1,721,482 research outputs found
Variable gamma-ray emission from the crab nebula: Short flares and long "waves"
No full textGamma-ray emission from the Crab Nebula has been recently shown to be unsteady. In this paper, we study the flux and spectral variability of the Crab above 100 MeV on different timescales ranging from days to weeks. In addition to the four main intense and day-long flares detected by AGILE and Fermi-LAT between 2007 September and 2012 September, we find evidence for week-long and less intense episodes of enhanced gamma-ray emission that we call "waves." Statistically significant "waves" show timescales of 1-2 weeks, and can occur by themselves or in association with shorter flares. We present a refined flux and spectral analysis of the 2007 September-October gamma-ray enhancement episode detected by AGILE that shows both "wave" and flaring behavior. We extend our analysis to the publicly available Fermi-LAT data set and show that several additional "wave" episodes can be identified. We discuss the spectral properties of the 2007 September "wave"/flare event and show that the physical properties of the "waves" are intermediate between steady and flaring states. Plasma instabilities inducing "waves" appear to involve spatial distances l ∼ 1016 cm and enhanced magnetic fields B ∼ (0.5-1) mG. Day-long flares are characterized by smaller distances and larger local magnetic fields. Typically, the deduced total energy associated with the "wave" phenomenon (Ew ∼ 1042 erg, where Ew is the kinetic energy of the emitting particles) is comparable with that associated to the flares, and can reach a few percent of the total available pulsar spin-down energy. Most likely, flares and waves are the product of the same class of plasma instabilities that we show acting on different timescales and radiation intensities. © 2013. The American Astronomical Society. All rights reserved
Feasibility study of O-X coupling for overdense plasma heating through O-X-B mode conversion in FTU
No full textThe subject of this paper is the feasibility study of electron Bernstein wave heating (EBWH) at 140 GHz through the O-X-B double mode conversion in a plasma of the Frascati Tokamak Upgrade (FTU) device. The high-density profiles of the FTU plasma are shown and the results of the simulations of O-X coupling carried out with a dedicated ray-tracing code are presented. The simulations are aimed at finding the optimal launching conditions and to calculating the maximum coupling efficiency achievable with the FTU parameters. Also the effects predicted by a recent theoretical model available in the literature, which accounts for a description of the inhomogeneities of the toroidal plasma more realistically than a slab model, are evaluated. The parameters of the launched beam considered in the paper are those permitted with the new electron cyclotron resonance heating front steering launcher of FTU. © 2013 IAEA, Vienna
Overview of the FTU results
No full textThis is the accepted manuscript of the following final paper: "Pucella, G., Alessi, E.,…, Calabrò, G., et al., Overview of the FTU results, (2019) Nuclear Fusion, 59 (11), art. no. 112015, https://doi.org/10.1088/1741-4326/ab19ef
Calibration of AGILE-GRID with in-flight data and Monte Carlo simulations
No full textContext. AGILE is a γ-ray astrophysics mission which has been in orbit since 23 April 2007 and continues to operate reliably. The γ-ray detector, AGILE-GRID, has observed Galactic and extragalactic sources, many of which were collected in the first AGILE Catalog. Aims. We present the calibration of the AGILE-GRID using in-flight data and Monte Carlo simulations, producing instrument response functions (IRFs) for the effective area (A eff), energy dispersion probability (EDP), and point spread function (PSF), each as a function of incident direction in instrument coordinates and energy. Methods. We performed Monte Carlo simulations at different γ-ray energies and incident angles, including background rejection filters and Kalman filter-based γ-ray reconstruction. Long integrations of in-flight observations of the Vela, Crab and Geminga sources in broad and narrow energy bands were used to validate and improve the accuracy of the instrument response functions. Results. The weighted average PSFs as a function of spectra correspond well to the data for all sources and energy bands. Conclusions. Changes in the interpolation of the PSF from Monte Carlo data and in the procedure for construction of the energy-weighted effective areas have improved the correspondence between predicted and observed fluxes and spectra of celestial calibration sources, reducing false positives and obviating the need for post-hoc energy-dependent scaling factors. The new IRFs have been publicly available from the AGILE Science Data Center since November 25, 2011, while the changes in the analysis software will be distributed in an upcoming release. © ESO, 2013
A unified model of density limit in fusion plasmas
