CNR Institute of Ionized Gas


CNR Institute of Ionized Gas

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Paccagnella R.,Consorzio RFX | Paccagnella R.,CNR Institute of Ionized Gas
Physics of Plasmas | Year: 2016

In this paper, a relaxation theory for plasmas where a single dominant mode is present [Bhattacharjee et al., Phys. Rev. Lett. 45, 347 (1980)], is revisited. The solutions of a related eigenvalue problem are numerically calculated and discussed. Although these solutions can reproduce well, the magnetic fields measured in experiments, there is no way within the theory to determine the dominant mode, whose pitch is a free parameter in the model. To find the preferred helical perturbation, a procedure is proposed that minimizes the "distance" of the relaxed state from a state which is constructed as a two region generalization of the Taylor's relaxation model [Taylor, Phys. Rev. Lett. 33, 1139 (1974); Rev. Mod. Phys. 58, 751 (1986)] and that allows current discontinuities. It is found that this comparison is able to predict the observed scaling with the aspect ratio and reversal parameter for the dominant mode in the Single Helical states. The aspect ratio scaling alone is discussed in a previous paper [Paccagnella, Nucl. Fusion 56, 046010 (2016)] in terms of the efficient response of a toroidal shell to specific modes (leaving a sign undetermined), showing that the ideal wall boundary condition, a key ingredient in relaxation theories, is particularly well matched for them. Therefore, the present paper altogether [Paccagnella, Nucl. Fusion 56, 046010 (2016)] can give a new and satisfactory explanation of some robust and reproducible experimental facts observed in the Single Helical Reversed Field Pinch plasmas and never explained before. © 2016 EURATOM.

Strauss H.,HRS Fusion | Sugiyama L.,Massachusetts Institute of Technology | Paccagnella R.,CNR Institute of Ionized Gas | Breslau J.,Princeton Plasma Physics Laboratory | Jardin S.,Princeton Plasma Physics Laboratory
Nuclear Fusion | Year: 2014

Toroidal rotation can be produced by disruptions, as observed in several experiments. There is a concern that rotating asymmetric forces during an ITER disruption might resonate with the blanket and other structures surrounding the plasma. Here it is shown, both computationally using the M3D code, and analytically, that toroidal rotation is produced by magnetohydrodynamic turbulence. In particular, rotation is produced during an asymmetric vertical displacement event (AVDE) disruption. Toroidal and poloidal rotation are also produced during edge localized modes (ELMs), and may be consistent with a scaling law found for intrinsic toroidal rotation in H-mode tokamaks. © 2014 IAEA, Vienna.

Strauss H.,HRS Fusion | Paccagnella R.,CNR Institute of Ionized Gas | Breslau J.,Princeton Plasma Physics Laboratory | Sugiyama L.,Massachusetts Institute of Technology | Jardin S.,Princeton Plasma Physics Laboratory
Nuclear Fusion | Year: 2013

A critical issue for ITER is to evaluate the forces produced on the surrounding conducting structures during plasma disruptions. We calculate the non-axisymmetric 'sideways' wall force Fx, produced in disruptions. Simulations were carried out of disruptions produced by destabilization of n = 1 modes by a vertical displacement event (VDE). The force depends strongly on γτwall, where γ is the mode growth rate and τwall is the wall penetration time, and is largest for γτwall = constant, which depends on initial conditions. Simulations of disruptions caused by a model of massive gas injection were also performed. It was found that the wall force increases approximately offset linearly with the displacement from the magnetic axis produced by a VDE. These results are also obtained with an analytical model. Disruptions are accompanied by toroidal variation of the plasma current Iφ. This is caused by toroidal variation of the halo current, as verified computationally and analytically. © 2013 IAEA, Vienna.

Paccagnella R.,Consorzio RFX | Paccagnella R.,CNR Institute of Ionized Gas
Nuclear Fusion | Year: 2016

In this paper a new idea to help to predict the aspect ratio scaling law for the toroidal mode number of the so called Single Helicity states in reversed field pinch (RFP) plasmas is presented. In particular it is shown that the pure electromagnetic response of a toroidal shell is 'optimal' when currents with specific pitches are excited on it and that this response depends on the shell aspect ratio. The pure electromagnetic passive toroidal shell response model, cannot however, be useful in predicting the handedness of the current which is related, instead, to the plasma magnetic field profiles and their on-axis stability properties. © 2016 IAEA, Vienna.

Strauss H.R.,HRS Fusion | Paccagnella R.,CNR Institute of Ionized Gas | Breslau J.,Princeton Plasma Physics Laboratory
Physics of Plasmas | Year: 2010

Nonlinear simulations with the M3D code [W. Park, Phys. Plasmas 6, 1796 (1999)] are performed of disruptions produced by large scale magnetohydrodynamic instabilities. The toroidally asymmetric wall forces produced during a disruption are calculated in an ITER [T. Hender, Nucl. Fusion 47, S128 (2007)] model. The disruption is produced by a vertical displacement event and a kink mode. Expressions are derived for the wall force, including the sideways force, using a thin conducting wall model. The scaling of wall force with γ τw is obtained, where γ is the kink growth rate and τw is the wall penetration time. The largest force occurs with γ τw ≈1. A theory is developed of the wall force produced by kink modes. The theory is in qualitative agreement with the simulations and Joint European Torus [V. Riccardo, Nucl. Fusion 49, 055012 (2009)] experiments. In particular, the theory and simulations give dependence of the sideways on γ τw, correlation of sideways force with sideways plasma displacement, and correlation of toroidally varying plasma current with toroidally varying vertical displacement. © 2010 American Institute of Physics.

