central plasma pressure gradient region of a linear plasma device. Tracking these packets reveals that they follow an outward directed spiral-shaped trajectory in the (r,θ) plane, are azimuthally stretched, and develop anisotropy as they approach an axisymmetric, radially sheared azimuthal flow located at the plasma boundary. Nonlinear energy transfer measurements and time-delay analysis confirm that structure absorption amplifies the sheared flow. Similar mechanisms likely operate at the edge of confined toroidal plasmas and should lead to the amplification of sheared flows at the boundary of these devices as well.     

激光等离子体中的强朗缪尔湍动谱分析

通过数值计算完全的萨哈罗夫方程,得到了朗缪尔波能量密度谱图。从一系列的谱图中可以看出,波的塌缩过程是沿着能量密度谱由小波数区域向大波数区域转移,朗缪尔波在塌缩过程中出现成丝现象。通过对获得的朗缪尔波平均能量密度谱进行拟合,得到了朗缪尔波能量密度谱函数。     

基于谱估计方法的等离子体湍流三波耦合数据处理算法

基于数字谱分析技术求解波耦合方程,进而计算与三波相互作用相关联的线性耦合系数和能量转移,以此开发了数据处理程序用于研究HL-2A 装置等离子体边缘湍流中的非线性能量传递过程。介绍了算法设计和开发的主要思想。应用该程序对与带状流相关的一次放电的实验数据进行了数据处理研究。结果表明,带状流是由等离子体湍流的能量逆级联所驱动的。     

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Based on the Ritz method to resolve the waves-coupling equation, the data processing code was developed to investigate the non-linear energy transfer progress in plasma edge turbulence in HL-2A tokamak. The arithmetic principle and the development methods of this code were introduced. By use of this program, the discharge data with relation to zonal flow were processed and studied. The results show that zonal flow was driven by the inverse energy cascade in the plasma turbulence.     

Spontaneous edge <Emphasis Type="Italic">E</Emphasis> × <Emphasis Type="Italic">B</Emphasis> sheared flow development studies in the TJ-II stellarator

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Tarragona, Spain, July 3–4, 2005.     

Weak turbulence for a vibrating plate: can one hear a Kolmogorov spectrum?

We study the long-time evolution of waves of a thin elastic plate in the limit of small deformation so that modes of oscillations interact weakly. According to the theory of weak turbulence (successfully applied in the past to plasma, optics, and hydrodynamic waves), this nonlinear wave system evolves at long times with a slow transfer of energy from one mode to another. We derive a kinetic equation for the spectral transfer in terms of the second order moment. We show that such a theory describes the approach to an equilibrium wave spectrum and represents also an energy cascade, often called the Kolmogorov-Zakharov spectrum. We perform numerical simulations that confirm this scenario.     

Electromagnetic Forces on Plasma Particles in the Presence of Resonant and Nonresonant Plasma Turbulence

Nonlinear forces on plasma particles in the presence of a test nonresonant wave and resonant plasma wave turbulence are calculated. The important feature of the considered nonlinear effect is that the forces due to the nonresonant test wave act on the plasma particles in the absence of linear and nonlinear resonances between the wave and the particles. Although in a closed plasma-wave system the process is balanced by the quasilinear interaction between the plasma resonant turbulence and plasma particles (leading to nonstationarity and inhomogeneity of the system), in open systems the effect can be significant.     

Mode-Coupling and Nonlinear Landau Damping Effects in Auroral Farley-Buneman Turbulence

Abstract

The fundamental problem of Farley-Buneman turbulence in the auroral E-region has been discussed and debated extensively in the past two decades. In the present paper we intend to clarify the different steps that the auroral E-region plasma has to undergo before reaching a steady state. The mode-coupling calculation, for Farley-Buneman turbulence, is developed in order to place it in perspective and to estimate its magnitude relative to the anomalous effects which arise through the nonlinear wave-particle interaction. This nonlinear effect, known as nonlinear “Landau damping” is due to the coupling of waves which produces other waves which in turn lose energy to the bulk of the particles by Landau damping. This leads to a decay of the wave energy and consequently a heating of the plasma. An equation governing the evolution of the field spectrum is derived and a physical interpration for each of its terms is provided.     

L-mode to H-mode transition triggered by sawtooth-induced heat flux in EAST

H-modes induced by sawtooth events can be often observed in discharges with marginal auxiliary power injection in EAST. Poloidal flow shear at the very plasma edge, increasing ∼25% up to the threshold value, is observed just before the L-H transition by means of a fast reciprocating probe array in EAST. This suddenly risen poloidal flow shear, caused by the increased turbulent driven Reynolds force, is motived by the heat pulse originally released by a sawtooth crash at the plasma core. Associated with the critical poloidal flow shear, the local turbulent decorrelation rate increases significantly. The increased turbulent decorrelation rate compensated by nonlinear energy transfer rate from the turbulence to the low-frequency shear flows, exceeding the turbulence energy input rate, is sustained for several hundred microseconds till the turbulence quench happening.     

Orthogonal interaction of bernstein mode with ion-acoustic wave through plasma maser effect

Interaction of Bernstein mode wave with ion-acoustic turbulence is treated as the plasma maser effect. Ion-acoustic turbulence is considered as resonant mode, propagating orthogonal to the test Bernstein mode. The external magnetic field, in the direction of ion-acoustic turbulence, is playing key role in transferring energy in upconversion process from resonant low frequency wave to the high frequency wave through modulated electric field. It is shown that the direct coupling term and the associated reverse absorbtion effect do not contribute individually to the growth or damping to the Bernstein mode. Only the polarization coupling term is found to be effective in the energy upconversion process as the external magnetic field is providing momentum in the direction of propagation of Bernstein mode. The polarization coupling term is identified to be the dominant upconversion factor in the plasma maser effect.     

超跨声涡轮叶栅尾缘激波系流动研究

本文通过分析超跨声涡轮尾缘波系的流动机理给出了详细的尾缘激波系流动图画,强调了尾缘分离激波的重要作用。经过分析,认为基底区压力是通过影响基底区宽度来影响涡轮叶栅尾缘激波强度的。最后将上述机理应用到某叶栅S1计算程序的改进工作中,数值计算和叶栅试验结果表明,改进工作有效提高了该程序对超跨声涡轮叶栅尾缘激波流动的模拟能力。