The Statistics of Edge-Localised Plasma Instabilities

Edge localised modes (ELMs), are presently common in high performance tokamak plasmas, but must be controlled or avoided in larger future tokamaks. They are quasiperiodic instabilities with a frequency that correlates with the plasma's energy confinement and the heat fluxes from ELMs to material surfaces, and have at least two distinct types that can be distinguished by the response of their frequency to heating. An improved characterisation of these instabilities can place constraints on theoretical models, and has the potential to reduce the experimental time presently required for the classification of ELMs and the development of scenarios. It can also provide new insights into the processes responsible for them. In a probability density function (pdf) for the "waiting" time intervals between ELMs was rigorously derived, with simple experimentally motivated assumptions leading to a pdf with the specific form of a Weibull distribution. The Weibull distribution arose from a simple model with a power law form, a more detailed model when evaluated would lead to a different pdf. To test the Weibull model a tool was developed that can detect ELMs in real-time and for non-steadystate data using the light radiation associated with ELMs. The method uses a single dimensionless threshold to determine whether an ELM has occured, which is set independently of the data and in advance of the analysis. Consequently large data sets can be studied in a quick and easy but rigorous manner. Because the derivation of the Weibull pdf assumed steady-state data, a database of JET plasmas with between 3 and 6 seconds of steady H-mode was formed.
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