Implementation of a hyperspectral filterscope for measuring impurities in fusion plasmas*

A novel flexible diagnostic for visible spectroscopy of fusion plasmas was prototyped and commissioned on the DIII-D tokamak at General Atomics in San Diego, California. The diagnostic consists of a tunable filterscope which leverages an acousto-optic tunable filter (AOTF) for fast wavelength selection within the range of 400 - 800 nm. Preliminary operations demonstrated measurements of multiple spectral emissions along the same light of sight within a single plasma discharge. Transmission efficiency, diffraction efficiency, spectral bandwidth, optical response time were measured to characterize the diagnostic. A wavelength calibration was performed to find the relationship between the applied frequency and diffracted wavelength. The filtering apparatus is connected to light collection optics on DIII-D through an input fiber and to an output photomultiplier tube from the currently installed filterscopes. From the signal detected during a plasma using cyclic wavelength hopping, the time evolution of multiple line emissions during each plasma discharge was constructed. The diagnostic expands the possibility of wavelength access, allowing measurements of non-standard emission or multiple charge states of different impurities along the same line of sight.