F. Porcelli

Politecnico di Torino | polito · DISAT - Department of Applied Science and Technology

Recent progress on the understanding of axisymmetric pertubations in tokamak plasmas is presented, with particular attention on Vertical Displacement Oscillatory Modes (VDOM) that can be driven unstable by their resonance with fast ion orbits, and on the impact of divertor X-points on the stability of vertical displacements.

Vertical displacement normal modes in shaped tokamak plasmas are studied analytically, based on the reduced ideal-magnetohydrodynamic model. With the help of quadratic forms, and using the appropriate eigenfunction for vertical displacements with toroidal mode number $n=0$ and dominant elliptical-angle mode number $m=1$ , a dispersion relation is d...

An analytic derivation of the relevant dispersion relation for vertical displacements in shaped tokamak plasmas is presented, valid for arbitrary values of the ellipticity parameter. The theory is developed within the framework of the reduced ideal-MHD model. A nearby, perfectly conducting wall can provide passive feedback stabilization of vertical...

A new type of fast particle instability involving axisymmetric modes in magnetic fusion tokamak plasmas is presented. The relevant dispersion relation involves three roots. One corresponds to a vertical plasma displacement that, in the absence of active feedback stabilization, grows on the wall resistivity time scale. The other two, oscillating clo...

Axisymmetric modes in shaped tokamak plasmas are normally associated with vertical displacement events. However, not enough attention has been given to the fact that these modes can be resonant in two different ways. Firstly, for a plasma bounded by a divertor separatrix, a generic n=0 ideal-MHD perturbation, ξ, is singular at the divertor X-point(...

Recent results within the framework of the collaborative project The Complete Geophysical Survey of the Valley of the Kings (VOK) (Luxor, Egypt) are reported in this article. In October 2018, a team of geomatics and geophysics researchers coordinated by the Polytechnic University of Turin worked side by side in the VOK. Topographic measurements in...

The existence of hidden chambers and corridors adjacent to Tutankhamun's tomb (code name KV62) has been long debated. In 2015 it was suggested that these chambers may host the as yet undiscovered burial of Nefertiti. In order to test this hypothesis, two Ground Penetrating Radar (GPR) surveys, conducted in 2015 and 2016 from inside KV62, were carri...

Theoretical and experimental considerations suggest that axisymmetric perturbations that are resonant at the X-point(s) of a magnetic divertor separatrix may play a role in the understanding of Edge Localized Modes in tokamak experiments and their active control via so-called vertical kicks. With this motivation in mind, the first step in the devel...

Electrical resistivity tomography (ERT) of the area surrounding Tutankhamun’s tomb (KV62) in the Valley of the Kings (Luxor, Egypt) reveals the presence of two anomalies located a few meters from Tutankhamun’s funerary chamber. The strategy for ERT data acquisition and the adopted methods for data analysis are discussed in detail in this article, t...

This report illustrates the data acquisition work carried out by the joint Egyptian-Italian collaboration within the framework of the project titled The Third KV62 Radar Scan.

Scholars have long discussed the introduction and spread of iron metallurgy in different civilizations. The sporadic use of iron has been reported in the Eastern Mediterranean area from the late Neolithic period to the Bronze Age. Despite the rare existence of smelted iron, it is generally assumed that early iron objects were produced from meteorit...

Abstract Since the last IAEA Conference JET has been in operation for one year with a programmatic focus on the qualification of ITER operating scenarios, the consolidation of ITER design choices and preparation for plasma operation with the ITER-like wall presently being installed in JET. Good progress has been achieved, including stationary ELMy...

The reasons that the electrical field can be concentrated on radius are considered. It is shown that due to ion accumulation in magnetic islands, large core plasma density, and strong anomalous transport in peripheral region the radial distribution of the electrical field can be narrow. At small ion accumulation the double layer type of radial elec...

We estimate the electron angular velocity shear ∂rωθo ,w hichcan be formed by plasma heating near the low- order rational surface with a poloidal chain of magnetic islands. We suppose that the plasma is heated sufficiently that its electrons start to miss the magnetic islands during their radial collisional shift and movement along the toroidal sur...

