Suppression of first-wall interaction in negative triangularity plasmas on TCV

Magnetically confined fusion plasmas with negative triangularity (delta) exhibit greater L-mode confinement than with positive delta. Recent experiments in the TCV and DIII-D tokamaks have correlated the confinement improvement to a reduction of fluctuations within the plasma core. We report on fluctuation measurements in the scrape-off layer (SOL) for -0.61 < delta < +0.64 in limited and diverted ohmic L-mode plasmas; these reveal a strong reduction in SOL fluctuation amplitudes at delta less than or similar to -0.25, and, surprisingly, an almost full suppression of plasma interaction with the main-chamber first-wall, which could have important implications for the prospects of using negative delta plasmas as a reactor solution. An exploration of several physical mechanisms suggests that a reduced connection length-intrinsic to negative delta plasmas-plays a critical role in the origin of this phenomenon.