Benchmark of multi-phase method for the computation of fast ion distributions in a tokamak plasma in the presence of low-amplitude resonant MHD activity

The transport of fast ions in a beam-driven JT-60U tokamak plasma subject to resonant magnetohydrodynamic (MHD) mode activity is simulated using the so-called multi-phase method, where 4 ms intervals of classical Monte-Carlo simulations (without MHD) are interlaced with 1 ms intervals of hybrid simulations (with MHD). The multi-phase simulation results are compared to results obtained with continuous hybrid simulations, which were recently validated against experimental data (Bierwage et al., 2017). It is shown that the multi-phase method, in spite of causing significant overshoots in the MHD fluctuation amplitudes, accurately reproduces the frequencies and positions of the dominant resonant modes, as well as the spatial profile and velocity distribution of the fast ions, while consuming only a fraction of the computation time required by the continuous hybrid simulation. The present paper is limited to low-amplitude fluctuations consisting of a few long-wavelength modes that interact only weakly with each other. The success of this benchmark study paves the way for applying the multi-phase method to the simulation of Abrupt Large-amplitude Events (ALE), which were seen in the same JT-60U experiments but at larger time intervals. Possible implications for the construction of reduced models for fast ion transport are discussed

Velocity-space resolved measurements of fast-ion losses due to magnetohydrodynamic instabilities in the ASDEX Upgrade tokamak.

The confinement of suprathermal ions in magnetically confined fusion plasmas is essential to ensure a good fusion performance. Auxiliary heating systems - and fusion reactions themselves - create fast-ion populations, which must be confined for long enough time to transfer their energy to the bulk of the plasma via Coulomb collisions. A good confinement of the fast-ions is needed to ensure a good plasma heating and current drive. Furthermore, if fast-ions are lost to the walls of the machine in a sufficiently intense and localized way, irreversible damage to plasma facing components can be provoked. Therefore, a deep understanding of the mechanisms leading to fast-ion transport and eventual losses is of paramount importance. The need to develop control tools to avoid these losses is now becoming a priority in the roadmap to future burning plasma experiments. In this sense, scintillator based fast-ion loss detectors (FILD) have been proven to be a powerful diagnostic to study the interaction between fast-ions and magnetohydrodynamic (MHD) instabilities, contributing to unravel the physics underlying the transport mechanisms. In this thesis the study of fast-ion losses in the presence of various MHD instabilities in the ASDEX Upgrade tokamak is presented. A comprehensive description of scintillator based FILDs response is given here for the first time, with a special focus on its velocity-space resolution. As any other instrument in physics, the resolution of the system is finite, in this case due to the size of the detector pinhole and the gyrophase distribution of the measured ions. The detector response is described in terms of a simple model based on a weight function formalism. The model allows to calculate synthetic FILD signals given a velocity-space distribution of fast-ions reaching the detector pinhole. This enables a direct comparison between simulations and experimental measurements, taking into account the response of the instrument. Velocity-space tomography techniques have been implemented, which allow to obtain the undistorted velocity-space distribution of fast-ions reaching the detector pinhole. The tool improves the velocity-space resolution of FILD measurements, which can potentially reveal additional details in the velocity-space dynamics of fast-ion losses. These improvements have been applied to the study of different MHD induced fast-ion losses. The first velocity-space resolved absolute measurement of fast-ion losses in the presence of a tearing mode in the ASDEX Upgrade tokamak is presented. An estimate of the different loss channels in absolute terms is given. These measurements, supported by simulations of fast-ion losses including the modelling of ICRF power deposition, suggest that MHDinduced fast-ion losses are responsible for the anomalously large heat load measured by the FILD detector, which is then damaged irreversibly. This case represents a perfect example of the potential consequences derived from a bad confinement of the fast-ion population. The velocity-space dynamics of fast-ion losses induced by edge localized modes (ELMs) are investigated. It is observed that, in low collisionallity discharges, a fastion population with energies well above the main neutral beam injection (NBI) - dubbed high-energy feature - is measured. The high-energy feature is correlated with the occurrence of ELMs. The pitch-angle structure of the high-energy feature is observed to change with the edge safety factor and the NBI source, which is found to be related with the topology of the orbits. The high-energy feature is also observed in mitigated ELM regimes, while not seen in ELM suppressed regimes. This observation is interpreted as the acceleration of beam ions during the ELM crash, when magnetic reconnection is believed to take place. A resonant interaction between the beam-ions and the parallel electric fields emerging during the ELM is proposed as a possible acceleration mechanism, and is observed to qualitatively agree with the main experimental results. The observation motivates a kinetic description of fast-ions in ELM models. Additionally, the finding might also be of interest to the astrophysics community, where acceleration of charged particles in plasmas is ubiquitous, in particular in solar flares, which show similarities with ELMs in tokamaks.Premio Extraordinario de Doctorado U

