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Turbulence in fluids is a ubiquitous, fascinating, and complex natural phenomenon that is not yet fully understood. Unraveling turbulence in high density, high temperature plasmas is an even bigger challenge because of the importance of electromagnetic forces and the typically violent environments. Fascinating and novel behavior of hot dense matter has so far been only indirectly inferred because of the enormous difficulties of making observations on such matter. Here, we present direct evidence of turbulence in giant magnetic fields created in an overdense, hot plasma by relativistic intensity (10 18 W∕cm 2 ) femtosecond laser pulses. We have obtained magneto-optic polarigrams at femtosecond time intervals, simultaneously with micrometer spatial resolution. The spatial profiles of the magnetic field show randomness and their k spectra exhibit a power law along with certain well defined peaks at scales shorter than skin depth. Detailed two-dimensional particle-in-cell simulations delineate the underlying interaction between forward currents of relativistic energy "hot" electrons created by the laser pulse and "cold" return currents of thermal electrons induced in the target. Our results are not only fundamentally interesting but should also arouse interest on the role of magnetic turbulence induced resistivity in the context of fast ignition of laser fusion, and the possibility of experimentally simulating such structures with respect to the sun and other stellar environments.intense laser matter interaction | high energy density | astrophysical simulations | filamentary structures T he largest terrestrially available magnetic fields are generated when an intense laser pulse (intensity above 10 18 W∕cm 2 ) irradiates a solid target (1-3). The high energy density produced by laser irradiation generates relativistic electron jets, through the process of wave breaking. These relativistic electron jets carry the laser energy deep into the target ionizing and heating the colder portions behind the laser generated plasma and exciting return shielding currents. In the laboratory, such heating is extremely important for fast ignition of highly compressed targets in laser fusion (4, 5), simulation of intra planetary matter existing at ultrahigh pressure (6), ultrafast X-ray pulses (7), as well as proton and ion acceleration up to the MeV-GeV levels (3). It also serves as an excellent tool for modeling astrophysical systems (8-10). The transport of relativistic electrons through hot dense matter is very complex and is barely understood (11,12). Simulations have shown that relativistic electron transport in plasma media is fraught with severe plasma instabilities particularly the Weibel instability (13), which leads to spatial separation of forward and backward currents and eventually to the emergence of turbulent structures (14) and rapid energy dissipation. A major physical parameter that mirrors this complex physics is the giant magnetic field-as high as hundreds of megagauss-generated in this interaction. In earlier st...

With the advances in micro-electronics, wireless sensor devices have been made much smaller and more integrated, and large-scale wireless sensor networks (WSNs) based the cooperation among the significant amount of nodes have become a hot topic. “Large-scale” means mainly large area or high density of a network. Accordingly the routing protocols must scale well to the network scope extension and node density increases. A sensor node is normally energy-limited and cannot be recharged, and thus its energy consumption has a quite significant effect on the scalability of the protocol. To the best of our knowledge, currently the mainstream methods to solve the energy problem in large-scale WSNs are the hierarchical routing protocols. In a hierarchical routing protocol, all the nodes are divided into several groups with different assignment levels. The nodes within the high level are responsible for data aggregation and management work, and the low level nodes for sensing their surroundings and collecting information. The hierarchical routing protocols are proved to be more energy-efficient than flat ones in which all the nodes play the same role, especially in terms of the data aggregation and the flooding of the control packets. With focus on the hierarchical structure, in this paper we provide an insight into routing protocols designed specifically for large-scale WSNs. According to the different objectives, the protocols are generally classified based on different criteria such as control overhead reduction, energy consumption mitigation and energy balance. In order to gain a comprehensive understanding of each protocol, we highlight their innovative ideas, describe the underlying principles in detail and analyze their advantages and disadvantages. Moreover a comparison of each routing protocol is conducted to demonstrate the differences between the protocols in terms of message complexity, memory requirements, localization, data aggregation, clustering manner and other metrics. Finally some open issues in routing protocol design in large-scale wireless sensor networks and conclusions are proposed.

A new metric for measuring the fault-tolerance capability of a multihop wireless sensor network is introduced in this paper. Most of the studies on fault-tolerance in sensor networks use connectivity as the metric of fault-tolerance. If the underlying network is k-connected, it can tolerate up to k − 1 failures. In measuring fault tolerance in terms of connectivity, no assumption regarding the locations of the failed sensor nodes is made -they may be very close to each other or very far from each other. In other words, the connectivity metric fails to capture any notion of locality of faults. However, in sensor networks, it is highly likely that the faults will be localized. This is particularly true in military applications, where an enemy bomb may inflict massive but localized damage to the sensor network. To capture the notion of locality in fault-tolerance, we introduce the notion of region-based connectivity. The region-based connectivity of a network may be informally defined to be the minimum number of nodes within a region whose failure will disconnect the network. Obviously, the notion of region-based connectivity is tied to the notion of a region. A region may be defined in several different ways and they are discussed in detail in the paper. The attractive feature of the region-based connectivity as the fault-tolerance metric is that it can achieve the same level of fault-tolerance as the traditional metric -connectivity, but requires much lower transmission power for the sensor nodes. We provide both analytical as well as extensive simulation results to support our claim.

IEEE 802.15.4 technology provides one solution for low-rate short range communications. Based on the integrated superframe structure of IEEE 802.15.4, a novel low-delay traffic-adaptive medium access control (LDTA-MAC) protocol for wireless body area networks (WBANs) is proposed in the paper. In LDTA-MAC, the guaranteed time slots (GTSs) are allocated dynamically according to the traffic load. At the same time, the active portion of superframe is kept to be a reasonable duration to decrease the energy consumption of the network devices. Moreover, for the successful GTS requests, the related data packets are transmitted in the current superframe instead of waiting more time to reduce the average packet delay. Simulations are conducted to evaluate the network performance and verify our protocol design. Comparing with IEEE 802.15.4, the results reveal LDTA-MAC accommodates more devices access to the network and reduces the packet delay obviously without the cost of more energy consumption.