No full textIn this work we identify by analytical and numerical means the conditions for the existence of a magnetic and thermal equilibrium of a cylindrical plasma, in the presence of Ohmic and/or additional power sources, heat conduction and radiation losses by light impurities. The boundary defining the solutions' space having realistic temperature profile with small edge value takes mathematically the form of a density limit (DL). Compared to previous similar analyses the present work benefits from dealing with a more accurate set of equations. This refinement is elementary, but decisive, since it discloses a tenuous dependence of the DL on the thermal transport for configurations with an applied electric field. Thanks to this property, the DL scaling law is recovered almost identical for two largely different devices such as the ohmic tokamak and the reversed field pinch. In particular, they have in common a Greenwald scaling, linearly depending on the plasma current, quantitatively consistent with experimental results. In the tokamak case the DL dependence on any additional heating approximately follows a 0.5 power law, which is compatible with L-mode experiments. For a purely externally heated configuration, taken as a cylindrical approximation of the stellarator, the DL dependence on transport is found stronger. By adopting suitable transport models, DL takes on a Sudo-like form, in fair agreement with LHD experiments. Overall, the model provides a good zeroth-order quantitative description of the DL, applicable to widely different configurations. © 2017 IAEA, Vienna
Calibration of AGILE-GRID with On-ground Data and Monte Carlo Simulations
No full textAGILE is a mission of the Italian Space Agency (ASI) Scientific Program dedicated to γ-ray astrophysics, and has operated in a low Earth orbit since 2007 April 23. It is designed to be a very light and compact instrument, capable of simultaneously detecting and imaging photons in the 18-60 keV X-ray energy band and in the 30 MeV-50 GeV γ-ray energy range with a good angular resolution (≈1 @ 1 GeV). The core of the instrument is the Silicon Tracker, supplemented by a CsI calorimeter and an AntiCoincidence system, which form the Gamma Ray Imaging Detector (GRID). Before launch, the GRID needed on-ground calibration with a tagged γ-ray beam to estimate its performance and validate the Monte Carlo simulation. The GRID was calibrated using a tagged γ-ray beam with energy up to 500 MeV at the Beam Test Facilities at the INFN Laboratori Nazionali di Frascati. These data are used to validate a GEANT 3-based simulation by comparing the data and the Monte Carlo simulation by measuring the angular and energy resolutions. The GRID angular and energy resolutions obtained using the beam agree well with the Monte Carlo simulation. Therefore the simulation, can be used to simulate the same performance on-flight with high reliability. © 2018. The American Astronomical Society. All rights reserved
Impact of divertor geometry on H-mode confinement in the JET metallic wall
No full textRecent experiments with the ITER-like wall have demonstrated that changes in divertor strike point position are correlated with strong modification of the global energy confinement. The impact on energy confinement is observable both on the pedestal confinement and core normalised gradients. The corner configuration shows an increased core density gradient length and ion pressure indicating a better ion confinement. The study of neutral re-circulation indicates the neutral pressure in the main chamber varies inversely with the energy confinement and a correlation between the pedestal total pressure and the neutral pressure in the main chamber can be established. It does not appear that charge exchange losses nor momentum losses could explain this effect, but it may be that changes in edge electric potential are playing a role at the plasma edge. This study emphasizes the importance of the scrape-off layer (SOL) conditions on the pedestal and core confinement. © 2017 CEA
Analysis of plasma termination in the JET hybrid scenario
No full textThis paper analyses the final phase of hybrid scenario discharges at JET, the reduction of auxiliary heating towards finally the Ohmic phase. The here considered Ohmic phase is mostly still in the current flattop but may also be in the current ramp down. For this purpose a database is created of 54 parameters in 7 phases distributed in time of the discharge. It is found that the occurrence of a locked mode is in most cases preceded by a radiation peaking after the main heating phase either in a low power phase and/or in the Ohmic phase. To gain insight on the importance of different parameters in this process a correlation analysis to the radiation peaking in the Ohmic phase is done. The first finding is that the further away in time the analysed phases are the less the correlation is. This means in the end that a good termination scenario might also be able to terminate unhealthy plasmas safely. The second finding is that remaining impurities in the plasma after reducing the heating power in the termination phase are the most important reason for generating a locked mode which can lead to a disruption. © EURATOM 2018
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