1,721,470 research outputs found
Studies of runaway electrons via Cherenkov effect in tokamaks
No full textThe paper concerns measurements of runaway electrons (REs) which are generated during discharges in tokamaks. The control of REs is an important task in experimental studies within the ITER-physics program. The NCBJ team proposed to study REs by means of Cherenkov-type detectors several years ago. The Cherenkov radiation, induced by REs in appropriate radiators, makes it possible to identify fast electron beams and to determine their spatial- and temporal-characteristics. The results of recent experimental studies of REs, performed in two tokamaks - COMPASS in Prague and FTU in Frascati, are summarized and discussed in this paper. Examples of the electron-induced signals, as recorded at different experimental conditions and scenarios, are presented. Measurements performed with a three-channel Cherenkov-probe in COMPASS showed that the first fast electron peaks can be observed already during the current ramp-up phase. A strong dependence of RE-signals on the radial position of the Cherenkov probe was observed. The most distinct electron peaks were recorded during the plasma disruption. The Cherenkov signals confirmed the appearance of post-disruptive RE beams in circular-plasma discharges with massive Ar-puffing. During experiments at FTU a clear correlation between the Cherenkov detector signals and the rotation of magnetic islands was identified. © Published under licence by IOP Publishing Ltd
Segmented superluminescent diode with linear power-current characteristics and adjustable radiation pattern
No full textThis study presents the modelling and experimental characterisation of 980 nm segmented-contact superluminescent diodes designed to produce linear power-current characteristics for applications requiring high output powers and readily reproducible operational characteristics, with adjustable beam width, radiation pattern and spectral width. The maximum measured output power was 160 mW at 4 A pulsed driving current, resulting in 2% wall-plug efficiency per pulse, with ripple-free broad-band output. Coupling into multi- and single-mode optical fibres was achieved with efficiencies of, respectively, 60 and 17%. © The Institution of Engineering and Technology 2013
Analysis of runaway electron expulsion during tokamak instabilities detected by a single-channel Cherenkov probe in FTU
No full textThe expulsion of runaway electrons (REs) during different types of tokamak instabilities is analysed by means of a Cherenkov probe inserted into the scrape-off layer of the FTU tokamak. One such type of instability, the well-known tearing mode, is involved in disruptive plasma termination events, during which the risk of RE avalanche multiplication is highest. The second type, known as anomalous Doppler instability, influences RE dynamics by enhancing pitch angle scattering. Three scenarios are analysed here, characterised by different RE generation rates and mechanisms. The main conclusions are drawn from correlations between the Cherenkov probe and other diagnostics. In particular, the Cherenkov probe permits the detection of fast electron expulsion with a high level of detail, presenting peaks with 100% signal contrast during tearing mode growth and rotation, and sub-peak structures reflecting the interplay between the magnetic island formed by the tearing mode, RE diffusion during island rotation and the geometry of obstacles in the vessel. Correlations between the Cherenkov signal, hard x-ray emission and electron cyclotron emission reveal the impulsive development of the anomalous Doppler instability with instability rise time in the microsecond scale resolved by the high time-resolution of the Cherenkov probe
Multi-Objective Modular Strategic Planning Framework for Low Altitude Missions Within the Urban Air Mobility Ecosystem
No full textThis paper introduces a multi-metric multi-constraint strategic path planning framework applicable to unstructured urban airspace. The planner is based on a modular and scalable approach to handle several information sources and aspects characterizing urban flight scenarios, such as risk and weather maps, landing site locations, navigation requirements, and mobile and fixed obstacle characteristics. This information is coupled with dynamic constraints and UAV specifications to derive a flyable and safe path connecting a start position and a destination. Strategies for data gathering and synthesis, used to keep a reduced computational burden, are described along with the path planner algorithm. The latter consists in three steps specifically developed to handle both static and time-varying information. A multi-objective cost function with variable weighting coefficients has been implemented so that the most relevant factors for the considered applications can be selected in an adaptive fashion. The performance of the developed algorithms is tested by investigating the planner behavior when changing its inputs as well as the cost function weighting coefficients, demonstrating the ability of the planner in returning an efficient and safe trajectory
