72 research outputs found
Three dimensional magnetohydrodynamics of fusion plasmas
No full textThe primary aim of the research on nuclear fusion is to obtain a new energy source to help satisfying a growing and sustainable consumption. This objective has to be reached through scientific research, both from the physics point of view and through the demonstration of the technological feasibility of a nuclear fusion reactor. The option on which the major efforts of the international community are focused is to obtain controlled nuclear fusion using a magnetic field to confine a plasma formed by
deuterium and tritium, in a vacuum chamber of toroidal shape. The most promising magnetic configuration is the so called tokamak configuration. The scientific community aims at addressing the remaining problems connected with physics performing the experiment ITER (International Thermonuclear Experimental Reactor) and to verify the technological feasibility of a nuclear fusion reactor with the DEMO experiment.
An important part of the scientific efforts is addressed to the study of configurations alternative to the tokamak, like the stellarator and the reversed-field pinch (RFP). These configurations achieve three dimensional helical states: in the RFP a global helical state is obtained spontaneously, due to the presence of a strong current flowing in the plasma, while currents flowing in external helically shaped coils generate a global helical state in the stellarator. Helical states can be obtained also in the tokamak configuration, for instance due to the presence of external magnetic field perturbations. The research activity of my PhD focuses on the study of the 3D nonlinear magnetohydrodynamics model applied to the numerical study of the RFP and tokamak helical configurations.
The main aim of my research is the characterization, under three different aspects later described, of the three dimensional helical states. These states are presently believed to provide possible scenarios for reducing dangerous MHD activity for both RFP (magnetic chaos transport reduction) and tokamak (sawtooth mitigation, disruption avoidance).
The research activity included the development and the exploitation of advanced numerical tools to deal with the numerical solution of the 3D nonlinear MHD model, while the interaction with the experimental environment provided the opportunity to develop tools for model-experiment comparison (validation) and benchmarking of numerical tools (verification).
The results obtained during my PhD provide a further step towards a predictive capability of the employed modelling tools. In fact, the boundary conditions are proved to be a key ingredient in bringing the comparison of MHD simulations with the experiment at a quantitative level. Moreover it recently inspired a successful and promising experimental activity in RFX-mod, the biggest RFP experiment in the world, located in Padova.
My PhD research activity and results can be divided into three main areas. The first is the dynamical simulation of a magnetically confined plasma through numerical solution of the 3D nonlinear visco-resistive MHD model. The second area of research consists in the topological study of the magnetic field configurations obtained from MHD simulations. The third area is the study of transport due to magnetic stochasticity in both tokamak and RFP states, with data coming from MHD simulations, gyrokinetics simulations and experimental results.
The first area of research deals with the simulation of the dynamical properties of a magnetically confined plasma, performed using the 3D nonlinear MHD codes SPECYL and PIXIE3D. The most important achievement is represented by the level of agreement between MHD simulation and experimental dynamics of the RFP, a degree of agreement obtained in simulations where, for the first time, a helical boundary condition is applied. It is also demonstrated that by imposing a finite helical radial magnetic field at the edge it is possible to induce a global helical regime with the chosen helicity. As for the tokamak configuration the study of helical boundary conditions shows that they can favour a steady helical equilibrium, thus mitigating the sawtooth dynamics typically detrimental for the confinement. This area of research leads to a unifying vision for the RFP and the tokamak, as the use
of helical boundary condition for the magnetic field seems to allow the easier establishment of a helical equilibrium in both configurations, with interesting properties for the configurations.
The second area of research is centred on the topological study of the magnetic configurations obtained from the MHD simulations of the RFP. The separatrix expulsion of the dominant helical mode has been studied analyzing the magnetic field topology with the field line tracing code N EMATO. Two so called paradigmatic cases, characterized by a simplified MHD dynamics, have been analyzed. In the first one it was shown that the dominant mode separatrix expulsion can reduce the level of magnetic field lines stochasticity remarkably, in the second case an “exotic” (before these studies) dynamics was considered, i.e. the development of a helical equilibrium from a non-resonant mode. These results confirmed older studies that placed separatrix expulsion in direct connection with helical RFP states obtained in RFX-mod, which develop internal transport barriers observed as electronic temperature steep gradients. Furthermore it showed that the helical equilibrium based on a non-resonant mode can result in particularly strong magnetic order. The favourable properties found led to the proposal to
experimentally drive QSH states built upon non-resonant MHD modes in the RFX-mod experiment: these states were successfully produced in the experiment, and the study of thermal properties is presently ongoing. Topological studies on more realistic cases coming from MHD simulations that show a quantitative agreements with the standard operation of the RFX-mod experiment are also tackled in this thesis. The results obtained underline the importance of the spectrum of secondary perturbations to the helical equilibrium.
