Particle Acceleration in Pulsar Wind Nebulae: PIC modelling

We discuss the role of particle-in-cell (PIC) simulations in unveiling the origin of the emitting particles in PWNe. After describing the basics of the PIC technique, we summarize its implications for the quiescent and the flaring emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be emerging that, in addition to the standard scenario of particle acceleration via the Fermi process at the termination shock of the pulsar wind, magnetic reconnection in the wind, at the termination shock and in the Nebula plays a major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae" edited by D. Torres for Springer, based on the invited contributions to the workshop held in Sant Cugat (Barcelona), June 14-17, 201

Recent EUROfusion Achievements in Support of Computationally Demanding Multiscale Fusion Physics Simulations and Integrated Modeling

Integrated modeling (IM) of present experiments and future tokamak reactors requires the provision of computational resources and numerical tools capable of simulating multiscale spatial phenomena as well as fast transient events and relatively slow plasma evolution within a reasonably short computational time. Recent progress in the implementation of the new computational resources for fusion applications in Europe based on modern supercomputer technologies (supercomputer MARCONI-FUSION), in the optimization and speedup of the EU fusion-related first-principle codes, and in the development of a basis for physics codes/modules integration into a centrally maintained suite of IM tools achieved within the EUROfusion Consortium is presented. Physics phenomena that can now be reasonably modelled in various areas (core turbulence and magnetic reconnection, edge and scrape-off layer physics, radio-frequency heating and current drive, magnetohydrodynamic model, reflectometry simulations) following successful code optimizations and parallelization are briefly described. Development activities in support to IM are summarized. They include support to (1) the local deployment of the IM infrastructure and access to experimental data at various host sites, (2) the management of releases for sophisticated IM workflows involving a large number of components, and (3) the performance optimization of complex IM workflows.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014 to 2018 under grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission or ITER.Peer ReviewedPostprint (published version

Stable Perfectly Matched Layers for a Cold Plasma in a Strong Background Magnetic Field

International audienceThis work addresses the question of the construction of stable perfectly matched layers (PML) for a cold plasma in the infinitely large background magnetic field. We demonstrate that the traditional, Bérenger's perfectly matched layers are unstable when applied to this model, due to the presence of the backward propagating waves. To overcome this instability, we use a combination of two techniques presented in the article. First of all, we consider a simplified 2D model, which incorporates some of the difficulties of the 3D case, namely, the presence of the backward propagating waves. Based on the fact that for a fixed frequency either forward or backward propagating waves are present, we stabilize the PML with the help of a frequency-dependent correction. An extra difficulty of the 3D model compared to the 2D case is the presence of both forward and backward waves for a fixed frequency. To overcome this problem we construct a system of equations that consists of two independent systems, which are equivalent to the original model. The first of the systems behaves like the 2D plasma model, and hence the frequency-dependent correction is added to the PML for the stabilization. The second system resembles the Maxwell equations in vacuum, and hence a standard Bérenger's PML is stable for it. The systems are solved inside the perfectly matched layer, and coupled to the original Maxwell equations, which are solved in a physical domain, on a discrete level through an artificial layer. The numerical experiments confirm the stability of the new technique

The geodesic acoustic mode in strongly-shaped tight aspect ratio tokamaks

This thesis presents comparison between experimental measurements from the spherical tokamak MAST, two-fluid simulation data and theory of the Geodesic Acoustic Mode (GAM) in tight aspect ratio strongly shaped tokamak plasmas. The first identification of a strong ~10kHz mode detected in both potential and density fluctuations of the edge plasma in MAST using a reciprocating probe is given. The mode is radially localised, with outer limit ~ 2cm inside the separatrix, and is affected on application of resonant magnetic perturbations (RMP) generated by external coils. A shift in frequency with plasma rotation is found, and a suppression of the mode is observed above a certain threshold. Non-linear coupling to high wave number turbulence is evident, and an increase in power of turbulence fluctuations is seen after suppression. These observations are then interpreted in the context of known low frequency plasma modes present in the toroidal configuration. The supposition that the observed mode is a geodesic acoustic mode is considered and motivated by experimental observations and numerical simulations

Simulations of ion cyclotron emission from energetic ions in DIII-D tokamak plasmas

Ion cyclotron emission (ICE) was the first collective radiative instability, driven by confined fusion-born ions, that was observed from deuterium-tritium plasmas in both JET and TFTR. The excitation mechanism for ICE is the magnetoacoustic cyclotron instability (MCI). The diagnostic exploitation of ICE in future experiments relies on the understanding of the MCI. Recent advances in computational plasma physics have led to substantial progress in developing models to study instabilities driven by fusion-born ions in fusion plasmas. ICE is a potential diagnostic for confined alpha-particles in ITER, where measurements of ICE could yield information on energetic ion behaviour supplementing that obtainable from other diagnostics. Furthermore, it may be possible to use ICE to study fast ion redistribution and loss due to MHD activity in ITER. One way to study the MCI is by using particle-in-cell simulations, where the electron and ion dynamics are evolved using the Lorentz force and the electromagnetic fields are solved using the full set of Maxwell’s equations. In this thesis, highly resolved particle in cell simulations of edge and central ICE have been performed for JET and the DIII-D tokamak. The results are found to be in good agreement with the experiment and the theory of the magnetoacoustic cyclotron instability

Studies of magnetised and non-local transport in laser-plasma interactions

The application of magnetic fields in inertial fusion experiments has led to renewed interest in fully understanding magnetised transport in laser-plasma regimes. This motivated the development of a new laser magnetohydrodynamic code PARAMAGNET, written to support investigations into classical magnetised transport phenomena and laser propagation in a plasma. This code was used to simulate laser-underdense plasma interactions such as the pre-heat stage of magneto-inertial fusion. Alongside these simulations, this thesis will present analytic focusing and filamentation models derived from magnetohydrodynamics extended with classical magnetised transport coefficients. These results showed the focal length and filamentation growth length shortened with magnetisation, a result of the magnetisation of the thermal conductivity. Further investigation of the transport properties using the diffusion approximation kinetic code IMPACT showed significant deviation of the growth rate at intermediate values of magnetisation and non-locality, inexplicable using fluid models. The kinetic code result motivated exploring the influence of the high-order anisotropies of the distribution function (in terms of spherical harmonics), ignored in conventional approximations. By using a recursive matrix inverse method, corrections to the transport coefficients including all orders of the electron distribution expansion were found. Analysis of the conductivity, resistivity and thermoelectric coefficients showed deviation by up to 50% from the classical form at intermediate magnetisation and nonlocality. The diffusive approximation of the IMPACT simulations was insufficient to capture the transport behaviour present in the theoretical high order calculation. Modern inertial fusion experiments work in regimes that are non-local and susceptible to significant focusing exacerbated by magnetisation. The resulting filamentation has detrimental implications to laser absorption and the modified non-local transport behaviour is a possible source of error in simulations. The complex interplay between non-locality and magnetisation in transport suggests using more terms of the spherical harmonic expansion in closures of plasma equations. Particular consideration is given to the implications to inertial fusion experiments. Together these results suggest the necessity of including non-local magnetised transport in the modelling of high-energy-density laser plasma experiments.Open Acces