Modelling and measurement of 3d fields in stellarators and tokamaks
Creators: name: Haskey, Shaun Robert
Publisher: name: The Australian National University
Year: 2015

description: This thesis presents advances in the modelling, analysis, and measurement of 3D fields, which have significantly improved the diagnosis and understanding of Alfven waves in the H-1NF stellarator and the role of non-axisymmetric magnetic perturbations (MPs) in edge localised mode (ELM) suppression on the DIII-D tokamak. Alfven waves and MPs impact on the performance of stellarators and tokamaks; consequently, improved measurement and understanding of these effects is critical to optimising the operation of these devices and the success of magnetic confinement fusion. To further understand the plasma fluctuations in H-1NF, a sixteen former, tri-axis helical Mirnov array (HMA) was designed and installed. The new array complements two existing poloidal Mirnov arrays by providing polarisation information, higher frequency response, and much improved toroidal resolution for spontaneously excited H-1NF fluctuations. The quantity and complexity of the data obtained from this new array led to the development of a periodic datamining algorithm which has been used to extract distinct plasma fluctuations in multi-channel oscillatory timeseries data. A state of the art synchronous imaging technique which overcomes frame rate and readout noise limitations of cameras is described and demonstrated on longer-lived H-1NF fluctuations. This technique can be used to image modes in the kHz - MHz range, opening up the possibility of high resolution imaging of high frequency MHD instabilities, which play an important role in stability and particle transport on both tokamaks and stellarators. In order to make the most of the data from the synchronous imaging technique, a 3D tomographic reconstruction technique which uses magnetic coordinates was developed for inversion of a set of high-resolution 2D visible light emission projections. Information about the modes, which was obtained using these diagnostic systems and analysis techniques is compared with calculations of the H-1NF continuum gap structure and several discrete normal modes within the framework of linearised ideal compressible MHD, using the CONTI and CAS3D codes. The observations have good agreement with modes where compressibility (beta induced Alfven eigenmodes) or interaction with sound modes is important (beta-induced Alfven-acoustic eigenmodes), in terms of their dominant mode numbers, radial structure, and polarisation. In tokamaks, an outstanding issue is the plasma response to externally applied 3D magnetic field perturbations and how the response leads to ELM suppression. Extensive modelling using the single fluid linear MHD code, MARS-F, shows the ability to tailor the plasma response through careful control of the applied MP field. Metrics are described, which allow the identification of regions of parameter space where the MP directly couples to the pitch-resonant harmonics or the kink mode. Several important parameteric dependencies such as [dollar]q_{u2022}[dollar], toroidal rotation, resistivity, and [dollar]\beta_N[dollar] are also discussed. Finally, recent experiments on DIII-D using the modelling presented here show the importance of coupling to the kink mode in order to maximise the residual pitch-resonant harmonics and achieve ELM suppression. This finding critically influences the way that MPs are optimised to achieve the vital goal of ELM suppression on tokamaks.