The third area of research focuses on the consequences of transport produced by the presence of magnetic stochasticity. Two specific cases relevant for the RFP and the tokamak are considered: the magnetic chaos produced by microtearing activity at the electron internal transport barrier in the RFP, and the case of edge magnetic stochasticity due to the action of edge helical magnetic perturbations in the tokamak. The tools to study transport were developed and used to calculate the energy diffusion coefficient and other meaningful quantities. Such tools are now available for further and more general
applications.
On a numerical ground two important activities were performed during the PhD. The parallelization of the field line tracing code NEMATO, during one month mobility at Oak Ridge National Laboratory, was fundamental for the speeding up of the research activity. The numerical verification of NEMATO and ORBIT was also performed. The verification gave a positive result, showing a satisfactory agreement, both qualitative and quantitative, on the features of the magnetic field topology in the RFP configuration
Formulation and numerical benchmark of improved magneto fluid-dynamics boundary conditions for 3D nonlinear MHD code SPECYL
No full text3D nonlinear MHD code SpeCyl is a spectral tool, operating in zero-β approximation and
in cylindrical geometry to advance in time the magnetic field and the plasma flow. The traditional
formulation of its boundary conditions would see the plasma as if in direct contact with an
ideally conducting shell. However, a recent reformulation introduced the presence of a rigid
thin shell and a tuneable-width vacuum region between the plasma and the outer conductor.
With suitable parameters choice, the resistive shell can be made transparent to the magnetic
field, so to simulate a free-interface between plasma and vacuum. Numerical benchmark performed
in this regime against current-driven linear MHD instabilities found good agreement
concerning the internal modes, yet quantitatively poor for external modes of MHD, motivating
a reformulation of fluid boundary conditions, as well.
We present here the resulting set of boundary conditions, which combines the chance for finite
flow at plasma edge with the already present thin shell-like modelling of magnetic plasmavacuum
matching conditions. We also illustrate numerical benchmarks, mainly against some
well known results of the theory of linear MHD instabilities. Finally, we include a mutualbenchmark
between our formulation of SpeCyl and another MHD nonlinear simulations code,
Pixie3D, with analogous physical assumptions at plasma edge. This extends the nonlinear
benchmark, already performed between the two codes in the past
Formulation and numerical benchmark of improved magneto-fluid boundary conditions for 3D nonlinear MHD code SPECYL
No full text3D nonlinear MHD code SPECYL boundary conditions BCs have been increasingly made more realistic, from traditional SpeCyl.1 (ideal conductor facing plasma) to modelling plasma vacuum interface as a resistive thin shell.
Linear benchmark against ideal MHD ( η_plasma , ν_plasma→0) suggested the need for finite plasma edge Vr. We present the resulting BCs, here dubbed SpeCyl.2.
Verification of SpeCyl.2 against another code (Pixie3D), enforcing analogous physical assumptions in BCs, is also shown. This amends some unphysical idealities of SpeCyl.1 and completes what already done in 2010, in a formerly published mutual benchmark of the two codes.
Linear benchmark against the theory of ideal MHD instabilities is presented, featuring a pseudo-vacuum low-density layer at plasma edge, in analogy to what is done in other codes
Implementation and benchmark of improved boundary conditions for 3D nonlinear MHD code SpeCyl
No full textAn improvement of the boundary conditions scheme of the 3D nonlinear MHD numerical code SpeCyl is presented. Boundary conditions have been shown to play a key role in the helical self-organization both in Reversed Field Pinch and tokamak plasmas.
Two different sets of boundary conditions have been extensively tested against ubiquitous relaxation phenomena induced by plasma current in toroidal devices: ideal kinks and tearing modes. The role of wall position and resistivity on linear perturbations profiles and their exponential growth rates was tested, motivating the need for a reformulation of fluid boundary conditions as well.