An analytic expression for the stability threshold of linear tearing modes is derived. The magnetized plasma is described in terms of a standard viscoresistive magnetohydrodynamic model. The analytic derivation requires an extension of the standard layer equation that represents an approximation of the full model in the vicinity of the reconnecting...

A simple formula for predicting the width of a saturated island, formed as a consequence of tearing perturbation of linear force-free fields in cylindrical geometry, is derived. The formula makes it possible to calculate the saturated island width in terms of the values of parameters characterizing the initial force-free equilibrium and can be appl...

The saturation of Neoclassical Tearing Mode islands in a periodic slab configuration is investigated. Several theoretical models, all based on a generalization of Rutherford's procedure, that aim at reducing the complete system to a single equation of the magnetic island width, are compared against numerical simulations. When the effects of the boo...

The linear stability of a class of force-free equilibria in cylindrical geometry is investigated. The class consists of cylindrically symmetric force-free equilibria for which the ratio μ between the parallel current density and the magnetic field is a step function of the radius. It is suggested that plasmas in reversed field pinches could be roug...

An innovative argument is presented in order to explain the formation of Quasi-Single-Helicity (QSH) states in Reversed-Field-Pinches (RFPs) as result of a tearing perturbation of a force-free equilibrium. In particular it is shown that force-free equilibria with a piecewise constant ratio between the current density and the magnetic field can be t...

Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document (1999 Nucl. Fusion 39 2137–2664), is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER basel...

The saturation of the tearing mode in a plasma column is investigated in the framework of the resistive magnetohydrodynamics approximation. In particular, a perturbative procedure is adopted to evaluate the structure of the magnetic island in three relevant physical conditions, depending on the model for the evolution of the resistivity, which may...

A study of saturated magnetic island equilibria on the basis of the resistive magneto-hydro-dynamic model is presented. A bifurcation in the sequence of equilibria is found as the ratio of the width of the current layer in the initial (non-reconnected) configuration over the island periodicity length reaches a critical threshold. Below this thresho...

Collisionless magnetic field line reconnection exhibits important similarities under widely different physical conditions, ranging from the magnetohydrodynamic regime, of interest for space and high temperature magnetically confined plasmas, to the electron-magnetohydrodynamic regime, of interest for the interaction of ultraintense, ultrashort lase...

Simulations of JET and TFTR discharges with the BALDUR integrated modeling code are used to test a sawtooth model that consists of sawtooth triggering mechanisms [Porcelli et al., Plasma Phys. Contolled Fusion 38, 2163 (1996)] together with a modified version of the Kadomtsev sawtooth reconnection model [ Kadomtsev, Sov. J. Plasma Phys. 1, 389 (197...

Recent efforts on the theory of magnetic reconnection in collisionless regimes are reviewed. The differences between dissipative and collisionless reconnection are discussed. The results of simulations obtained with a new, initial value, finite volume code are presented. The initial equilibrium is a tanh x for the magnetic field component along y,...

We examine the transport of momentum across an island using the reduced magnetohydrodynamic (RMHD) model. We find that the Reynolds stress gives rise to a momentum source proportional to the resistivity. As a result of the frozen-in property the transport of momentum can be described by a one-dimensional equation, even for islands with finite aspec...

New, rigorous results for the tearing island saturation problem are presented. These results are valid for the realistic case where the magnetic island structure is non-symmetric about the reconnection surface and the electron temperature, on which the electrical resistivity depends, is evolved self-consistently with the island growth.

The nonlinear behavior of reconnecting modes in three spatial dimensions (3D) is investigated, on the basis of a collisionless fluid model in slab geometry, assuming a strong constant guide field in one direction. Unstable modes in the so-called large {delta}{sup '} regime are considered. Single helicity modes, i.e., modes with the same orientation...

We consider the problem of reconnection in weakly collisional plasmas in the strong guide field limit. In this regime the standard resistive Ohm's law is modified to include electron compressibility and electron inertia effects. Despite the increased complexity of the governing equations, we show that analytic steady state solutions, like those dis...

Magnetic reconnection in collisionless regime is investigated in two- and three-dimensional configurations. In the framework of two-dimensional configurations some recent results concerning the difference between dissipative and collisionless reconnection are reviewed. The results of numerical simulation of three-dimensional configurations are pres...

