Spatio-temporal variation of edge neutral pressure in the LAPD

A recent upgrade to the plasma source of the Large Plasma Device (LAPD) at UCLA enables access to higher plasma density and temperature operation. As a result, the behavior of neutrals in the device has changed, going from a regime where the plasma is effectively transparent to neutrals (leading to little spatial variation in the neutral density) to a regime with a short neutral penetration length (and hence the potential for strong spatial variation of the neutral density).   Ion gauges with millisecond response times are positioned at various axial locations along the Large Plasma Device. Each is on a plenum with a large conductance to the chamber, so within limits can measure rapid changes in local pressure. Results are presented for a wide variety of plasma conditions, including a comparison between static gas fill and fast gas puffing using a piezoelectric valve. Gas puffing at different axial locations changes the neutral dynamics of the plasma and the observed time dependence of pressure variation along the machine. Results of conductance calculations are used to improve the time resolution with plasma at the expense of signal to noise ratio. These results are combined with fast framing camera imaging of visible light from the end of the machine using a filter for a Helium I spectral line to attempt to deduce neutral behavior inside the plasma.