Bursty Behavior Research Articles

About bursty behaviour, coherent structures, wide scrape-off layer and large parallel flows in the edge of the Tore Supra tokamak

We present the measurements of plasma characteristics in the scrape-off layer (SOL) of the Tore Supra tokamak performed by means of reciprocating Langmuir probe. The probe is inserted into the machine from top. As the radial distance from last closed flux surface (LCFS) increases, ion saturation current exhibits stronger bursty character and its probability density function becomes increasingly skewed towards positive values. At the same time, burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of coherent turbulent structures of different size. The results of two-dimensional fluid modelling based of flux-driven interchange instability mechanism are in excellent agreement with experimental results. We obtained clear experimental evidence that most of the coherent structures are formed in poloidally localized region of the SOL around the outboard midplane. If the probe is magnetically connected to this region, the SOL is very wide and we detect bursty behaviour in the far SOL. On the other hand, if the probe is not magnetically connected to the outboard midplane region (magnetic field lines are intercepted by the outboard limiter), then the SOL is very thin and bursty behaviour is much less prominent. Detection of bursty behaviour in the far SOL is correlated with existence of wide SOL pointing on important role of bursty transport by means of coherent turbulent structures in establishing the width of the SOL in tokamaks. The measurements of parallel flow in the SOL shows that plasma particle radial flux coming from confinement region to the SOL is mostly poloidally localized around the outboard midplane. Our estimations show that more than 80% of plasma particle radial flux is coming from confinement region to the SOL in poloidally localized region — approximately ±15° — around the outboard midplane.

Transition-Region Explosive Events: Reconnection Modulated by p-Mode Waves

Transition-region explosive events (TREEs) have long been proposed as a consequence of magnetic reconnection. However, several critical issues have not been well addressed, such as the location of the reconnection site, their unusually short lifetime (about one minute), and the recently discovered repetitive behaviour with a period of three to five minutes. In this paper, we perform MHD numerical simulations of magnetic reconnection, where the effect of five-minute solar p-mode oscillations is examined. UV emission lines are synthesised on the basis of numerical results in order to compare with observations directly. It is found that several typical and puzzling features of the TREEs with impulsive bursty behaviour can only be explained if there exist p-mode oscillations and the reconnection site is located in the upper chromosphere at a height range of around 1900 km < h < 2150 km above the solar surface. Furthermore, the lack of proper motions of the high-velocity ejection may be due to a rapid change of temperature along the reconnection ejecta.

Bursty Behaviour of Turbulent Particle and Energy Fluxes in Edge Plasma of HT-7 Tokamak

High time resolution measurements of the electrostatic fluctuations and the turbulent particle and energy fluxes have been performed with a Langmuir probe array in the edge plasma in HT-7 tokamak. Bursty behaviour was observed in the time resolved turbulent fluxes with positive skewness and large kurtosis. The contribution of the large sporadic bursts to the transport losses were estimated. The analysis shows that the turbulent fluxes have different behaviour in different frequency domains and the probability distribution functions (PDFs) of the particle and energy fluxes present two distinct scaling ranges. All these are essentially consistent with the predictions of the self-organized criticality (SOC) model, though further studies are needed.

Data network models of burstiness

We review characteristics of data traffic which we term stylized facts: burstiness, long-range dependence, heavy tails, bursty behavior determined by high-bandwidth users, and dependence determined by users without high transmission rates. We propose an infinite-source Poisson input model which supplies traffic in adjacent time slots. We study properties of the model as slot width decreases and traffic intensity increases. This model has the ability to account for many of the stylized facts.

Data network models of burstiness

We review characteristics of data traffic which we termstylized facts: burstiness, long-range dependence, heavy tails, bursty behavior determined by high-bandwidth users, and dependence determined by users without high transmission rates. We propose an infinite-source Poisson input model which supplies traffic in adjacent time slots. We study properties of the model as slot width decreases and traffic intensity increases. This model has the ability to account for many of the stylized facts.