We review some recent results that have been obtained in the investigation of collisionless reconnection in two dimensional magnetic configurations with a strong guide field in regimes of interest for laboratory plasmas. First we adopt a two-fluid dissipationless plasma model where the plasma evolution is described by the advection of two Lagrangia...

Recent theoretical work on magnetic reconnection in hot plasma confinement devices is reviewed. The presentation highlights the common aspects of reconnection phenomena, and current research trends are emphasized. Progress in understanding the dynamics of slowly evolving modes of the tearing family, based on advanced analytic techniques and numeric...

A simple scenario for two-dimensional reconnection is reexamined in the context of a fluid drift model which treats the pressure and parallel ion velocity independently. The rapid nonlinear establishment of an arbitrarily thin current sheet is unaffected for expected values of the plasma β of a few percent. Eventually the sound wave dynamics should...

We present an analytical and numerical study of forced magnetic reconnection about a magnetic X-point. The physical model is obtained by linearization of the two-dimensional, two-fluid MHD equations with respect to a current free equilibrium. The model is then further reduced (by using a suitable frequency ordering )leading, in the limit of no guid...

The effect of the Hall term in the process of forced magnetic reconnection for an X point magnetic field configuration, both in the collisionless limit and in the case of weak resistive dissipation, is investigated in this paper. In the case of no guide field, a two-fluid (four fields) linearized model is derived and solved as a forced initial-valu...

A set of reduced equations governing nonlinear, two-dimensional, two-fluid, collisionless magnetic reconnection with arbitrary guide field is derived. These equations represent an improvement on the existing nonlinear reduced equations generally used to investigate collisionless reconnection, which are only valid in the large guide-field limit. The...

Nonlinear drift-tearing magnetic islands are studied numerically with a four-field model that includes the parallel ion dynamics. In certain regions of the parameter space, multiple solutions are found. In particular, linearly stable drift-tearing perturbations can grow to finite size magnetic islands when the amplitude of the initial perturbation...

Small-scale quasi-coherent oscillations of the x-ray emissivity and poloidal magnetic field perturbations are identified in high density plasmas in the T-10 tokamak with the use of a toroidal view x-ray imaging system, in-vessel hard x-ray detector and fast magnetic probes. Small-scale x-ray oscillations are associated with forward bremsstrahlung e...

A model for the observed improvement in confinement when a stationary MARFE develops is proposed. It is based on the fact that field-aligned flows are naturally created in association to a MARFE, which, when coupled to the field line curvature, give rise to plasma spin-up. The resulting radially-sheared poloidal rotation about the layer affected by...

A simple nonlinear treatment of the reduced magnetohydrodynamics equations, that describes the saturation of small width magnetic island, is presented. This procedure is an extension of Rutherford's theory. It is found that the tearing mode saturates at a finite amplitude, w(s), which is proportional to the linear stability parameter, Delta('). The...

The stability of rotating magnetic islands in homogenous plasmas is studied taking into account the ion polarization current and ion sound effects. Coupling to the ion sound dynamics occurs due to the modification of the ion plasma density that is no longer a function of the magnetic flux surfaces when the ion polarization drift is included. It is...

Critical stability issues involving magnetic reconnection that are likely to influence the successful operation of burning plasma experiments are addressed. In particular, we discuss: (1) the stabilization of drift-tearing modes in weakly collisional regimes, with particular reference to nonlinear stability, including neo-classical effects; (2) the...

The main magnetohydrodynamic (MHD) activities affecting the Frascati Tokamak Upgrade (FTU) high-field plasmas with limiter configuration are sawtooth relaxations and tearing modes. The period of sawtooth relaxations can be increased in FTU both by electron heating and by pellet particle deposition near the sawtooth inversion radius; both methods le...

Injection of frozen deuterium pellets from the magnetic low field side into discharges of the Frascati Tokamak Upgrade with high current and broad temperature profile gives rise to strong central fuelling, whereas pellet ablation calculations predict that pellets should be almost completely sublimated outside the q = 1 magnetic surface. Enhanced ce...

