Correlation between the relative blob fraction and plasma parameters in NSTX

Abstract

This paper presents a simple analysis of the fraction of blobs observed in the scrape-off layer of NSTX and calculates the correlation between this blob fraction and various plasma parameters. The measurements were made using the gas puff imaging (GPI) diagnostic on a database of 103 shots from 2010 with neutral beam power PNBI = 0-6 MW. The blob fraction ranged from fb = 0.1% to 4.8%, where fb is defined as the fraction of time the GPI signal was larger than three times its average value within 0-7.5 cm outside the separatrix. The blob fraction generally decreased with increasing neutral beam injection power and was lower on average in H-mode plasmas than Ohmic and L-mode plasmas. The blob fraction had its highest correlation with the measured poloidal turbulence velocity and its radial gradient just inside the separatrix. This is in part consistent with a model in which the blob generation rate increases with the poloidal velocity shear.