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

The objective of the Porous Plug Injector (PPI) is to obtain calibrated spectroscopic measurements of dissociation fragments resulting from hydrocarbon influx in a tokamak divertor. This is done by admitting methane through a porous surface, $>$90\% graphite, such that the injected molecules ``see" a local carbon surface, similar to the re-deposition environment seen by molecules emitted by chemical sputtering. Following its initial use in DIII-D DiMES, the PPI has been significantly upgraded to include a finer porous graphite cap, a small orifice flow restrictor for precise gas control, and a built-in Langmuir probe to make direct measurement of possible perturbation to the local plasma as a consequence of gas injection. Design of these and other improvements are presented.