124,259 research outputs found

    Tributes to Prof David Sanders

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    Obituary within letters to the editor for Professor David Sander

    Resonant interaction of fast particles with Alfven waves in spherical tokamaks

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    The Spherical Tokamak (ST) concept has become one of the main avenues in magneticnuclear fusion research since STs successfully demonstrated plasma operationat [Beta] = 2P[mu]0=B2~1. Next step ST machines aiming at achieving burning plasmaconditions in high [Beta] plasmas are being planned, such as the Spherical TokamakPower Plant (STPP) and the Component Testing Facility (CTF). Instabilities offast particle-driven Alfven eigenmodes are often observed in present-day STs. Suchinstabilities, driven by fusion-born alpha particles as well as by fast ions producedwith auxiliary heating schemes, in the next step STs may pose a major problem asthese instabilities may affect confinement and losses of the fast ions.A theory of compressional Alfven eigenmodes (CAE) with frequencies above thedeuterium cyclotron frequency,[omega] > [omega]cD, is developed for plasma parameters of aSTPP, and modes in the ion-ion hybrid frequency range, [omega]cT < [omega] < [omega]cD, are alsoinvestigated in order to assess the potential of diagnosing the deuterium-tritium(D-T) ratio. For the 1-D character of a STPP equilibrium with [Beta]~1 , a `hollowcylinder toroidal plasma model is employed for studying CAEs with arbitrary valuesof the parallel wave-vector k[||] = k[.]B/|B|. The existence of weakly-damped CAEs,free of mode conversion, is shown to be associated with the `well in the magneticfield profile, B = B (R), that can exist at the magnetic axis.A significant part of this thesis focusses on the experimentally observed effectsof resonant wave-particle interaction between Alfven waves and fast particles in theMega Amp Spherical Tokamak (MAST) device at the Culham Laboratory, UK, andin the LArge Plasma Device (LAPD) in the University of California, Los-Angeles,USA. New robust experimental scenarios for exciting CAEs in the MAST spherical tokamak are developed, and interpretation of the observed CAEs in the frequencyrange [omega]cD/3 < [omega] < [omega]cD is given in the context of the 1-D ST model and the Dopplershifted cyclotron resonance. The e ciency of the Doppler resonance between coand counter directed fast ions and left and right hand polarised Alfven waves isfurther assessed experimentally on the LAPD device, with probe ions injected inthe presence of Alfv en waves launched by an external antenna.The developed theory of CAEs is then applied to a calculation of the linear kineticdrive of CAEs in the MAST experiments. A model representation of the fast iondistribution function, produced by neutral beam injection (NBI), is used by fittingto the TRANSP Monte-Carlo NBI modelling results. The main free energy sourcesassociated with temperature anisotropy and bump-on-tail are estimated analytically,and the CAE stability boundary is qualitatively assessed.In order to explain the experimentally observed difference between steady-stateand pulsating Alfvenic modes, the non-linear theory of fast particle driven modesnear marginal stability is extended to include dynamical friction (drag). For thebump-on-tail problem, the drag is shown to always give an explosive amplitudeevolution in contrast to diffusion in velocity space in the vicinity of the wave-particleresonance. This is then extended to the case of experimentally observed NBI-driventoroidal Alfven eigenmodes (TAEs) in the MAST machine. The experimentallyobserved differences between TAEs driven by fast ions produced with ion cyclotronresonance heating (ICRH) and NBI are then interpreted. The problem of dragdominated collisions for modes excited by fusion-born alpha particles in burningplasmas such as a STPP and ITER is underlined.Imperial Users onl

    Future wireless applications for a networked city: services for visitors and residents

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    Future wireless networks will offer near-ubiquitous high-bandwidth communications to mobile users. In addition, the accurate position of users will be known, either through network services or via additional sensing devices such as GPS. These characteristics of future mobile environments will enable the development of location-aware and, more generally, context-sensitive applications. In an attempt to explore the system, application, and user issues associated with the development and deployment of such applications, we began to develop the Lancaster GUIDE system in early 1997, finishing the first phase of the project in 1999. In its entirety, GUIDE comprises a citywide wireless network based on 802.11, a context-sensitive tour guide application with, crucially, significant content, and a set of supporting distributed systems services. Uniquely in the field, GUIDE has been evaluated using members of the general public, and we have gained significant experience in the design of usable context-sensitive applications. We focus on the applications and supporting infrastructure that will form part of GUIDE II, the successor to the GUIDE system. These developments are designed to expand GUIDE outside the tour guide domain, and to provide applications and services for residents of the city of Lancaster, offering a vision of the future mobile environments that will emerge once ubiquitous high-bandwidth coverage is available in most cities

    More Challenges to RF Democrats: Interview with Father Gleb Bakunin

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    Private Ordering and Orphan Works: Our Least Worst Hope?

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    The political capture of copyright law by industry groups has inadvertently led to orphan works problems arising in less organized industries, such as publishing. Google Book Search (GBS) is a prime example of how private ordering can circumvent legislative inefficiencies. Digital technologies such as GBS can open up a new business model for publishers and other content industries, centered around aggregated rights holdings. However, the economic inertia that private ordering represents may pose a threat to the knowledge-oriented goals of copyright law

    Measurement: everywhere and nowhere in secondary mathematics

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    School mathematics is commonly structured into number, algebra, geometry and statistics. This raises the issue of where to place ideas within the topic of measurement since some aspects of measurement (such as measuring length or area) have a geometrical component, while other aspects of measurement (such as time or money) are about number. Furthermore, when actual measures are unknown, relationships between measures can be expressed – and this is one of the roots of algebra. Additionally, probability can be thought of as a form of measure (of uncertainty) and the various measures of data variation, such as standard deviation, can also be viewed as a form of measurement. All these considerations mean that the placing of measurement in the mathematics curriculum can be problematic for curriculum designers and policy makers; and equally tricky for teachers to teach in the most effective way. Informed by a review of the research basis for teaching key ideas in secondary school mathematics, this paper argues that measurement is both everywhere and nowhere in secondary mathematics; that is, measurement occurs across the topics that comprise secondary school mathematics, but the ideas of measurement are so scattered that the teaching of measurement in secondary school mathematics may lack some focus that might store up problems for learners as they progress with mathematics

    Status and Prospects of Planetary Transit Searches: Hot Jupiters Galore

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    The first transiting extrasolar planet, orbiting HD209458, was a Doppler wobble planet before its transits were discovered with a 10 cm CCD camera. Wide-angle CCD cameras, by monitoring in parallel the light curves of tens of thousands of stars, should find hot Jupiter transits much faster than the Doppler wobble method. The discovery rate could easily rise by a factor 10. The sky holds perhaps 1000 hot Jupiters transiting stars brighter than V=13. These are bright enough for follow-up radial velocity studies to measure planet masses to go along with the radii from the transit light curves. I derive scaling laws for the discovery potential of ground-based transit searches, and use these to assess over two dozen planetary transit surveys currently underway. The main challenge lies in calibrating small systematic errors that limit the accuracy of CCD photometry at milli-magnitude levels. Promising transit candidates have been reported by several groups, and many more are sure to follow.Comment: To appear in: Scientific Frontiers in Research on Extrasolar Planets, 18-21 Jun 2002, Washington D.C., ASP Conference Series Vol ???, 2003, D.Deming and S.Seager, eds v2: update notes ogle-tr-56b discover