The beam emission spectroscopy diagnostic on the DIII-D tokamak

A beam emission spectroscopy system has been installed on DIII-D to provide localized density fluctuation measurements for long-wavelength turbulent modes with $\text{k⩽3\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ which are typically associated with anomalous radial transport. High signal-to-noise fluctuations measurements are accomplished through use of high speed electronics to maintain a frequency response of over 500 KHz and cryogenically cooled amplifiers and detectors to reduce electronic noise. The optics and neutral beam-sightline geometry have been optimized to allow for spatial resolution of $\text{Δr⩽1\hspace{0.17em}}\mathrm{cm}.$ In addition, a half-scale two-dimensional (2D) fiber array to measure the 2D turbulent density field, necessary to measure the full ${\mathrm{S(k}}_r{\mathrm{,k}}_{\theta })$ wavenumber spectra, has been implemented and initial results obtained.