A Bayesian model of filamentary dynamics in MAST

A Bayesian model of filamentary dynamics in MAST

A Bayesian model of filamentary dynamics in MAST 150 150 UKAEA Opendata
UKAEA-CCFE-PR(20)65

A Bayesian model of filamentary dynamics in MAST

A novel approach using Bayesian inference has been implemented to interpret the filamentary dynamics measured by a Langmuir probe fixed to a reciprocating assembly on MAST. The model describes the system as a superposition of time-displaced filaments and a fixed background component. Each filament is parameterised in terms of a characteristic rise and fall time and maximum amplitude centred on local maxima in the measured data time-series. A distinctive feature of the approach is that no minimum threshold is set for the existence of filaments. Furthermore, the model uncertainty is provided as an additional free parameter. Data is analysed in short sub-sampled intervals characterising fixed positions in the SOL and core plasma corresponding to the position of the reciprocating probe assembly. The results obtained achieve a t to the model with an error of  10%. The MAP background signal is found to be zero in over 95% of subsample intervals. Results of Markov chain Monte Carlo sampling of the posterior distribution provide uncertainties on all model parameters. Differences in the MAP values for the rise and fall are small compared to the computed uncertainties in these parameters. This result is in line with previously reported findings that filaments on MAST are symmetric. It is observed that whereas large amplitude filaments are well characterised in terms of rise times, smaller amplitude filaments are often unconstrained by the data and are limited by the details of the prior. Based on these findings, a new definition for the plasma filaments is proposed based on the uncertainty in the filament rise times. The remaining filaments together with the constant background component forms a new time-dependent signal referred to as the computed background fluctuation signal. The characteristics of these signals (for the plasma filaments and for the background fluctuations) are reported in terms of their spatial variation as the probe moves through the SOL and into the core plasma. It is shown that the pdf of both the waiting times and signal amplitude in the SOL for the computed plasma filaments are Poisson distributed. The pdf of waiting times for the background fluctuations are Poisson-distributed throughout the plasma core and the SOL. The mean waiting times of the background fluctuations are constant in the core plasma and SOL regions, with the magnitude in the SOL approximately 50% the magnitude in the plasma core. The mean waiting times of the plasma filaments is constant throughout the plasma core and SOL regions even though details of the pdf change from a Poisson-like distribution in the SOL to a more symmetric distribution in the plasma core.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP (Institute of Physics)
Published date:
19/01/2021
The published version of this paper is currently under embargo and will be available on 20/10/2021