The Alpha Effect: The Connection between Cyclonic Events and Current Helicity

In a magnetized plasma, resistive diffusion of large-scale magnetic fields can be suppressed or even overcome by a turbulently generated electromotive force. For a plasma in which the turbulence is homogeneous and isotropic this EMF is characterized by the ensemble average = ?B0, where ?v and ?b represent the turbulent fields and B0 defines the large-scale field. Determination of the statistical properties of the turbulence that are required to generate a finite alpha effect, as it has become known, is one of the central subjects of dynamo theory. Parker has shown that helical velocity fluctuations possessing a net amount of kinetic helicity are capable of dynamo action. These "cyclonic events" produce electromagnetic fluctuations characterized by their own statistical properties. Within the context of "mean-field electrodynamics" we show that these fluctuations possess a net amount of current helicity, and find that a necessary condition for dynamo action is that the turbulent current helicity and the current helicity in the large-scale field be of opposite sign.     

Dynamics of plasma blobs in a shear flow

The global dynamic of plasma blobs in a shear flow is investigated in a simple magnetized torus using the spatial Fourier harmonics (k-space) framework. Direct experimental evidence of a linear drift in k space of the density fluctuation energy synchronized with blob events is presented. During this drift, an increase of the fluctuation energy and a production of the kinetic energy associated with blobs are observed. The energy source of the blob is analyzed using an advection-dissipation-type equation that includes blob-flow exchange energy, linear drift in k space, nonlinear processes, and viscous dissipations. We show that blobs tap their energy from the dominant ExB vertical background flow during the linear drift stage. The exchange of energy is unidirectional as there is no evidence that blobs return energy to the flow.     

Wavelet transform modulus maxima based fractal correlation analysis

We study the intermittent behavior of the energy decay and the linear magnetic response of a glassy system during isothermal aging after a deep thermal quench, using the Edward-Anderson spin glass model as a paradigmatic example. The large intermittent changes in the two observables occur in a correlated fashion and through irreversible bursts, `quakes', which punctuate reversible and equilibrium-like fluctuations of zero average. The temporal distribution of the quakes is a Poisson distribution with an average growing logarithmically on time, indicating that the quakes are triggered by record sized fluctuations. As the drift of an aging system is to a good approximation subordinated to the quakes, simple analytical expressions [Sibani et al. Phys Rev B 74, 224407 (2006)] are available for the time and age dependence of the average response and average energy. These expressions are shown to capture the time dependencies of the EA simulation results. Finally, we argue that whenever the changes of the linear response function and of its conjugate autocorrelation function follow from the same intermittent events a fluctuation-dissipation-like relation can arise between the two in off-equilibrium aging.     

The turbulent Alfvénic aurora

It is demonstrated from observations that the Alfvénic aurora may be powered by a turbulent cascade transverse to the geomagnetic field from large MHD scales to small Alfvén wave scales of several electron skin depths and less. We show that the energy transport through the cascade is sufficient to drive the observed acceleration of electrons from near-Earth space to form the aurora. We find that regions of Alfvén wave dissipation, and particle acceleration, are localized or intermittent and embedded within a near-homogeneous background of large-scale MHD structures.     

Superstatistics and renewal critical events

An approach to intermittent systems based on renewal processes is reviewed. The Waiting Times (WTs) between events are the main variables of interest in intermittent systems. A crucial role is played by the class of critical events, characterized by Non-Poisson statistics and non-exponential WT distribution. A particular important case is given by WT distributions with power tail. Critical events play a crucial role in the behavior of a property known as Renewal Aging. Focusing on the role of critical events, the relation between superstatistics and non-homogeneous Poisson processes is discussed, and the role of Renewal Aging is illustrated by comparing a Non-Poisson model with a Poisson one, both of them modulated by a periodic forcing. It is shown that the analysis of Renewal Aging is sensitive to the presence of critical events and that this property can be exploited to detect Non-Poisson statistics in a time series. As a consequence, it is claimed that, apart from the characterization of superstatistical features such as the distribution of the intensive parameter or the separation of the time scales, the Renewal Aging property can give some effort to better determine the role of Non-Poisson critical events in intermittent systems.      

Transport induced by large scale convective structures in a dipole-confined plasma

Convective structures characterized by E×B motion are observed in a dipole-confined plasma. Particle transport rates are calculated from density dynamics obtained from multipoint measurements and the reconstructed electrostatic potential. The calculated transport rates determined from the large-scale dynamics and local probe measurements agree in magnitude, show intermittency, and indicate that the particle transport is dominated by large-scale convective structures.     

Temporally heterogeneous dynamics in granular flows

Granular simulations are used to probe the particle scale dynamics at short, intermediate, and long time scales for gravity-driven, dense granular flows down an inclined plane. On approach to the angle of repose, where motion ceases, the dynamics become intermittent over intermediate times, with strong temporal correlations between particle motions-temporally heterogeneous dynamics. This intermittency is characterized through large-scale structural events whereby the contact network periodically spans the system. A characteristic time scale associated with these processes increases as the stopped state is approached. These features are discussed in the context of the dynamics of supercooled liquids near the glass transition.     

