Design and Construction of Neutral Gas Handling Systems on WHAM*

Neutral gas plays an important role in seeding and fueling a magnetic mirror plasma, but could also contribute to fast ion losses via charge exchange. This necessitates a flexible neutral gas handling system to maximize control over neutral gas distribution and composition.

In the WHAM experiment, gas is supplied from a manifold capable of mixing and switching four different input gases to four gas puff valves independently. Two solenoid valves located at the end cells provide a pre-fill, while two piezoelectric valves located in high magnetic field are capable of proportional control of fuel and impurity injection on the millisecond timescale. Of the latter two, the fuel valve is heavily baffled to limit background neutral gas density, while the impurity injector is mounted on a swivel feedthrough to improve its accessibility to the plasma edge. A time-dependent Molflow+ simulation informed the design of gas baffles, while the optimization of a 3D printed ceramic convergent-divergent nozzle was achieved with computational fluid dynamics using COMSOL Multiphysics. 

For neutral pressure measurement, WHAM relies on a combination of hot cathode ion gauges, high-accuracy capacitance manometers, residual gas analyzers and Pirani gauges. A high vacuum is sustained by a combination of cryogenic, turbomolecular, titanium sublimation, and soon, tantalum-gettering pumps. For diagnostics of neutral-plasma interactions,  high-speed imaging and impurity line radiation spectroscopy allow the measurement of neutral gas penetration with edge temperature and density. Furthermore, charge exchange losses are measured with foil bolometers. Finally, an overview of the electronic control and data logging system will be provided.