Sensitivity study for N-NB-driven modes in JT-60U: boundary, diffusion, gyroaverage, compressibility

The sensitivity of the growth and nonlinear evolution of fast-ion-driven modes is examined with respect to the choice of particle boundary conditions, diffusion coefficients, fast ion gyroradii and bulk compressibility. The primary purpose of this work is to justify the choice of parameters to be used in the self-consistent long-time simulations of fast ion dynamics using global MHD-kinetic hybrid codes that include fast ion sources and collisions. The present study is conducted for a scenario based on the N-NB-driven JT-60U shot E039672, which is subject to abrupt large events (ALE). We use realistic geometry, a realistic fast ion distribution, and focus on experimentally observed harmonics with low toroidal mode numbers n  =  1, 2, 3. The use of realistic boundary conditions and finite Larmor radii for the fast ions is shown to be essential. The usual values μ0η＝υ＝χ～10-6υΑ０R0 used for resistivity, viscosity and thermal diffusivity, and Τ=5/3used for the specific heat ratio (controlling the effect of compressibility) are shown to be reasonable choices. Our method for performing the parameter scans around the threshold for the onset of convective amplification is proposed as a strategy for nonlinear benchmark studies

Full-orbit studies of wave-particle interaction on the Mega Ampere Spherical Tokamak

Energetic particles with super-Alfv\'enic speeds could potentially drive Alfv\'enic instabilities in a magnetically confined plasma. The driven waves can influence the fast particle distribution function as energetic particles are redistributed or lost to the vessel wall leading to a reduction in energetic particle confinement and heating efficiency. This thesis investigates the interaction between particles and waves via full orbit numerical simulations. The work presented herein takes steps towards the development of a capability to assess whether future reactor scenarios will be susceptible to these adverse effects or not. A full orbit particle tracking code has been developed to calculate particle trajectories and more importantly to compute particle orbital frequencies as they are followed in the simulation. Based on the wave-particle resonance condition, resonant particles are identified using this code for realistic tokamak geometries. Experimental observations of fast-ion driven waves on the MAST tokamak are presented. Magnetic perturbations in the kilo-Hertz range are detected by a set of high resolution Mirnov coils during the neutral beam injection heating phase where the mode frequency is observed to chirp downwards over the course of a magneto-hydrodynamics (MHD) burst. A decrease in fast-ion deuterium alpha signals is found to be correlated with the electromagnetic bursts indicating fast ion redistribution during the MHD activity. Simulation results suggest that the increase in plasma pressure is disproportional to the increase in NBI heating power in the presence of MHD modes. The effect of instabilities on energetic particle behaviour has been analysed by calculating resonance maps and resonant particle orbits. Full orbit calculations show that the chirping frequency broadens the wave-particle resonance region which can result in enhanced particle transport. Preliminary attempts have been made to evaluate fast particle transport induced by chirping modes using the non-linear full orbit \texttt{HALO} code. The chirping behaviour of the mode frequency is simulated by an ad-hoc function similar to experimental measurement. Calculations are performed for a simple cylindrical tokamak geometry and a mocked-up alpha particle distribution. An $n=6$ toroidal Alfv\'en eigenmode (TAE) is found numerically for this equilibrium. The results of the simulations show that fast particles are transported outwards from the plasma centre when chirping modes are present while no significant particle transport is seen when the mode frequency is constant. The level of transport is affected by either mode amplitude or chirping rate. These results suggest that the inclusion of a chirping effect is necessary to study particle redistribution in the presence of fast-ion modes when considering plasma scenarios in the future