Effects of plasma current on nonlinear interactions of ITG turbulence, zonal flows and geodesic acoustic modes

The mutual interactions of ion temperature gradient (ITG) driven modes, zonal flows and geodesic acoustic modes (GAM) in tokamak plasmas are investigated using a global nonlinear gyrokinetic formulation with totally unconstrained evolution of temperature gradient and profile. A series of numerical simulations with the same initial temperature and density profile specifications is performed using a sequence of ideal MHD equilibria differing only in the value of the total plasma current, in particular with identical magnetic shear profiles and shapes of magnetic surfaces. On top of a bursty or quasi-steady state behaviour the zonal flows oscillate at the GAM frequency. The amplitude of these oscillations increases with the value of the safety factor q, resulting in a less effective suppression of ITG turbulence by zonal flows at a lower plasma current. The turbulence-driven volume-averaged radial heat transport is found to scale inversely with the total plasma current.

Two-dimensional convection and interchange motions in fluids and magnetized plasmas

In this contribution some recent investigations of two-dimensional thermal convection relevant to ordinary fluids as well as magnetized plasmas are reviewed. An introductory discussion is given of the physical mechanism for baroclinic vorticity generation and convective motions in stratified fluids, emphasizing its relation to interchange motions of non-uniformly magnetized plasmas. This is followed by a review of the theories for the onset of convection and quasi-linear saturation in driven-dissipative systems. Non-linear numerical simulations which result in stationary convective states reveal the process of laminar scalar gradient expulsion, leading to the formation of temperature plumes and vorticity sheets. These dissipative structures are demonstrated to result in temperature profile consistency and power law transport scaling far from the threshold. The last part of this paper deals with the generation of differential rotation by fluctuating motions through tilting of the convective structures. The role of kinetic energy transfer and shearing due to differential advection is pointed out. Numerical simulations for strongly driven systems reveal turbulent states with a bursty behaviour of the fluctuation level which is associated with relaxation oscillations in the kinetic energy of the azimuthally mean flows. This leads to a state of large-scale intermittency manifested by exponential tails in the single-point probability distribution function of the dependent variables. The global bursting is interpreted in terms of a predator–prey regulation from the point of view of energetics. Finally, a discussion is given of the relevance of these phenomena to a variety of magnetized plasma experiments.

Switching for IP-Based Multimedia Satellite Communications

This paper discusses the structure and performance of an Internet protocol (IP)-based satellite communications system to provide multimedia services. Uplink scheduling and switching to support IP differentiated services (DiffServ) traffic in a multibeam environment are addressed. End-to-end performance of a multibeam satellite communications system using an on-board switch is evaluated using simulation. Aggregate real-time and non-real-time traffic using different DiffServ classes is considered and the effects of their burstiness and long-range dependent behavior on the queueing performance are examined. Multiple-frequency time-division multiple-access is used on the uplink in conjunction with a dynamic capacity allocation scheme. Higher priority is given to voice and video real-time traffic to avoid delay variation. On-board downlink queue for non-real-time traffic is provided to achieve high statistical multiplexing gain.

Dynamical interplay between fluctuations, electric fields and transport in fusion plasmas

A view of recent experimental results and progress in the characterization of the statistical properties of electrostatic turbulence in magnetically confined devices is given. An empirical similarity in the scaling properties of the probability distribution function (PDF) of turbulent transport has been observed in the plasma edge region in fusion plasmas. The investigation of the dynamical interplay between fluctuation in gradients, turbulent transport and radial electric fields has shown that these parameters are strongly coupled both in tokamak and stellarator plasmas. The bursty behaviour of turbulent transport is linked with a departure from the most probable radial gradient. The dynamical relation between fluctuations in gradients and transport is strongly affected by the presence of sheared poloidal flows which organized themselves near marginal stability. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way to characterize transport based on the computation of effective transport coefficients

Application-directed voltage scaling

Clock (and voltage) scheduling is an important technique to reduce the energy consumption of processors that support voltage scaling. It is difficult, however, to achieve good results using only statistics from the operating system level when applications show bursty (unpredictable) behavior. We take the approach that such applications must be made power-aware and specify their average execution time (AET) and the deadline to the scheduler controlling the clock speed and processor voltage. This paper describes our energy priority scheduling (EPS) algorithm supporting power-aware applications. EPS orders tasks according to how tight their deadlines are and how often tasks overlap. Low-priority tasks are scheduled first, since they can be easily preempted to accommodate for high-priority tasks later. The EPS algorithm does not always yield the optimal schedule, but has a low complexity. We have implemented EPS on a StrongARM-based variable-voltage platform. We conducted experiments with a modified video decoder that estimates the AET of each frame. Measurements show that application-directed voltage scaling reduces processor power consumption with 50% for the bursty video decoder without missing any frame deadlines.

