Enhanced Sensitivity Beam Emission Spectroscopy System for Nonlinear   Turbulence Measurements
  An upgraded Beam Emission Spectroscopy (BES) system has been deployed to access low amplitude turbulence regions near internal transport barriers on the DIII-D tokamak. Sixteen high sensitivity channels are being installed. A significant increase in total signal to noise is achieved by: 1.) Increased spatial volume sampling tailored to known turbulence characteristics; 2.) An increased throughput spectrometer assembly to isolate the local beam fluorescence, coupled to new large-area photoconductive photodiodes; 3.) A new sharp edge interference filter designed to optimize detection of the beam emission plus a significant fraction of the thermal deuterium charge exchange. A new data acquisition system has been installed, providing an 8 times increase in integration time or an increased sample rate. Preliminary results from the upgraded system show a signal enhancement of greater than an order of magnitude. A clear broadband density fluctuation signal is observed in H-mode discharges with the upgraded BES system, demonstrating the significant performance enhancement. 
Deepak K Gupta, Raymond J Fonck, George R McKee, David J Schlossberg   and Morgan W Shafer