Self-consistent interaction of fast particles and ICRF waves in 3D applications of fusion plasma devices

Tokamaks and stellarators are the most promising reactor concepts using the magnetic confinement to contain the plasma fuel. Reactors capable of sustaining deuterium-tritium (D-T) fusion reactions requires the confinement of a very high temperature plasma (above 100 millions kelvin). In addition to external heating methods, the slowing down of alpha particles (helium-4 nuclei) born from D-T fusion reactions on the background plasma represents a significant source of plasma heating. The good confinement of fast particles is therefore one of the most important aspect of magnetic fusion devices. Furthermore, long plasma operation in future fusion reactors requires the control of inherent plasma instabilities. These instabilities are particularly dangerous in tokamaks because of the large plasma current necessary to establish the confining magnetic field. In this thesis we use the numerical code package SCENIC to study the application of Ion Cyclotron Range of Frequency (ICRF) waves to tokamak and stellarator devices. This numerical tool was built to self-consistently solve, in three dimensional configurations, the plasma magnetohydrodynamic (MHD) equilibrium, the ICRF wave propagation and the resonant ion distribution function. SCENIC is used to interpret how the sawtooth instability can be controlled in tokamaks by appropriate application of ICRF waves. This control method was successfully tested in the JET tokamak and it is foreseen to be applied in the future ITER tokamak. Such plasma degrading instabilities are not, however, expected in stellarators because they operate with no plasma current. The recently started stellarator Wendelstein 7-X (W7-X) must however prove experimentally that fast particles particles can be confined in an optimised quasi-isodynamic magnetic configuration. An efficient auxiliary source of fast ions is required in W7-X since it is not designed to procude alpha particles via D-T fusion reactions. In this thesis, we address the possibility of generating a significant fast ion population with ICRF waves in W7-X. SCENIC simulations are employed in order to identify relevant fast ion loss channels that may still exist in the W7-X quasi-omnigenous equilibrium. These simulations show that ICRF minority heating may not be suitable for producing fast ions in W7-X plasmas. It is found that a high energy tail is more likely to be developed if a three-ion species scheme is applied

The Berk-Breizman Model as a Paradigm for Energetic Particle-driven Alfven Eigenmodes

The achievement of sustained nuclear fusion in magnetically confined plasma relies on efficient confinement of high-energy ions produced by the fusion reaction. Such particles can excite Alfven Eigenmodes (AEs), which significantly degrade their confinement and threatens the vacuum vessel of future reactors. To develop diagnostics and control schemes, a better understanding of linear and nonlinear features of resonant interactions between plasma waves and high-energy particles, is required. In the case of an isolated single resonance, the problem is homothetic to the so-called Berk-Breizman (BB) problem, which is an extension of the classic bump-on-tail electrostatic problem, including external damping to a thermal plasma, and collisions. A semi-Lagrangian simulation code, COBBLES, is developed to solve the initial-value BB problem. The nonlinear behavior of instabilities in experimentally-relevant conditions is categorized into steady-state, periodic, chaotic, and frequency-sweeping (chirping) regimes. The chaotic regime is shown to extend into a linearly stable region, and a mechanism for such subcritical instabilities is proposed. Analytic and semi-empirical laws for nonlinear chirping characteristics, such as sweeping-rate, lifetime, and asymmetry, are developed and validated. Long-time simulations demonstrate the existence of a quasi-periodic chirping regime. Collisional drag and diffusion are shown to be essential to reproduce the alternation between major chirping events and quiescent phases, which is observed in experiments. Based on these findings, a fitting procedure between COBBLES simulations and chirping AE experiments is developped. This procedure, which yields local linear drive and external damping rate, is applied to Toroidicity-induced AEs (TAEs) on JT-60U and MAST tokamaks. This suggests the existence of TAEs relatively far from marginal stability