Stochastic modelling of blob-like filaments in the scrape-off layer

A stochastic model has been developed in order to describe the dynamics of intermittent fluctuations due to radial motion of blob-like filaments in the scrape-off layer (SOL) of magnetically confined plasma [1-3]. Uncorrelated pulses move radially outwards with a random distribution of amplitudes, sizes and velocities. The pulses have a fixed shape and an exponentially decaying amplitude due to parallel drainage towards the divertor plates. In its simplest form, the model predicts exponentially decreasing average radial profiles [2].



More generally, we investigate the implications of a distribution of blob velocities and correlations with the amplitudes. When pulse velocities depend on the instantaneous amplitude, the filaments will eventually stagnate in the SOL. A broad distribution of pulse velocities leads to non-exponential profiles with strongly increased intermittency and large-amplitude fluctuations in the far-SOL [3].



It is demonstrated that this explains many features from experimental measurements. This includes the formation of a shoulder structure in the average density profile with broad far-SOL profiles, high relative fluctuation levels throughout the SOL, and strong intermittency with positively skewed and flattened probability density functions in the far-SOL. These results will be discussed in the context of experimental measurements in tokamak plasmas.



References

[1] O. E. Garcia, Phys. Rev. Lett. 108, 265001 (2012)

[2] O. E. Garcia et al., 23, 052308 (2016)

[3] J. M. Losada et al., Phys. Plasmas 30, 042518 (2023)