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Description
In L-mode plasmas, cross-field transport in the scrape-off layer (SOL) is predominantly driven by turbulent filamentary structures known as blobs [1]. Blobs contribute to enhanced particle and heat transport, affecting edge plasma confinement, divertor heat loads, and plasma-wall interactions. While blob dynamics at the midplane have been studied extensively [2, 3], their downstream impact at the divertor and the dependence of fluctuation statistics on plasma parameters remains underexplored. Even current 3D edge (SOL) turbulence simulations do not yet fully reproduce the observed divertor intermittency statistics [4, 5].
The COMPASS tokamak is equipped with Langmuir and ball-pen probes at both the midplane and divertor, enabling simultaneous measurements of plasma fluctuations with microsecond time resolution [6, 7]. In this work, fluctuations measured by reciprocating (RCP) probes, divertor probes, and Li-beam emission spectroscopy (Li-BES) are analyzed across a line-averaged density scan. Midplane and divertor are mapped using EFIT reconstruction corrected by the two-point model [8]. The statistical properties of the RCP measurements are found to be consistent with the stochastic Poisson process model [2]. Divertor ion saturation current fluctuations exhibit the same characteristic intermittency signatures, including Gamma-like probability density functions, parabolic skewness–kurtosis scaling, and exponential waiting-time distributions, indicating that blob statistics are largely preserved along magnetic field lines from the midplane to the divertor. In contrast, Li-BES measurements show weaker intermittency, reflecting the reduced sensitivity of the diagnostic to blob statistics. The results further indicate that radial position has a stronger influence on intermittency than line-averaged density over the investigated parameter range. These results establish divertor probe measurements as a robust tool for quantitative studies of SOL blob transport and provide an experimental benchmark for validating edge turbulence simulations. Future work will extend this statistical analysis to electron temperature and plasma potential fluctuations to assess whether the same stochastic description applies to other plasma parameters.
[1] D.A. D'Ippolito et al., Physics of Plasmas 18(6) (2011)
[2] O.E. Garcia, Physical review letters, 108(26), 265001 (2004)
[3] S. Ahmed, PhD thesis, The Arctic University of Norway (2023)
[4] D.S. Oliveira et al., Nuclear Fusion 62, 096001 (2022)
[5] P. Macha et al., accepted to Nuclear Fusion (2026)
[6] D. Cipciar et al., Plasma Physics Controlled Fusion 64(5), 055021 (2022)
[7] J. Adamek et al, Nuclear Fusion, 57(11), 116017 (2017)
[8] P. Stangeby, The plasma boundary of magnetic fusion devices. Series in Plasma Physics (2000)