3D Simulations of Scrape-Off Layer Filaments

In the Scrape-Off Layer (SOL) of magnetic confinement devices, cross-field transport of particles is dominated by the convection of filamentary plasma structures via self-generated E×B velocity fields. This thesis investigates the dynamics of such filaments using three dimensional simulations to further theoretical understanding of SOL transport. A new 3D SOL simulation code called STORM3D has been developed using the BOUT++ framework to implement an isothermal drift-reduced fluid model in a slab geometry. Verification and validation exercises are documented to demonstrate that the code has been implemented correctly and that the physical model adequately reproduces experimental observations. A comprehensive characterisation of how a filament’s initial geometry affects its subsequent dynamics is provided via a series of 3D simulations of isolated filaments. In particular the size of a filament in the plane perpendicular to the magnetic field, δ⊥, is shown to have a strong influence on its motions, as it determines which currents balance the filament’s pressure-driven diamagnetic currents, which in turn determines its E×B velocity. At small δ⊥, this balance is predominantly provided by polarisation currents and the filament’s radial velocity is observed to increase with δ⊥. In contrast, at large δ⊥, parallel currents closing through the target are found to be dominant, and the radial velocity decreases with δ⊥. Comparisons are made between 3D simulations and 2D simulations using different parallel closures; namely the sheath dissipation closure, which neglects parallel gradients, and the vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure is found not to replicate the 3D perpendicular dynamics well and overestimates the initial radial velocity of all filaments studied. A more satisfactory comparison is obtained with the sheath dissipation closure, even in the presence of significant parallel gradients, where the closure is no longer valid. The vorticity advection closure’s poor performance occurs because in the 3D case parallel currents closing through the sheath play an important role in reducing the extent to which polarisation currents are driven. In a conduction-limited or detached SOL regime however, low plasma temperatures and high neutral densities near the divertor will produce significantly higher resistivity values in the region than that used in the aforementioned 3D simulations. Therefore the effect of increasing the normalised plasma resistivity in the last quarter of the domain nearest the targets is examined using 3D simulations. Whilst small δ⊥ filaments are observed to be relatively unaffected by this quantity, large δ⊥ filaments exhibit faster radial velocities at higher resistivity values due to two mechanisms. Firstly, parallel currents are reduced meaning that polarisation currents are necessarily enhanced and secondly, a potential difference forms along the parallel direction so that higher potentials are produced in the region of the filament for the same amount of current to flow into the sheath. This indicates that broader SOL profiles could be produced at higher values of normalised resistivity, and hence at larger reference SOL densities and at colder temperatures

Overview of recent physics results from MAST

New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp-up, models are not able to correctly predict the current diffusion. Experiments have been performed looking at edge and core turbulence. At the edge, detailed studies have revealed how filament characteristics are responsible for determining the near and far scrape off layer density profiles. In the core the intrinsic rotation and electron scale turbulence have been measured. The role that the fast ion gradient has on redistributing fast ions through fishbone modes has led to a redesign of the neutral beam injector on MAST Upgrade. In H-mode the turbulence at the pedestal top has been shown to be consistent with being due to electron temperature gradient modes. A reconnection process appears to occur during edge localized modes (ELMs) and the number of filaments released determines the power profile at the divertor. Resonant magnetic perturbations can mitigate ELMs provided the edge peeling response is maximised and the core kink response minimised. The mitigation of intrinsic error fields with toroidal mode number n  >  1 has been shown to be important for plasma performance

The effects of shape and amplitude on the velocity of scrape-off layer filaments

A complete model of the dynamics of scrape-off layer filaments will be rather complex, including temperature evolution, three dimensional geometry and finite Larmor radius effects. However, the basic mechanism of ExB advection due to electrostatic potential driven by the diamagnetic current can be captured in a much simpler model; a complete understanding of the physics in the simpler model will then aid interpretation of more complex simulations, by allowing the new effects to be disentangled. Here we consider such a simple model, which assumes cold ions and isothermal electrons and is reduced to two dimensions. We derive the scaling with width and amplitude of the velocity of isolated scrape-off layer filaments, allowing for arbitrary elliptical cross-sections, where previously only circular cross-sections have been considered analytically. We also put the scaling with amplitude in a new and more satisfactory form. The analytical results are extensively validated with two dimensional simulations and also compared, with reasonable agreement, to three dimensional simulations having minimal variation parallel to the magnetic field