Preliminary results of such new boundary conditions are also presented, along with a summary of the relevant theoretical framework underlying linear MHD instabilities
Non-linear visco-resistive MHD modelling of reversed-field pinch fusion plasmas: viscosity coefficient studies
No full textKinematic viscosity estimates in reversed-field pinch fusion plasmas
No full textThis paper concerns the kinematic viscosity in reversed-field pinch fusion plasmas, including both the study of numerical magneto-hydrodynamics (MHD) simulations and the analysis of RFX-mod experimental data. In the first part, we study the role of non-uniform time-constant radial viscosity profiles in 3D non-linear visco-resistive MHD simulations. The new profiles induce a moderate damp (for the velocity field) and a correspondent enhancement (for the magnetic field) of the spectral components resonating in the regions where the viscosity is higher. In the second part, we evaluate the kinematic viscosity coefficient on a wide database of RFX-mod shots according to the transport theories of Braginskii (considering parallel, perpendicular and gyro viscosity coefficients), considering the action on viscosity of ITG modes (ion temperature gradient) and according to the transport theory of Finn. We then exploit the comparison with the visco-resistive MHD simulations (where the visco-resist..
Detection of magnetic barriers in a chaotic domain: First application of finite time Lyapunov exponent method to a magnetic confinement configuration
No full textMagnetic field lines embedded in a plasma confinement system are often characterized by
a chaotic motion. This weakens the confinement properties of any magnetic configuration.
However, even in case of chaotic domains, magnetic barriers can emerge and limit the field
line motion itself. In the context of the numerical simulation of a Reversed-Field Pinch
configuration a new magnetic topology analysis, borrowed from previous fluid dynamic
studies, is discussed. This methodology relies on the behavior of the Finite Time Lyapunov
Exponent (FTLE) associated with the magnetic field. By referring to a previous work in which
the magnetic field is given in terms of analytical function the FTLE field shows the presence of ridges, special gradient lines normal to the
direction of minimum curvature, forming magnetic barriers. These ridges can be recognized
as Lagrangian Coherent Structures (LCSs) for the system, actually opposing the penetration
of magnetic field lines across them. In this article a more general numerical scheme for the
detection of the LCSs has been adopted that allows analysis of realistic cases in which the
magnetic fields are numerically known on a discrete mesh. After a validation test performed
on the analytical case, a first application to a numerical magnetohydrodynamics simulation
of the RFP, characterized by a broad chaotic region, has been performed. A strong magnetic
barrier has been observed that effectively limits the field lines motion inside the chaotic sea
Weak Chaos in the Plasma of a Fusion Device
No full textIt is known that the magnetic field which is used to confine hot plasmas in nuclear fusion devices can be described in Hamiltonian form. The time variable which appears in Hamilton equations is replaced by an angular variable around the torus. Small field errors applied on purpose, or naturally occurring instabilities of the plasma, often break good KAM surfaces and determine the presence of stochastic motion of magnetic field lines. This topic has practical implications, since particles follow magnetic field lines, and, if the former are chaotic, particles are rapidly lost towards the vacuum vessel of the device, and can damage the plasma facing components. From a theory point of view, various models have been applied to describe this experimental situation: the most common one is the analysis of the connection length Lc,w of field lines to the wall, which can be directly compared to camera images of the damage caused by energetic particles hitting the wall of the device. More sophisticated tools have been applied recently, including the Lagrangian Coherent Structures (LCS) and the Poincaré Recurrence Time, which map the presence of more or less chaotic regions inside the plasma
Structures in Reversed Field Pinch Magnetic Self-Organization, Insights and Prospects from 3D Nonlinear MHD
No full textSince the early 90ties, 3D nonlinear MHD studies have been developing a fundamental framework for the understanding of the Reversed Field Pinch (RFP) helical self-organization. Indeed, this process began to manifest itself clearly only in late 90ties in high current experiments. Within 3D MHD, it was identified as a route to helical equilibrium, governed by viscoresistive dimensionless parameters, going beyond the well-known magnetic Taylor’s magnetic relaxation theory application to the RFP. In this presentation, we will discuss the impact of self-organization on the transport properties, and the key role of boundary conditions (seed edge Magnetic Perturbations) in favoring the transition to the quasi-helical regimes. In particular, the transition leads to a significant reduction of the edge chain of 3D m~0 magnetic islands in favor of a macroscopic order characterized by a core helical structure. Seed edge Magnetic Perturbations -for the first time in advanced simulations- allowed for realistic description of the experimental quasi-helical regime, with remnant sawtoothing cycle, typical of medium current RFP discharges. In addition, it has been shown the capability of forcing various pitches of the helical plasma shape (with different character of the safety factor profile), leading to the discovery of new RFP helical regimes, whose transport properties are presently under investigation, and will be further experimentally studied in the RFX-mod2 experiment in Italy, which will start operation in 2024
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