Streamer discharges are often operated in a repetitively pulsed mode and are therefore influenced by species left over from the previous discharge, especially free electrons and ions. We have investigated these effects by applying two consecutive positive high voltage pulses of 200-700 ns duration to a point-plane gap in artificial air, pure nitrogen, other nitrogen-oxygen mixtures and pure argon at pressures between 67 and 533 mbar. The pulses had pulse-to-pulse intervals ( t) between 200 ns and 40 ms. We imaged both discharges with two ICCD cameras and combined this to a compound image. We observe for values of t below 0.5-15 µs (at 133 mbar, with t depending on gas mixture) that during the second pulse the streamers continue the paths of the first-pulse streamers. We call the maximal time for which this continuation still occurs the continuation time. For N 2 -O 2 mixtures, this time has a maximum at an oxygen concentration of about 0.2%. According to our plasma-chemical modelling this maximum is determined by the electron loss rate which has a minimum around this oxygen concentration. Depending on oxygen concentration the dominant recombining positive ion is N + 4 , O + 2 or O + 4 where O + 2 dominates around 0.2% O 2 and recombines slowest. For increasing values of t we observe that after the continuation phase first no new streamers occur at all, then new streamers show up that avoid the entire pre-ionized region. Next we see new thin streamers that follow the edges of the old channels. For larger t (10-200 µs) the new streamers start to increase in size and move to the centre of the old channels. Finally, around millisecond timescales the new channels are completely independent of the old channels.Together this points to the combination of two mechanisms: streamers search the proximity of regions with increased electron density, but cannot penetrate regions with very high electron density.

During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet, China, a long-duration count enhancement associated with thunderclouds was detected by a solar-neutron telescope and neutron monitors installed at the Yangbajing Comic Ray Observatory. The event, lasting for $40 min , was observed on July 22, 2010. The solar-neutron telescope detected significant -ray signals with energies >40 MeV in the event. Such a prolonged high-energy event has never been observed in association with thunderclouds, clearly suggesting that electron acceleration lasts for 40 min in thunderclouds. In addition, Monte Carlo simulations showed that >10 MeV rays largely contribute to the neutron monitor signals, while >1 keV neutrons produced via a photonuclear reaction contribute relatively less to the signals. This result suggests that enhancements of neutron monitors during thunderstorms are not necessarily clear evidence for neutron production, as previously thought.

The role of free electrons in the guiding of positive streamers View the table of contents for this issue, or go to the journal homepage for more 2016 Plasma Sources Sci. Technol. 25 044001

In contrast to conventional hardware where the structure is irreversibly fixed in the design process, evolvable hardware (EHW) is designed to adapt to changes in task requirements or changes in the environment, through its ability to reconfigure its own hardware structure dynamically and autonomously. This capacity for adaptation, achieved by employing efficient search algorithms based on the metaphor of evolution, has great potential for the development of innovative industrial applications. This paper introduces EHW chips and six applications currently being developed as part of MITI's Real-World Computing Project; an analog EHW chip for cellular phones, a clock-timing architecture for Giga hertz systems, a neural network EHW chip capable of autonomous reconfiguration, a data compression EHW chip for electrophotographic printers, and a gate-level EHW chip for use in prosthetic hands and robot navigation.

here is a flurry of activity in the networking community developing advanced services networks. Although the focus of these efforts varies widely from per-flow service definitions like integrated services (IntServ) [1,2] to service frameworks like Xbind [3], they share the overall goal of evolving the Internet service model from what is essentially a basic bitway pipe to a sophisticated infrastructure capable of supporting novel advanced services.In this article we consider a network environment that comprises not only communication services, but storage and computation resources as well. By packaging storage/computation resources together with communication services, value-added service providers will be able to support sophisticated services such as intelligent caching, video/audio transcoding and mixing, virtual private networking, virtual reality games, and data mining. In such a service-oriented network, value-added services can be composed in a hierarchical fashion: applications invoke high-level service providers, which may in turn invoke services from lower-level service providers. Providers in the top of the hierarchy will typically integrate and add value to lower-level services, while the lowest-level services will supply basic communication and computational support. Since services can be composed hierarchically, both applications and service providers will be able to combine their own resources with resources or services delivered by other service providers to create a high-quality service for their clients. The design of such a service-oriented network poses challenges in several areas, such as resource discovery, resource management, service composition, billing, and security. In this article we focus on the resource management architecture and algorithms for such a network.Service-oriented networks have several important differences from traditional networks that make existing network resource management inadequate. First, while traditional communication-oriented network services are provided by switches and links, value-added services will have to manage a broader set of resources that includes computation, storage, and services from other providers. Moreover, interdependencies between differ-0890-8044/01/$10.00 AbstractThe Internet is rapidly changing from a set of wires and switches that carry packets into a sophisticated infrastructure that delivers a set of complex value-added services to end users. Services can range from bit transport all the way up to distributed value-added services like video teleconferencing, virtual private networking, data mining, and distributed interactive simulations. Before such services can be supported in a general and dynamic manner, we have to develop appropriate resource management mechanisms. These resource management mechanisms must make it possible to identify and allocate resources that meet service or application requirements, support both isolation and controlled dynamic sharing of resources across services and applications sharing physical resources, a...

Streamer discharges are a primary mode of electric breakdown in thunderstorms and high voltage technology; they are generally believed to grow along electric field lines. However, we here give experimental and numerical evidence that streamers can propagate nearly perpendicularly to the background electric field. These streamers are guided by pre-ionization that is orders of magnitude lower than the ionization density in a streamer channel, hardly affecting the background field. Positive streamers could be guided in nitrogen with 0.5% of oxygen or less, but not in air. This observation also tests the role of photoionization in gas mixtures with varying nitrogen-oxygen ratio.

We demonstrate the amplification of a femtosecond signal pulse in an underdense plasma by a novel mechanism called superradiant amplification. The pulse is amplified by a counterpropagating few picosecond long pump pulse. In the superradiant regime, the ponderomotive forces exceed the electrostatic forces and arrange the plasma electrons to reflect the pump light into the signal pulse. We found a significant amplification in energy and intensity. The time structure of the amplified signal pulse carries intrinsic features of the superradiant regime. Sub-10-fs pulses of petawatt power appear feasible.

Irradiation with a KrF laser controlled the positive streamer branching in atmospheric argon gas. This laser irradiation changed the amount of background ionization before the streamer discharge. Measuring the ionization current allowed us to evaluate the initial electron density formed by the KrF laser. We observed characteristic feather-like branching structure and found that it was only suppressed in the irradiated region. The threshold of ionization density which can influence the branching was evaluated to be 5 × 105 cm−3. The relationship between the size of avalanche head and mean distance between initial electrons explained this suppression behaviour. These experimental results support that the feather-like structure originates from the branching model of Loeb–Meek, a probabilistic merging of individual avalanches.