Investigation into the influence of magnetic field structure on the dynamical and spatial properties of plasma edge turbulence in the stellarator TJ-K

Abstract

Nuclear fusion could provide the solution to our rising energy demands in a world where greenhouse gas emissions must be rapidly reduced (Germany plans a 55 % reduction by 2030, compared to 1990 levels). In order to have sufficient rates of fusion a high-temperature, relatively dense plasma is required. Containing such a plasma for long enough for fusion to occur at sufficient rates is challenging. One solution is to confine the plasma with strong toroidal magnetic fields. Whilst this drastically improves the confinement, particles and heat are still transported towards the reactor walls, reducing the temperature and density, and posing a threat to wall components. The main contribution to outward transport in fusion-relevant reactors comes from turbulent fluctuations in the plasma potential and density. The aim of this thesis is to investigate the effect of magnetic field geometry on the spatial, statistical and dynamical properties of turbulence in the confined region of the plasma, dominated by drift-wave turbulence, and the region close to the reactor wall (scrape-off layer), where the turbulence has an intermittent character due to outward-travelling high-amplitude pressure perturbations, often called blobs. A high proportion of scrape-off layer transport towards the reactor wall is due to blob filaments. The generation of blobs is known to be linked to drift-wave turbulence, and so the intermittent nature of scrape-off layer turbulence could have its origin in the confined plasma. To gain an understanding of the intermittent properties of drift-wave turbulence, simulations were carried out with a simple slab geometry model and compared to experimental measurements at the stellarator TJ-K. It was found that the density fluctuations become more intermittent with increasing plasma collisionality, due to a progressive decoupling of density and potential fluctuations. An extended version of the model, accounting for magnetic curvature, predicted more intermittent density fluctuations on the outboard side of the reactor, where the normal curvature is negative. Experiments showed that the most intermittent fluctuations are in fact shifted into the region where the normal curvature is negative but the geodesic curvature is positive. This shift could be understood by considering the spatial properties of the turbulence, which enter into the density-potential coupling terms in the model. Furthermore, the local magnetic shear was found to have a damping effect on drift-wave turbulence, which locally reduces the cross-sectional areas of turbulent structures. Blob dynamics were studied using a three-dimensional gyrofluid model, in which blob filaments are generated self-consistently. An effect of the magnetic curvature was found not only on the radial blob speeds, which is well known, but also on the binormal velocity component. This result was also found in experiments at TJ-K, where the geodesic curvature drive was shown to contribute significantly to poloidal blob speeds. In addition, the structure of blob filaments was studied in the experiment. It was found that the magnetic shear does not have a deforming effect, and that blobs retain the filamentary form that they have near their generation region.