Experimental observations of bifurcated power decay lengths in the near Scrape-Off Layer of tokamak plasmas

TL;DR

This study reports the bifurcation of power decay lengths in the scrape-off layer of a spherical tokamak, revealing two distinct groups linked to different physical scales and turbulence levels.

Contribution

It provides the first experimental observation of bifurcated decay lengths in the SOL, connecting them to ion Larmor radii and turbulence suppression.

Findings

Two groups of decay lengths observed, matching ion Larmor scales.

Narrow decay lengths associated with suppressed magnetic fluctuations.

Scaling of decay lengths aligns with existing H-mode models.

Abstract

The scrape-off layer parallel heat flux decay lengths measured at ST40, a high field, low aspect ratio spherical tokamak, have been observed to bifurcate into two groups. The wide group matches closely with the scale of ion poloidal Larmour radius and follows existing H-mode scalings, while the narrow group falls up to 10 times below scalings, on the scale of ion total Larmour radius. The onset of the narrow scrape-off layer width is observed to be associated with suppressed magnetic fluctuations, suggesting reduced electromagnetic turbulence levels in the SOL.