Multiple UAVs trajectory generation and waypoint assignment in urban environment based on DOP maps
No full textThis paper tackles strategic path planning for a multi-UAV routing problem in low altitude urban environment, where GNSS coverage challenges typically affect navigation performance and thus autonomous flight capabilities. These issues are addressed with a multi-step strategy that includes automated definition of GNSS-challenging volumes based on a georeferenced three-dimensional environment model, derivation of candidate obstacle-free paths between waypoints, waypoint assignment and definition of time-tagged trajectories for all UAVs. In all the steps, attention is paid to limiting the computational burden, thus ensuring applicability in real mission scenarios. Going beyond binary logics which consider GNSS-challenging volumes as obstacles, a discrete number of dilution of precision levels is considered, leading to different three-dimensional maps of the environment defined as “DOP layers”. Then, the basic idea to ensure that the designed trajectories are “flyable” with given positioning accuracy requirements is to take navigation performance into account at waypoint assignment level, using propagated covariance as a metric. The approach thus combines “navigation aware” planning with multi-vehicle task assignment, generating a solution that depends on the sensors embarked onboard the UAVs, and can be naturally extended to account for multi-sensor-based navigation architectures and ground-infrastructure support. The algorithm is tested in simulations based on a real world scenario, considering a wide combination of input parameters in terms of positioning error threshold, maximum UAV velocity, number of UAVs, navigation sensors performance, and mission epoch
Improving Navigation in GNSS-challenging Environments: Multi-UAS Cooperation and Generalized Dilution of Precision
Full text linkThis paper presents an approach to tackle navigation challenges for Unmanned Aircraft Systems flying under non nominal GNSS coverage. The concept used to improve navigation performance in these environments consists in using one or more cooperative platforms and relative sensing measurements (based on vision and/or ranging) to the navigation aid. The paper details the cooperative navigation filter which can exploit multiple cooperative platforms and multiple relative measurements, while also using partial GNSS information. The achievable navigation accuracy can be predicted using the concept of "generalized dilution of precision", which derives from applying the idea of dilution of precision to the mathematical structure of the cooperative navigation filter. Values and trends of generalized dilution of precision are discussed as a function of the relative geometry in common GNSS-challenging scenarios. Finally, navigation performance is assessed based on simulations and on multi-drone flight tests
Multi-drone cooperation to improve navigation integrity in low altitude urban environments
No full textThis paper uses multi-UAV cooperation to enhance the positioning performance of a swarm when GNSS measurements are affected by faults. Only pseudorange observables are accounted for and a centralized extended Kalman filter, which can deal with both cooperative and non cooperative measurements, has been developed. The filter is complemented with a fault detection and elimination strategy based on Mahalanobis distance, which removes pseudoranges that are deemed faulty. Contribution of relative sensing sources (camera or ranging devices) and formation geometry to cooperative GNSS integrity mitigation is investigated, and the concept of cooperative slope is introduced as a metric to define the best cooperative formation geometry. Results demonstrate, by simulating spoofing and multipath phenomena, that cooperation enhances the navigation performance of the UAVs in a multi vehicle formation only if the formation geometry is correctly selected with the aid of the cooperative slope metric
Navigation-aware Path Planning for Multiple UAVs in Urban Environment
No full textThis paper tackles the problem of multi-VAV routing in urban environment. Due to the heterogenous GNSS coverage characterizing these environments, path selection and waypoint assignment should account for the navigation performance of the UAVs, so as to guarantee flyable trajectories within an autonomous flight framework. Therefore, this paper accounts for navigation capability and onboard sensor performance while assigning waypoints to the VAVs. A centralized strategic flight planner is developed, which adapts to navigation performance levels and can also handle fleets of heterogeneous UAVs. The proposed path planning algorithm is tested in simulations using a 3D real-world scenario. The impact on path planning efficiency of different onboard sensors performance and of eventual ground infrastructure support is discussed
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