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

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.

Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

“…Recently, a stochastic model was introduced, describing the fluctuations as a super-position of uncorrelated pulses with an exponential shape and constant duration [16][17][18][19][20]. Predictions of this model, including the probability density function and the frequency power spectral density, are in excellent agreement with Langmuir probe and gas puff imaging (GPI) measurements obtained in ohmic and low confinement modes (L-modes) of several tokamak devices [21][22][23][24][25][26][27][28].…”

The diffusion limit of ballistic transport in the scrape-off layer

“…The auto-correlation function of the fluctuations in the SOL can be well fitted by an exponentially decaying function as shown for example in TCV [28], JET [29] and COMPASS [30]. The exponential correlation function is given by…”

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

“…A recent model by Walkden [42] relating SOL filament characteristics to SOL shoulders also pointed towards reduced parallel flows out of the SOL as an important mechanism for SOL density shoulder formation. The model compares parallel resistivity, upstream ionization and drainage as mechanisms for shoulder formation based on a stochastic framework for turbulence characteristics.…”