High sensitivity beam emission spectroscopy for core plasma turbulence imaging (invited)

An upgraded beam emission spectroscopy (BES) diagnostic has been developed and deployed at the DIII-D tokamak to achieve a dramatic increase in sensitivity to small-scale density fluctuations. This upgraded BES diagnostic system incorporates high-throughput silica optical fiber bundles (1.62mm2-ster per channel), ultra fast spectrometer collection optics, custom-designed high-transmission interference filters, and large-area photodiodes. The fiber bundle images are optimized to match measured radial and poloidal asymmetries of turbulent eddies. These enhancements result in an approximately one order of magnitude increase in measured beam emission signal. As a consequence of the relative contributions of photon and electronic noise, the net increase in sensitivity to plasma density fluctuation power ranges from ten to 30, allowing for the observation of small-amplitude (ñ∕n⩾0.1%) density fluctuations associated with turbulence and energetic particle modes in the core of high-performance discharges. An array of 16 BES channels has been arranged in a 4×4 grid to provide modest-field (3.2×5cm), high-sensitivity imaging capability that extends over multiple turbulence correlation lengths. This diagnostic capability allows for the first direct imaging of turbulence in the core region of low-confinement and high-confinement tokamak plasmas.