# Interpretation of scrape-off layer profile evolution and first-wall ion flux statistics on JET using a stochastic framework based on filamentary motion

## TL;DR

This study uses a stochastic filament-based model to interpret and replicate the evolution of the scrape-off layer profile and ion flux statistics on JET, revealing the importance of localised sink reduction and additional physics in shoulder formation.

## Contribution

The paper introduces a novel stochastic modelling approach to interpret SOL profile evolution and identifies key physical mechanisms involved in shoulder formation on JET.

## Key findings

- Stochastic model matches experimental profile and fluctuation data.
- Localised reduction in density sink triggers shoulder formation.
- Additional physics needed for shoulder evolution beyond sink reduction.

## Abstract

This paper presents the use of a novel modelling technique based around intermittent transport due to filament motion, to interpret experimental profile and fluctuation data in the scrape-off layer (SOL) of JET during the onset and evolution of a density profile shoulder. A baseline case is established, prior to shoulder formation, and the stochastic model is shown to be capable of simultaneously matching the time averaged profile measurement as well as the PDF shape and autocorrelation function from the ion-saturation current time series at the outer wall. Aspects of the stochastic model are then varied with the aim of producing a profile shoulder with statistical measurements consistent with experiment. This is achieved through a strong localised reduction in the density sink acting on the filaments within the model. The required reduction of the density sink occurs over a highly localised region with the timescale of the density sink increased by a factor of 25. This alone is found to be insufficient to model the expansion and flattening of the shoulder region as the density increases, which requires additional changes within the stochastic model. An example is found which includes both a reduction in the density sink and filament acceleration and provides a consistent match to the experimental data as the shoulder expands, though the uniqueness of this solution can not be guaranteed. Within the context of the stochastic model, this implies that the localised reduction in the density sink can trigger shoulder formation, but additional physics is required to explain the subsequent evolution of the profile.