Gas puff imaging diagnostic for fluctuation studies in alternative divertor configurations in the ASDEX Upgrade tokamak.

Controlling power and particle exhaust is a critical challenge for magnetic-confinement fusion devices. Therefore, the dynamics of the transient filamentary transport in the divertor region need to be well characterized, especially in the newly designed upper divertor of ASDEX Upgrade (AUG). To address this, we have developed a two-color gas puff imaging (GPI) system optimized for the new experimental capabilities. The diagnostic employs a fast in-vessel helium valve, a compact periscope with a fiber-optic image guide, and a single camera that simultaneously records two helium lines. This arrangement yields a spatial resolution of 2-5 mm across a 15-20 cm field of view and temporal resolution up to 150 kFPS, enabling field-aligned imaging of filamentary structures near the magnetic X-point. First experimental tests carried out in AUG demonstrate sufficient GPI signal (SNR ≥ 3-4) in many cases, with signal-to-background ratios limited primarily by neutral deuterium background. One of the highest tested helium injections was found to raise the core helium concentration by about 1.5 percentage points without causing a regime transition, detachment, or an increase in radiation. The system is used to resolve I-phase bursts in the divertor, revealing fluctuations propagating from the outer leg to the X-point and subsequent spread into the private flux region. In addition, a novel line ratio technique is presented and applied to determine the electron pressure from the two brightest helium lines. These results demonstrate the diagnostic's novel capabilities for the validation of divertor dynamics in turbulence codes.