Description
In the Scrape-Off Layer (SOL) of tokamaks plasmas, turbulence is known to self-organize into coherent density structures (“blobs”) that propagate ballistically, intermittently and radially outward toward the wall [1]. In this study, we unravel a novel nonlinear and non-Markovian behavior of turbulence-driven blobs. To this end, we use the Tokam2d code [3] that features collisional drift-wave and interchange instabilities – which are the main candidates for SOL turbulence – in the plane transverse to the magnetic field lines. In the considered flux-driven setting, turbulence self-organizes under the forcing of a prescribed particle source and the self-consistent evolution of zonal profiles and fluctuations. When a blob propagates, it produces a wake in the density and vorticity fields, which generates in turn a poloidally aligned electric potential dipole that has the ability to guide and attract other nearby structures, increasing radial flux [3]. This nonlocal interaction occurs when structures synchronize poloidally, and ultimately leads to the emergence of self-sustaining preferential paths where transport is channelled. These transport paths are elongated in the radial direction and exhibit a long correlation time with respect to the lifetime of the individual structures from which they stem. We show that these transport paths substantially enhance SOL transport by increasing the propagation length of density blobs by up to 30% compared with cases where such paths do not form. An effective Prandtl number rules this transport channel: long-lasting paths are correlated with a high momentum-to-density diffusivity ratio. In addition, it is shown that the interchange instability – known to advect structures radially [4] – favours the emergence and sustainment of such lanes, while the drift-wave instability – known to twist structures [4] – tends to hinder them. This nonlocal coupling mechanism is believed to play a key role in blob dynamics, and could be instrumental in SOL transport modelling.
[1] D. A. D’Ippolito, Physics of Plasmas 18, 2011.
[2] P. Ghendrih, Journal of Physics: Conference Series 2397, 2022.
[3] G. Decristoforo, Physics of Plasmas 27, 2020.
[4] R. Varennes, Plasma Physics and Controlled Fusion 66, 2024.