Strong Self-Similar Fluctuations of Target Fragments in Ring-like Events in Ultra-Relativistic Nuclear Collision

The intermittent fluctuation of target evaporated particles is studied in both ring-like and jet-like events emitted in ^32 S-emulsion interactions at 200 A Ge V within the framework of multi-dimensional factorial moment methodology using the concept of the Hurst exponent. It is observed that the intermittent fluctuation in the ring-like event is self-similar, whereas in the jet-like event fluctuation is self-affine. However, study indicates that the strength of fluctuation in the ring-like events is much stronger than that in the jet-like events.     

Assessment of LES of intermittent soot production in an aero-engine model combustor using high-speed measurements

Soot production in turbulent flames is an extremely intermittent phenomenon since it is the result of specific thermochemical conditions occasionally occurring in space and time. In realistic configurations such as the swirling flames used in gas-turbines, the presence of large-scale flow motions can additionally affect soot formation processes, leading to even more pronounced intermittency. Classically, the validation of numerical simulations is performed by comparing time-averaged results with experimental data of the phenomenon under investigation. This comparison can be considered as rigorous only if a statistically converged numerical representation is obtained. In case of sporadic events such as intermittent soot formation in turbulent flames, this means to perform the simulation over thousands of milliseconds of physical time, which can have extremely high CPU demands when performing Large Eddy Simulation (LES). In this work, a possible strategy to overcome this issue is proposed based on the use of high-speed measurements and numerically synthesized signals from LES. To illustrate the approach, numerical and experimental soot light scattering signals are considered here by looking at the model aero-engine combustor developed at DLR for the study of pressurized swirled sooting flames. The light scattering signal is numerically synthesized from an LES. Experimental high-speed measurements are used to statistically account for the high temporal and spatial variability of soot when considering time intervals similar to what is today achievable with LES. The feasibility of this approach is finally demonstrated by comparing numerical results to the ensemble of possible soot production states observed experimentally in the DLR burner allowing to eventually validate the present LES results.     

Hole with diluted density in the presence of self-generated magnetic fields

The collapse behavior of the fields and hole with diluted density described by the nonlinear coupling equations in laser-produced plasma are investigated in the condition of non-static limit. The results show that the nonlinear plasma currents give rise to intermittent magnetic fields, leading to collapsing hole with diluted density. The self-generated magnetic field is very important for the formation of hole with diluted density. The resulting density perturbation rate is very similar to that observed experimentally.     

混沌动力学方法在等离子体尾迹流场研究中的应用

用混沌动力学方法对多道扫描静电探针的离子饱和电流信号进行分析,研究了等离子体尾迹流场.通过对相关维、Renyi熵和最大Lyapunov指数的分析,得到了近尾流场的分层结构.利用最大Lyapunov指数,观测到了在x>10D以后的远尾流场与自由流场相似.结合探针信号的自相关函数,研究流场湍流结构,发现近尾可能存在大涡拟序结构,而在远尾则没有湍流.观察到了流场具有一定的间歇特征,认为这种间歇性与湍流有关.结果还表明,混沌动力学的分析方法对信号中非周期成分十分敏感,在研究等离子体尾迹流场这一类非线性系统时,它具有明显的优越性 关键词： 混沌动力学 尾迹 等离子体湍流 静电探针     

Experimental observation of intermittent route to chaos in magnetized filamentary discharge plasma due to the cylindrical plasma bubble

We delineate an experimental observation of the effect of the magnetic field along with mesh grid biasing in the presence of a cylindrical plasma bubble in a filamentary discharge magnetised plasma system. The cylindrical mesh grid of 80% optical transparency has been negatively biased and introduced in the plasma for creating a plasma bubble. Plasma floating potential fluctuations have been taken outside (LP1) and inside (LP2) of the plasma bubble. It has been noticed that as the external magnetic field is increased the oscillation pattern shows intermittent route to chaos as the system evolved from regular type of relaxation oscillations (of larger amplitude) to an irregular type of oscillations (of smaller amplitude) We have used recurrence quantification analysis (RQA) to the observed intermittency to chaos in the plasma. The main measures of RQA are laminarity (LAM) and determinism (DET). The laminarity measure can be associated with the average time between the chaotic burst in the intermittency. It has also been observed that the DET depends on the control parameter and decreases exponentially, features like a dip in skewness and a hump in the kurtosis with the variation of control parameter have been noticed, which are the strong evidence of intermittent behaviour of the system. Further, a numerical model has been developed to the observed experimental analysis of the intermittent route to chaos.     

Crossover from intermittent to continuum dynamics for locally driven colloids

We simulate a colloid with charge q(d) driven through a disordered assembly of interacting colloids with charge q and show that, for q(d) approximately q, the velocity-force relation is nonlinear and the velocity fluctuations of the driven particle are highly intermittent with a 1/f characteristic. When g(d) >q , the average velocity drops, the velocity-force relation becomes linear, and the velocity fluctuations are Gaussian. We discuss the results in terms of a crossover from strongly intermittent heterogeneous dynamics to continuum dynamics. We also make several predictions for the transient response in the different regimes.     

