Description
Understanding the dynamics of turbulent transport in the scrape-off layer (SOL) of magnetically confined plasmas is critical for optimizing plasma confinement and mitigating plasma–surface interactions in fusion devices. In this study, we present a comprehensive analysis of blob-like filament dynamics in the SOL of the Alcator C-Mod tokamak, focusing on velocity scaling regimes and their implications for profile broadening and flattening.
Using gas puff imaging (GPI) diagnostics, we analyzed blob sizes and velocities in a series of Ohmically heated, lower single-null diverted plasmas with varying core plasma densities. Our results reveal a monotonic increase in both average blob size and radial velocity with increasing core plasma density. This transition corresponds to a shift from sheath-connected to resistive velocity scaling regimes as the density limit is approached. Notably, the normalized blob size in Alcator C-Mod is significantly larger than reported for other devices, while the normalized velocities remain comparable.
The study highlights the role of collisionality in SOL transport, showing that increased collisionality enhances radial blob velocities and contributes to the broadening and flattening of the far-SOL plasma density profile. These findings are consistent with theoretical predictions of resistive blob dynamics and provide valuable insights into the interplay between filamentary transport and plasma–surface interactions.
Both the time-average radial profile and the fluctuations in particle density are shown to be in excellent agreement with a stochastic model describing them as a super-position of uncorrelated blob structures moving radially outwards.
This work advances our understanding of SOL turbulence and its implications for plasma confinement and material interactions.
Alcator C-Mod measurement data was generated under the US DoE award DE-FC02-99ER54512.