Dalla Palma M.,CNR Institute of Ionized Gas
Fusion Engineering and Design | Year: 2016

The integrity assessment of structures subjected to cyclic loading must be verified with regard to cyclic type damage including time-independent fatigue and progressive deformation or ratcheting.Cyclic damage is verified simulating the material elastic-plastic loop and looking at the accumulated net plastic strain during each cycle at all points of the structure subjected to the complete time history of loadings.This work deals with the development of a numerical model producing the Chaboche hardening parameters starting from stress-strain data produced by testing of materials. Then, the total plastic strain can be simulated using the Chaboche inelastic constitutive model requested for finite element analyses. This is particularly demanding for pressure vessels, pressurised piping, boilers, and mechanical components of nuclear installations made of stainless steels.A design optimisation by iterative analyses is developed to approach the stress-strain test data with the Chaboche model. The parameters treated as design variables are the Chaboche parameters and the objective function to be minimised is a combination of the deviations from test data. The optimiser calls a macroinstruction simulating cyclic loading of a sample for different material temperatures.The numerical model can be used to produce hardening parameters of materials for inelastic finite element verifications of structures with complex joints like elbows subjected to a combination of steady sustained and cyclic loads. © 2016 Elsevier B.V.

Paccagnella R.,CNR Institute of Ionized Gas
Physics of Plasmas | Year: 2014

This paper presents a model for an ohmically heated plasma in which a feedback exists between thermal conduction and transport, on one side, and the magneto-hydro-dynamical stability of the system, on the other side. In presence of a reconnection threshold for the magnetic field, a variety of periodical or quasi periodical oscillations for the physical quantities describing the system are evidenced. The model is employed to interpret the observed quasi periodical oscillations of electron temperature and perturbed magnetic field around the so called "Single Helical" state in the reversed field pinch, but its relevance for other periodical phenomena observed in magnetic confinement systems, especially in tokamaks, is suggested. © 2014 Euratom.

Bellan P.M.,Applied Materials | Paccagnella R.,CNR Institute of Ionized Gas
Physics of Plasmas | Year: 2015

The value of the safety factor on the magnetic axis of a finite-beta spheromak is shown to be a function of beta in contrast to what was used in Bellan, Phys. Plasmas 9, 3050 (2002); this dependence on beta substantially reduces the gradient of the safety factor compared to the previous calculation. The method for generating finite-beta spheromak equilibria is extended to generate equilibria describing toroidal magnetic "bubbles," where the hydrodynamic pressure on the magnetic axis is less than on the toroid surface. This "anti-confinement" configuration can be considered an equilibrium with an inverted beta profile and is relevant to interplanetary magnetic clouds as these clouds have lower hydrodynamic pressure in their interior than on their surface. © 2015 AIP Publishing LLC.

Dalla Palma M.,CNR Institute of Ionized Gas | Pomaro N.,CNR Institute of Ionized Gas | Trevisan L.,CNR Institute of Ionized Gas
Fusion Engineering and Design | Year: 2011

The SPIDER test facility is under construction in Padova to verify the ion beam characteristics and to test the beam source operation under conditions consistent with the ITER neutral beam requirements. Thermal measurements will be performed on the beam source by installing thermocouples that will be used for different purposes: (a) diagnostic, such as for the reconstruction of the thermal map on the component surface for the evaluation of the thermal load distribution, plasma generation and beam extraction uniformity, (b) operational for the recognition of different experiment conditions such as the determination of the beginning of plasma extraction and acceleration related to caesium monolayer deposition, and (c) protective, for the verification of the components thermal reliability under the allowed maximum temperatures. The thermal measurements will be made with N type thermocouples based on mineral insulated cables 0.8 mm outer diameter, with grounded (not insulated) junctions. This specific thermocouple type guarantees minimal rise time and also ensures that the electric potential of the thermal sensor is always the same as that of the component, thus avoiding dangerous over-voltages in particular during grid-to-grid breakdowns. Rationale of the thermocouples design and layout is presented in this work. The paper also describes the thermocouple fixing systems that have been developed in detail and tested with prototypes in order to guarantee the thermal contact between the sensing junction and the component material. The signal isolation and conditioning systems have been designed considering the different voltages of the components with respect to ground and the response time of thermal sensors. Furthermore, voltage measurements integrated with thermal measurements will be acquired in order to develop a robust measurement system by correlating signals obtained from different transducers. © 2011 Consorzio RFX Associazione Euratom ENEA sulla Fusione. Published by Elsevier B.V. All rights reserved.

Paccagnella R.,CNR Institute of Ionized Gas
Physics of Plasmas | Year: 2013

In this work, the magneto-hydro-dynamic stability of pressure driven modes in the reversed field pinch has been analyzed. It is shown that at low and intermediate β's, i.e., typically for values below 20-25%, the tearing parity is dominant, while only at very high β, well above the achieved experimental values, at least part of the modes are converted to ideal interchange instabilities. Before their transition to ideal instabilities, according to their Lundquist number scaling, they can be classified as resistive-g modes. © 2013 EURATOM.

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