Bursty events and incremental diffusion in a local diffusion and multi-scale convection system

A one-dimensional cellular automaton is defined without the critical gradient rule (Δh>Δhc) which is essential to the existence of avalanches in self-organized criticality (SOC) models. Instead, only the local diffusion rule is used, however, the characteristics of SOC, such as the bursty behaviour, power-law decay in fluctuation spectra, self-similarity over a broad range of scales and long-time correlations, are still observed in these numerical experiments. This numerical model is established to suggest that the bursty events and the incremental diffusion observed universally in fusion experiments do not necessarily imply the submarginal dynamics.

Intermittent Structures in the High Field Side Boundary of the HYBTOK-II Tokamak

The spatial and temporal characteristics of the turbulence in the inboard and outboard scrape-off layer (SOL) of tokamak HYBTOK-II have been studied using poloidal probe array. Bursty behaviour with intermittent bursts was observed for both outboard and inboard SOL. In the inboard (high field side), high level of density fluctuations has been observed. The fluctuations in the high field side and low field side are identical in statistics that is non-Gaussian one.

Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.

Effectiveness of gain control in EDFAs against traffic with different levels of bursty behaviour

Previous publications have addressed the impact of bursty, self-similar traffic on erbium-doped fibre amplifiers (EDFA). Several gain control techniques have been suggested to combat the gain fluctuations of EDFA with this type of traffic. The effectiveness of two such methods are investigated, namely highly inverted amplifiers and all-optical gain clamping, while varying the parameters characterising the burstiness of the sources. While previous publications have focused on the effect of the average load or activity factor, the paper further investigate the dependence on the relative variability of the packet burst lengths (ON times), and lengths of the interburst idle times (OFF times). Both gain control methods reduce the output power and signal-to-noise ratio excursions with respect to a standard amplifier chain. The authors find that, for an eight channel WDM system and a cascade of six EDFAs, all-optical gain clamping can reduce the variations by a factor of five, provided adequate power is present in the clamping laser, while highly inverted amplifiers have variations reduced by a factor of two. The authors find that, in a clamped chain, the source activity factor does not uniquely determine the required level of lasing power. The authors also deduce that information on the relative variability of ON and OFF times is essential.

Modeling multipath fading channel dynamics for packet data performance analysis

The multipath fading channel modeling traditionally focuses on physical level dynamics such as signal strength and bit error rate. In this paper we characterize multipath fading channel dynamics at the packet level and analyze the corresponding data queueing performance in various environments. The integration of wireless channel modeling and data queueing analysis provides us a unique way to capture important channel statistics with respect to various wireless network factors such as channel bandwidth, mobile speed and channel coding. The second order channel statistics, i.e. channel power spectrum, is found to play an important role in the modeling of multipath fading channels. The data queueing performance is largely dependent on the interaction between the channel power spectrum and the data arrival power spectrum; whichever has lower frequency power will have more impact on queueing performance. Note that the data arrival power spectrum provides a measure of burstiness and correlation behavior of data packet arrivals. Throughout the paper, we use the Markov chain modeling technique to match the measured important channel statistics for both channel modeling and queueing analysis.

Probe experiment characterizing 30‐MHz radio wave scatter in the high‐latitude ionosphere

A probe experiment, consisting of radio links between a common 30‐MHz transmitter located at Murmansk, Russia, and two receivers used as the imaging riometer (two‐dimensional 64 multiple‐beam antenna) located at Ny Ålesund, Svalbard, and Tjornes, Iceland, was carried out to characterize wave scatter in the high‐latitude ionosphere. They are nearly aligned with and perpendicular to the geomagnetic meridian, respectively. In experiments conducted in March–April 1994, the 30‐MHz probe signals were identified at nighttime more frequently than during the day at both receiver stations during periods of increased geomagnetic activity near the path midpoints, indicating that a relationship between the propagation path and the location of the auroral oval controls signal identification. For the nighttime propagation paths within or crossing through the auroral oval, duty cycles of the probe signals were roughly correlated with increases in geomagnetic activity. Their arrival directions showed a spread with a dominant power on the low elevation and a normal distribution in azimuth. These results indicate that the probe signals are characterized as nonmeteoric “auroral E” scatter caused by irregular, large‐scale profiles of electron density enhancements at the lower edge of the ionosphere. However, on 2 days of weak geomagnetic activity, strong probe signals with bursty behavior were identified by an extremely high duty cycle (∼98%) for the nighttime meridian path only, and their arrival directions showed an isotropic spread in azimuth. Such nonmeteoric probe signals are characterized as “coherent” scatter caused by small‐scale (∼5 m) field‐aligned irregularities in electron density in the E region ionosphere, related to “sporadic E” occurrence.

