Understanding the link between scrape-off layer and pedestal conditions at JET using density profile measurements by reflectometry

The confinement of the plasma edge regions, the pedestal and the Scrape-Off Layer (SOL), has a large impact on the stability and efficiency of the entire system but the physics behind them are complex and partly outside first-principles theory. Empirical scaling laws are a crucial tool in understanding these processes, and the JET reflectometer provides a unique opportunity to study the pedestal and SOL, as it has high temporal and spatial resolutions.

Methods were developed to process and validate the reflectometer data and to extract and condense each shot to a set of physical parameters. Models that accurately describe all edge profile regions were created and improved. It was found that the density at the end of the high gradient region had a weak correlation with the separatrix density. A large database of shots was created, from which a selection of scans was chosen, to cover a large range of plasma parameters.

By studying the parameter correlations in the different scans, it was observed that the SOL confinement improved in the corner configuration and with higher triangularity and plasma current, and degraded with the increase in gas fuelling and heating power. Only triangularity was seen to have a positive impact on the pedestal confinement, with other parameters having a neutral or negative effect.

A temporal analysis of Edge Localised Modes (ELMs) was performed on several shots, to analyse the timescales of an ELM and how ELMs are affected by the different parameters. It was observed that the pedestal density has a longer recovery (around 30ms) than both pedestal and SOL widths (around 5ms).

It was also seen that in certain conditions ELMs can have a long tail, associated with a period with near-constant values of pedestal width and gradient, and a larger energy loss than normal ELMs. The largest impact of the plasma parameters on ELMs was found to be the ELM shape and timescale. Increased gas fuelling also showed a reduced impact of ELMs on the profile.