Improved Density Control in the Pegasus Toroidal Experiment using Internal Fueling

Routine density control up to and exceeding the Greenwald limit is critical to key Pegasus operational scenarios, including non-solenoidal startup plasmas created using single-point helicity injection and high $\beta $ Ohmic plasmas. Confinement scalings suggest it is possible to achieve very high $\beta $ plasmas in Pegasus by lowering the toroidal field and increasing n$_{e}$/n$_{g}$. In the past, Pegasus achieved $\beta \quad \sim $ 20{\%} in high recycling Ohmic plasmas without running into any operational boundaries.\footnote{ Garstka, G.D. \textit{et al.}, Phys. Plasmas \textbf{10}, 1705 (2003)} However, recent Ohmic experiments have demonstrated that Pegasus currently operates in an extremely low-recycling regime with R $<$ 0.8 and Z$_{eff }\sim $ 1 using improved vacuum conditioning techniques, such as Ti gettering and cryogenic pumping. Hence, it is difficult to achieve n$_{e}$/n$_{g}>$ 0.3 with these improved wall conditions. Presently, gas is injected using low-field side (LFS) modified PV-10 valves. To attain high n$_{e}$/n$_{g}$ operation and coincidentally separate core plasma and local current source fueling two new gas fueling capabilities are under development. A centerstack capillary injection system has been commissioned and is undergoing initial tests. A LFS movable midplane needle gas injection system is currently under design and will reach r/a $\sim $ 0.25. Initial results from both systems will be presented.