Measurements and observation of bifurcated near scrape-off layer power decay lengths in ST40

ST40 is a high-field spherical tokamak operated by Tokamak Energy Ltd, located near Oxford, Oxfordshire, UK [1]. A database has been assembled for all diverted discharges with viable IR camera data, and augmented with Thomson Scattering measured average and edge electron temperature and density as well as magnetic fluctuations measured with fast poloidal field loops. Statistical analysis reveals two classes of heat flux profiles, one with the typical single exponential drop off in the SOL, and another with two distinct decay lengths. The widths of the single exponential profiles fall within 50% of existing H-mode scalings. When two exponentials were observed, the near SOL decay width falls up to 10 times below existing scalings, between 0.3 – 0.8 mm, which is comparable to the ion Larmour radius. The onset of very narrow near SOL strongly correlated with the reduction of fluctuations in radial magnetic fields. Experimental and modeling results will be discussed.