A channel model for characterization of the error data recovered from compact discs

The data recovered from compact discs are contaminated with errors that exhibit a bursty behavior. A channel model is developed to characterize the error bursts and good-data gaps that occur in actual data recovered from compact discs. The probability of having m erroneous symbols in a data block of length n symbols is calculated using the channel model. This probability can be used to evaluate the performance of the error-correction code (ECC) used in the compact disc system.

Providing traffic smoothness and service guarantees within ATM networks through a rate-controlled service discipline

With the introduction of diverse rate requirements under a variety of statistical multiplexing schemes, traffic burstiness behavior of a source stream and its quality-of-service (QoS) performances within the ATM networks become difficult to model and analyze. In this paper, we address this issue and propose a rate-controlled service discipline that provides control of the traffic burstiness while maintaining QoS guarantees for traffic flows with various rate requirements. According to our analysis, traffic streams from different connections can be well regulated at the output of each network node based on their rate requirements. Traffic envelope and the associated burstiness behavior inside the network can thus be effectively characterized. In addition, by assuming a leaky-bucket constrained input source, we prove that the proposed scheme can provide end-to-end delay and jitter bounds for each connection passing through a multi-hop network. Further, due to the low traffic burstiness, only a small buffer space is required at the internal switches for guaranteeing QoS requirements.

Two-way TCP traffic over rate controlled channels: effects and analysis

We study the performance of bidirectional TCP/IP connections over a network that uses rate-based flow and congestion control. An example of such a network is an asynchronous transfer mode (ATM) network using the available bit rate (ABR) service. The sharing of a common buffer by TCP packets and acknowledgment (acks) has been known to result in an effect called ack compression, where acks of a connection arrive at the source bunched together, resulting in unfairness and degraded throughput. It has been the expectation that maintaining a smooth flow of data using rate-based flow control would mitigate, if not eliminate, the various forms of burstiness experienced with the TCP window flow control. However, we show that the problem of TCP ack compression appears even while operating over a rate-controlled channel. The degradation in throughput due to bidirectional traffic can be significant. For example, even in the simple case of symmetrical connections with adequate window sizes, the throughput of each connection is only 66.67% of that under one-way traffic. By analyzing the periodic bursty behavior of the source IP queue, we derive estimates for the maximum queue size and arrive at a simple predictor for the degraded throughput, for relatively general situations. We validate our analysis using simulation on an ATM network using the explicit rate option of the ABR service. The analysis predicts the behavior of the queue and the throughput degradation in simple configurations and in more general situations.

Effects of user mobility on the capacity of a CDMA cellular network

AbstractIn the recent technical literature on cellular networks Code Division Multiple Access (CDMA) has received a large attention as a promising radio interface access technique. Many studies. are devoted to the capacity evaluation of the radio interface adopting CDMA, but seemingly none of them accounts for the variability of the user spatial density due to user mobility. This should be a concern especially in a microcellular environment, where it cannot be relied upon a significant spatial average over the radio coverage area of a Base Station. The major aim of this work is a preliminary assessment of the effects of “bursty” user mobility on the capacity of the CDMA radio interface. To this end. we introduce a user mobility model apt to describe large fluctuations of the number of users in a radio cell area. A reference model of a CDMA network is used to evaluate the effects of user mobility on the capacity for a wide range of the model parameters, by means of simulations accounting for shadowing, call attempt process, voice activity and antenna directivity and assuming perfect power control. We show that user mobility can remarkably affect the CDMA capacity, mainly because of the resulting highly bursty behaviour of self‐noise. A simple traffic control scheme is devised to increase the capacity of the CDMA network, under a joint constraint on link availability, call blocking and call dropping probabilities. It is pointed out that the concepts introduced in this paper are independent of the specific CDMA implementation, i.e. they hold for any asynchronous CDMA based cellular network.