From quantum fluctuations to large-scale structures

An acceleration phase in the early universe allows microscopic quantum fluctuations inside a causal domain to expand into macroscopic ripples in the spacetime metric. These, in turn, can evolve into large-scale structures in the universe. After its generation from quantum fluctuations, a ripple in the metric spends a long period outside the causal domain where its evolution is characterized by a conserved amplitude, a fact closely related to the large-scale Friedmann-like evolution of the perturbed Friedmann universe. We show that, under the assumption of linear processes, the generation and evolution of large-scale structures can be described quite simply.     

Inertial ranges for turbulent solutions of complex Ginzburg-Landau equations

We consider the spatially periodic, complex Ginzburg-Landau (CGL) equation in regimes close to that of a critical or supercritical focusing non-linear Schrödinger (NLS) equation, which is known to have solutions that exhibit self-similar blow-up. We use the NLS blow-up solutions as a template to develop a theory of how nearly self-similar intermittent burst events can create a power-law inertial range in the time-averaged wave-number spectrum of CGL solutions. Numerical experiments in one dimension with a quintic (critical) and septant (supercritical) non-linearity show a that power-law inertial range emerges which differs from that predicted by the theory. However, as one approaches the NLS limit in the supercritical case, a second power-law inertial range is seen to emerge that agrees with the theory.     

On the reflection of a radiowave beam from a plasma layer with large-scale inhomogeneities

Using the method of parabolic equation (MPE), we obtain transfer equations for the mean field, the space-coherence function, and the ray intensity of a radiowave beam as it is reflected from a plasma layer with random inhomogeneities. The general solutions of these equations are found. Special attention is given to the case of radiowave beam reflection from a linear plasma layer with large-scale electron-density inhomogeneities. If a weakly directed transceiving SW antenna is used, the shortwave scattering can lead to a pronounced (of the order of 3 dB) decrease in the intensity of a vertical-sounding signal reflected from the ionospheric F2 layer only under the conditions of abnormally strong ionospheric electron-density perturbations. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 955–965, August, 1998.     

Observation of intermittency in edge plasma of SUNIST tokamak

The temporal intermittency of the fluctuation-driven particle transport fluxes is analysed by using data obtained from Langmuir probe array in the edge of the Sino-United Spherical Tokamak (SUNIST). The conditional statistics analysis indicates that the intermittent structures have a characteristic time width of about 30μs, which is the typical fluctuation time scaling. It is also found that the transport fluxes have a multifractal character over the fluctuation time scales, and exhibit a long-time-range correlation character with self-similar parameter H>0.5 in the plasma confinement time scales. Furthermore, the analyses show that the level of the intermittency and the long-range correlation of the fluxes vary with increasing plasma density. These observations are consistent with the prediction of the avalanche-like model.     

跳动模式对微球CH涂层表面粗糙度的影响

 采用低压等离子体化学气相沉积方法（LPPCVD）,结合反弹盘系统制备了微球CH涂层,研究了跳动模式对微球CH涂层表面形貌的影响。利用光学显微镜和扫描电镜(SEM)对微球涂层表面形貌进行了分析;利用原子力显微镜（AFM）测定了微球CH涂层表面均方根粗糙度（RMS）并对球形度进行了表征;利用X光照相技术对同心度进行了表征。结果表明：采用间歇跳动模式可有效改善微球CH涂层的表面形貌,降低中高模数的粗糙度。在间歇跳动模式下,减小占空比,可使CH涂层的表面粗糙度得到进一步降低。在占空比为1/4的间歇跳动模式下制备的厚度为30 mm的CH涂层,其表面均方根粗糙度低于30 nm,碳氢-聚苯乙烯（CH-PS）微球的球形度与同心度均优于99%。     

Detecting vorticity filaments using wavelet analysis: About the statistical contribution of vorticity filaments to intermittency in swirling turbulent flows

Swirling turbulent flows display intermittent pressure drops associated with intense vorticity filaments. Using the wavelet transform modulus maxima representation of pressure fluctuations, we propose a method of characterizing these pressure drop events from their time-scale properties. This method allows us to discriminate fluctuations induced by just formed (young) as well as by burst (old) filaments from background pressure fluctuations. The statistical characteristics of these filaments (core size, waiting time) are analyzed in details and compared with previously reported experimental and numerical findings. Their intermittent occurrence is found to be governed by a pure Poisson's law, the hallmark of independent events. Then we apply the wavelet transform modulus maxima (WTMM) method to the background pressure fluctuations. This study reveals that, once removed all the filaments, the “multifractal” nature of pressure fluctuations still persists. This is a clear indication that the statistical contribution of the filaments is not important enough to account for the intermittency phenomenon in turbulents flows. Received 27 July 1998 and Received in final form 23 November 1998