UFSAD: couplage d'un generateur de systemes experts et d'un systeme de gestion de base de donnees relationnel

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 77919 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Real-time-capable prediction of temperature and density profiles in a tokamak using RAPTOR and a first-principle-based transport model

The RAPTOR code is a control-oriented core plasma profile simulator with various applications in control design and verification, discharge optimization and real-time plasma simulation. To date, RAPTOR was capable of simulating the evolution of poloidal flux and electron temperature using empirical transport models, and required the user to input assumptions on the other profiles and plasma parameters. We present an extension of the code to simulate the temperature evolution of both ions and electrons, as well as the particle density transport. A proof-of-principle neural-network emulation of the quasilinear gyrokinetic QuaLiKiz transport model is coupled to RAPTOR for the calculation of first-principle-based heat and particle turbulent transport. These extended capabilities are demonstrated in a simulation of a JET discharge. The multi-channel simulation requires ∼0.2 s to simulate 1 second of a JET plasma, corresponding to ∼20 energy confinement times, while predicting experimental profiles within the limits of the transport model. The transport model requires no external inputs except for the boundary condition at the top of the H-mode pedestal. This marks the first time that simultaneous, accurate predictions of Te, Tiand nehave been obtained using a first-principle-based transport code that can run in faster-than-real-time for present-day tokamaks

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks - A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

This paper presents a survey of the experiments on runaway electrons (RE) carried out recently in frames of EUROFusion Consortium in different tokamaks: COMPASS, ASDEX-Upgrade, TCV and JET. Massive gas injection (MGI) has been used in different scenarios for RE generation in small and medium-sized tokamaks to elaborate the most efficient and reliable ones for future RE experiments. New data on RE generated at disruptions in COMPASS and ASDEX-Upgrade was collected and added to the JET database. Different accessible parameters of disruptions, such as current quench rate, conversion rate of plasma current into runaways, etc have been analysed for each tokamak and compared to JET data. It was shown, that tokamaks with larger geometrical sizes provide the wider limits for spatial and temporal variation of plasma parameters during disruptions, thus extending the parameter space for RE generation. The second part of experiments was dedicated to study of RE generation in stationary discharges in COMPASS, TCV and JET. Injection of Ne/Ar have been used to mock-up the JET MGI runaway suppression experiments. Secondary RE avalanching was identified and quantified for the first time in the TCV tokamak in RE generating discharges after massive Ne injection. Simulations of the primary RE generation and secondary avalanching dynamics in stationary discharges has demonstrated that RE current fraction created via avalanching could achieve up to 70-75% of the total plasma current in TCV. Relaxations which are reminiscent the phenomena associated to the kinetic instability driven by RE have been detected in RE discharges in TCV. Macroscopic parameters of RE dominating discharges in TCV before and after onset of the instability fit well to the empirical instability criterion, which was established in the early tokamaks and examined by results of recent numerical simulations

Runaway electron beam control

Post-disruption runaway electron (RE) beams in tokamaks with large current can cause deep melting of the vessel and are one of the major concerns for ITER operations. Consequently, a considerable effort is provided by the scientific community in order to test RE mitigation strategies. We present an overview of the results obtained at FTU and TCV controlling the current and position of RE beams to improve safety and repeatability of mitigation studies such as massive gas (MGI) and shattered pellet injections (SPI). We show that the proposed RE beam controller (REB-C) implemented at FTU and TCV is effective and that current reduction of the beam can be performed via the central solenoid reducing the energy of REs, providing an alternative/parallel mitigation strategy to MGI/SPI. Experimental results show that, meanwhile deuterium pellets injected on a fully formed RE beam are ablated but do not improve RE energy dissipation rate, heavy metals injected by a laser blow off system on low-density flat-top discharges with a high level of RE seeding seem to induce disruptions expelling REs. Instabilities during the RE beam plateau phase have shown to enhance losses of REs, expelled from the beam core. Then, with the aim of triggering instabilities to increase RE losses, an oscillating loop voltage has been tested on RE beam plateau phase at TCV revealing, for the first time, what seems to be a full conversion from runaway to ohmic current. We finally report progresses in the design of control strategies at JET in view of the incoming SPI mitigation experiments