Porous Plug Injection System for Studies of Hydrocarbon Dissociation and Transport in DIII-D

Calibrated spectroscopic measurements of hydrocarbon dissociation fragments in a tokamak divertor were obtained by admitting methane through a porous graphite surface, such that fragments of the injected molecules returned to a carbon surface in a similar way to fragments due to natural chemical sputtering. The porous surface was made from a graphite plate with 1004 holes 0.25 mm in diameter spread over a 3 cm region, and was viewed by calibrated spectrometers. The gas flow rate was 7-40$\times$10$^{17}$ CH$_4$/s, simulating expected chemical erosion yields. Intensities of CD band and CI and CII line emissions were recorded. It was thereby established that chemical sputtering contributed a minority of the carbon atoms naturally sputtered from the outer target.