Design and Implementation of a Gas Puff Imaging System on TCV*

Gas Puff Imaging (GPI) diagnostics routinely measure the spatially-resolved edge fluctuations in tokamak plasmas. We plan to install a GPI system on TCV that will address multiple physics topics, such as turbulence characteristics during confinement transitions, and neutral density shadowing effect in GPI. The system’s 120 views comprise a 10x12 array (43x52 mm view), with light detection accomplished by APD arrays with a 1 MHz bandwidth. Parametric scans of the gas puff design parameters - including flow rate of the He or D₂ gas puff (FR), spacing between the nozzle and the LCFS (Δ), nozzle design, and gas species - were performed using the DEGAS 2 neutral transport code to model and maximize the expected time-averaged emission within a practical design space. DEGAS 2 results predict that the system will give a brightness above a minimum usable level (2.0 mW/cm2/ster) with He puff and FR=6×10¹⁹ atoms/s, up to 2 mm inside the LCFS. The brightness and penetration are increased with higher FR or shorter Δ or collimated puffs, and the penetration will be deeper for D₂ puffs. Installation of the system on TCV is planned for summer/fall 2018.