Blob motion and flattened far-SOL profiles in Alcator C-Mod: evidence in support of stochastic modeling

Abstract Fluctuations in the boundary region of the Alcator C-Mod device are investigated with mirror Langmuir probe and gas puff imaging measurements in a series of experiments with a scan in core plasma density. This reveals the familiar broadening and flattening of the radial electron density profile in the scrape-off layer (SOL). A time delay estimation method is applied to deduce blob velocities at various radial positions within the boundary region. In discharges characterized by low core plasma density, the dynamics of blobs manifest in the far SOL, exhibiting radial velocities reaching up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi>km</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi mathvariant="normal">s</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . As the core plasma density approaches the empirical discharge density limit, blob dynamics come to dominate the entire SOL, extending even inside the last closed magnetic flux surface with radial velocities exceeding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi>km</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi mathvariant="normal">s</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . The fluctuations measured at any given location within the gas puff imaging field-of-view display pronounced intermittency in the region primarily governed by blob structures. Both the radial profiles and the fluctuations are in agreement with a stochastic model describing the blobs as a super-position of uncorrelated pulses moving radially outwards, predicting a particle density profile e-folding length given by the product of the radial blob velocity and the parallel loss time. With increasing core plasma density, the blobs move faster, the average particle density in the far SOL increases and the fluctuations become more intermittent, which will result in enhanced plasma–surface interactions.