Microwave pulse compression using a helically corrugated waveguide

There has been a drive in recent years to produce ultrahigh power short microwave pulses for a range of applications. These high-power pulses can be produced by microwave pulse compression. Sweep-frequency based microwave pulse compression using smooth bore hollow waveguides is one technique of passive pulse compression, however, at very high powers, this method has some limitation due to its operation close to cutoff. A special helical corrugation of a circular waveguide ensures an eigenwave with strongly frequency dependent group velocity far from cutoff, which makes the helically corrugated waveguide attractive for use as a passive pulse compressor for very high-power amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a particle in cell particle-in-cell (PIC) code are presented. In the experiments, a 70-ns 1-kW pulse from a conventional traveling-wave tube (TWT) was compressed in a 2-m-long helical waveguide. The compressed pulse had a peak power of 10.9 kW and duration of 3 ns. In order to find the optimum pulse compression ratio, the waveguide's dispersion characteristics must be well known. The dispersion of the helix was calculated using the PIC code Magic and verified using an experimental technique. Future work detailing plans to produce short ultrahigh power gigawatt (GW) pulses will be discussed

Experimental results from backward wave oscillators driven by a pseudospark-generated electron beam

The University of Strathclyde has investigated the applications of electron beams emitted by pseudospark (PS) discharges with an emphasis on their use in the production of radiation in the low terahertz range. To this end, backward wave oscillators (BWO), a form of vacuum tube which utilises the interaction between an axial electron beam and a slow-wave structure, have been designed and modelled using the particle-in-cell code MAGIC before construction of experiments. Successful millimetre radiation generated from BWOs at W-band (75GHz to 110GHz) driven by a 14-gap pseudospark discharge and G-band (140GHz to 220GHz) driven by a 4-gap pseudospark discharge will be presented

Atmospheric electrification in dusty, reactive gases in the solar system and beyond

Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) Brown Dwarfs which are amongst the oldest objects in the universe. Despite of this diversity, solar system planets, extrasolar planets and Brown Dwarfs have broadly similar global temperatures between 300K and 2500K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emission. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emission that potentially originate from accelerated electrons on Brown Dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions