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Militello F.,EURATOM | Borgogno D.,Polytechnic University of Turin | Grasso D.,CNR Institute for Complex Systems | Marchetto C.,CNR Institute for Plasma Physics Piero Caldirola | Ottaviani M.,CEA Cadarache Center
Physics of Plasmas | Year: 2011

The linear stability of the tearing mode (TM) in a plasma column is investigated in the presence of viscosity and finite equilibrium current density gradients (i.e., asymmetries). It is shown that for low β, both effects are essential in order to properly describe the mode behaviour close to marginality. In particular, the theory introduces a critical threshold for the destabilization, such that the perturbation grows only if δ′ > δ′ cr. The value of δ′cr depends on the equilibrium configuration and on the plasma parameters. Most importantly, δ′cr can take negative values, thus allowing unstable tearing modes for δ′ < 0 (even in the absence of bootstrap current). © 2011 American Institute of Physics.

Del Sarto D.,CNRS Jean Lamour Institute | Marchetto C.,CNR Institute for Plasma Physics Piero Caldirola | Pegoraro F.,University of Pisa | Califano F.,University of Pisa
Plasma Physics and Controlled Fusion | Year: 2011

We provide numerical evidence of the role of finite Larmor radius effects in the nonlinear dynamics of magnetic field line reconnection in high-temperature, strong guide field plasmas in a slab configuration, in the large Δ′ regime. Both ion and electron temperature effects introduce internal energy variations related to mechanical compression terms in the energy balance, thus contributing to regularize the gradients of the ion density with respect to the cold regimes. For values of the Larmor radii that are not asymptotically small, the two temperature effects are no longer interchangeable, in contrast to what is expected from linear theory, and the differences are measurable in the numerical growth rates and in the nonlinear evolution of the density layers. We interpret such differences in terms of the change, due to ion temperature effects, of the Lagrangian advection of the 'plasma invariants' that are encountered in the cold-ion, warm-electron regime. The different roles of the ion and ion-sound Larmor radii in the reconnection dynamics near the X- and O-points are evidenced by means of a local quadratic expansion of the fields. © 2011 IOP Publishing Ltd.

Croci G.,CERN | Croci G.,CNR Institute for Plasma Physics Piero Caldirola | Alfonsi M.,CERN | Ropelewski L.,CERN | And 4 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2013

Neutron GEM-based detectors represent a new frontier of diagnostic devices in neutron-linked physics applications such as detectors for fusion experiments (Croci et al., 2012 [1]) and spallation sources (Murtas et al., 2012 [2]). Besides, detectors installed in HEP experiments (like LHC at CERN) are dip in a high flux neutron field. For example, the TOTEM T2 GEM telescope (Bagliesi et al., 2010 [3]) at LHC is currently installed very close to the beam pipe where a high intensity (>104ncm-2s-1) neutron background is present. In order to assess the capability (particularly related to discharge probability) of working in intense neutrons environment, a 10×10cm2 Triple GEM detector has been tested using a high flux (105ncm-2s-1) neutron beam. The neutron-induced discharge probability PDisch was measured to be 1.37×10 -7 at an effective gain G=5×104. In addition, the different types of neutron interactions within the detector were fully explained through a GEANT4 simulation. © 2013 Elsevier B.V.

Alfonsi M.,CERN | Croci G.,CERN | Croci G.,CNR Institute for Plasma Physics Piero Caldirola | Duarte Pinto S.,CERN | And 5 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2012

The charging up effect is well-known in detectors containing dielectric materials and it is due to electrons and ions liberated in an avalanche and collected on the dielectric surfaces. In particular in Gas Electron Multiplier (GEM) based detectors, charges can be captured by the Kapton that separates top and bottom electrodes. The collection of a substantial number of charges on the dielectric surfaces induces a modification of the field inside the GEM holes that implies important consequences on some fundamental parameters such as the electron transparency and the effective gain. The correct simulation of this effect opens new ways to the detailed study of the processes that happens in a GEM-based detector and gives the possibility to optimise the GEM geometry in order to avoid it. This paper compares results of the measurements and the simulations, with and without the introduction of the charging-up effect, of the GEM electron transparency in the case of a single GEM detector. The introduction of the charging up effect in the simulation resulted to be crucial in order to get the proper agreement with the measurements. The measurements and simulations of the GEM effective gain will be the subject of a future work. © 2011 Elsevier B.V. All rights reserved.

Nemanic V.,Jozef Stefan Institute | Zajec B.,Jozef Stefan Institute | Dellasega D.,Polytechnic of Milan | Dellasega D.,CNR Institute for Plasma Physics Piero Caldirola | And 2 more authors.
Journal of Nuclear Materials | Year: 2012

We report results on long-term hydrogen outgassing and permeation studies of structurally highly disordered tungsten films, deposited on 40 mm diameter highly permeable Eurofer substrates, using the Pulsed Laser Deposition technique. Hydrogen interaction with tungsten is becoming a highly relevant topic since tungsten was recognized as the most promising candidate for the first wall of future fusion reactors. Prediction of hydrogen isotopes migration and their abundance after plasma exposure is uncertain due to a great role played by structural disorder that is formed on the W surface. Vacancy sites are theoretically predicted to trap multiple H atoms exothermically, but their density and their potential influence on permeability has not been experimentally investigated yet. In our work, permeability of W films having different thicknesses (1 and 10 μm) was initially extremely low, and was gradually increasing over a several-day campaign. The final values at 400 °C, lying between P = 1.46 × 10 -15 mol H 2/(m s Pa 0.5) and P = 4.8 × 10 -15 mol H 2/(m s Pa 0.5), were substantially lower than those known for well ordered films. Surprisingly, the 10 μm thick W film initially contained a very high amount of hydrogen, ∼0.1 H/W, which was gradually releasing during the twenty-day campaign. © 2012 Elsevier B.V. All rights reserved.

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