It is shown that local features of the equilibrium current density profile, i.e., its gradient and curvature evaluated at the resonant magnetic surface, can modify considerably the linear behavior of tearing modes. These features can become particularly important in cylindrical geometry, at moderate values of the electrical resistivity and of the t...

Injection of frozen deuterium pellets from the magnetic low field side into discharges of the Frascati Tokamak Upgrade with high current and broad temperature profile gives rise to strong central fuelling, whereas pellet ablation calculations predict that pellets should be almost completely sublimated outside the q = 1 magnetic surface. Enhanced ce...

The linear stability of high-toroidal-number drift-ballooning modes in tokamaks is investigated with a model that includes resistive and viscous dissipation, and assumes the mode frequency to be comparable to both the sound and diamagnetic frequencies. The coupled effect of ion drift waves and electron drift-acoustic waves is shown to be important,...

The magnetohydrodynamic stability of a plasma column with an elliptical cross section to axisymmetric, rigid plasma displacements is analyzed. The considered plasma equilibrium, characterized by the presence of a double-null magnetic separatrix, is found to be unstable. An application of the well-known Energy Principle [I. B. Bernstein, E. A. Friem...

Current ramping to achieve reversed shear confinement enhancement and peaked density profiles are important for achieving ignition conditions in the high-field tokamak IGNITOR [Coppi et al., Phys. Scri. 45, 112 (1992)]. Previous transport simulations used either fixed density profiles or obtained flat density profiles leading to a conclusion that a...

One of the recurring problems in magnetic reconnection is the identification of the appropriate generalized Ohm's law. In weakly collisional plasmas with a strong magnetic guide field component, a fluid model may be adopted, where electron inertia and the electron pressure gradient play important roles. In the absence of collisions, electron inerti...

The particle and energy transport properties of the high fusion performance JET pulses that were obtained before and during the first tritium experiments are discussed. The particle diffusion coefficient of tritium is determined by monitoring the decay of a small quantity of injected tritium in a deuterium background plasma. A good simulation of th...

When an ion cyclotron resonance heating (ICRH) antenna array is phased ( Delta phi not=0 or pi ), the excited asymmetric k// spectrum can drive non-inductive currents by interaction of fast waves both with electrons (transit time magnetic pumping (e-TTMP) and Landau damping (e-LD)) and with ions at minority (fundamental) or harmonic cyclotron reson...

Measurements of the toroidal magnetic field inside the JET tokamak plasma by means of the motional Stark effect are reported. In the vicinity of the resonance layer ion cyclotron resonance heating (ICRH) generates a population of energetic ions. For the first time, the diamagnetic change in the toroidal magnetic field due to these ICRH fast ions in...

We investigate collisionless magnetic reconnection on the basis of a Hamiltonian 3D model, derived in Ref. [1]. This model describes drift-Alfvén perturbations in a plasma immersed in a strong, uniform, externally imposed magnetic field, where spatial variations along the field lines are assumed to be small compared to variations normal to it. This...

The MHD effects observed in the hot ion H modes in the pre-divertor configuration of JET, including those generated in the preliminary tritium experiment, are described. Some observations were found to be similar to those in high beta regimes while others are new and appear to be pertinent to high performance discharges only. The high performance p...

The temporal evolution of the sawtooth crash in ASDEX Upgrade Ohmic and ECRH heated discharges is analysed using a heuristic reconnection model to simulate measured ECE signals. The method turns out to be well suited to detect islands of width above 0.1-0.2 times the mixing radius; for smaller island widths, we cannot distinguish between the resist...

In the limit where the electron drift-wave frequency exceeds the electron-ion collision frequency, drift-tearing modes are found to grow with a linear growth rate independent of resistivity and proportional to the product of the electron inertial skin depth and the ion sound Larmor radius. The stabilization of these modes in collisionless and semi-...

The mechanism for fast-ion-induced toroidal plasma rotation in the absence of external momentum input reported in Perkins et al (Perkins F.W. et al 2001 Phys. Plasmas 8 2181) is examined in detail, at the level of single particle orbits and their collisional evolution, with the aim at clarifying the physical processes involved. This mechanism may b...

