Design and Prospects of High-Speed Gas Puff Imaging Diagnostic at the HSX Stellarator

HSX is a quasi-symmetric optimized stellarator that has demonstrated excellent neoclassical transport properties [1]. In the outer plasma region, however, confinement is degraded by strong anomalous electron heat fluxes, which are likely explained by density gradient-driven Trapped-Electron-Mode turbulence [2]. To monitor turbulent fluctuations near the plasma edge of HSX, a new gas puff imaging system has been designed. The system is equipped with a high-speed camera (Phantom V12, up to 1 MHz) and will allow for studies of edge turbulence [3]. To provide spatial localization of the measurements a low Z neutral gas is planned to be puffed perpendicular to the B field and within the view of the camera. The viewing system will use a periscope with a single mirror to view the separatrix where the gas will be puffed. The periscope will be adjustable to obtain ideal alignment along the field line of interest and sufficient spatial resolution to view edge turbulence. The system is planned to be installed and commissioned during the upcoming experimental campaign of HSX and the first measurement results will be disseminated.

[1] J. M. Canik et al., Phys. Rev. Lett. 98, 085002 (2007)

[2] B. J. Faber et al., Phys. Plasmas 22, 072305 (2015)

[3] S. J. Zweben et al., Rev Sci Instrum 22, 041101 (2017)