A two-dimensional, two-fluid model is used to investigate driven magnetic reconnection in collisionless or semicollisional plasmas. The reconnection is driven by externally induced plasma flows in a background magnetic configuration that has a hyperbolic null component in the reconnection plane and a strong component, the so-called guide component,...

An independent assessment of the physics of Ignitor is presented. Ignitor is a physics demonstration experiment with the main goal of achieving thermonuclear ignition, defined as the regime where fusion alpha heating compensates for all forms of energy losses. Simulations show that a pulse of alpha particle power of up to 10–20 MW is produced for a...

Ion viscosity and particle diffusivity effects on the linear stability and mode structure of drift-tearing modes are investigated. If only resistivity and diamagnetic effects are taken into account, the drift-tearing mode can exist only as a propagating wave packet, which is resolved numerically as an initial value problem. Ion viscosity, particle...

Previous ohmic tokamak experiments show that improved confinement regimes called IOC require peaked density profiles. The performance of IGNITOR has been shown to depend critically on the density profile (W. Horton, F. Porcelli, P. Zhu, A. Aydemir, Y. Kishimoto, and T. Tajima, to appear in Nucl. Fusion). One problem of the previous IGNITOR simulati...

The article presents experimental and theoretical investigations on the dynamics of sawteeth and neoclassical tearing modes in tokamak plasmas. The main findings of these investigations are: (1) The experimental validation, on the basis of FTU and TCV data, of a model for the prediction of the sawtooth period and amplitude; (2) the importance of...

The nonlinear evolution of a Hamiltonian magnetic field line reconnection in a two-dimensional fluid plasma leads to a macroscopic equilibrium with a finite-size island and fine-scale spatial structures. The latter arise from the phase mixing of the Lagrangian invariant fields. This equilibrium is the analog of the Bernstein-Greene-Kruskal equilibr...

Different types of central relaxation oscillations are observed in the presence of ECH depending on the location of the deposited power. In the TCV tokamak, normal sawteeth, i.e. triangular sawteeth similar to ohmic sawteeth, and saturated sawteeth are observed with central ECH power deposition, while giant sawteeth and 'humpback' oscillations occu...

The problem of the collisional evolution of fast-ion guiding centre orbits in magnetically confined plasmas is analysed in great detail. Regimes where the radial excursion of a particle orbit from a given magnetic surface is large compared with its minimum distance from the magnetic axis are specifically addressed. These non-standard orbits give ri...

An interpretation of non-standard, central MHD events in the TCV tokamak during localized ECRH is presented. It is shown that the non-standard behaviour is a consequence of specific features in the electron temperature profile produced when ECRH power is deposited close to the q = 1 surface and by the advection and mixing of electron thermal ene...

On TCV (Tokamak à Configuration Variable, R=0.88m, a

Diamagnetic effects are known to affect importantly the linear and nonlinear stability of tearing modes. An outstanding question is the persistence of these effects for magnetic island widths exceeding the linear reconnection layer width. If the answer is affirmative, a new term should be included in the nonlinear evolution equation for neoclassica...

The stabilisation of plasma macroscopic instabilities by high energy particles is shown to be a consequence of the conservation of the third adiabatic invariant, i.e. the magnetic flux encircled by the precessional drift orbits. The author discusses the stabilisation mechanism, the physical processes that determine the boundaries of the stable regi...

The nonlinear evolution of collisionless magnetic field line reconnection is investigated numerically in plasma regimes where the effects of the electron and ion temperatures are important. These effects modify the structure of the current and vorticity layers that are formed during the onset of the reconnection instability. The results of investig...

Magnetic reconnection in two dimensional (2D), collisionless, non-dissipative regimes is investigated analytically and numerically by means of a finite difference code in the nonlinear regime where the island size becomes macroscopic. The cross-shaped structure of the reconnection region, originally reported by Cafaro et al (1998 Phys. Rev. Lett. 8...

Physics knowledge (theory and experiment) in energetic particles relevant to design of a reactor scale tokamak is reviewed, and projections for ITER are provided in this Chapter of the ITER Physics Basis. The review includes single particle effects such as classical alpha particle heating and toroidal field ripple loss, as well as collective instab...