Low-noise, high-speed detector development for optical turbulence fluctuation measurements for NSTX

A new beam emission spectroscopy (BES) diagnostic is under development. Photon-noise limited measurements of neutral beam emissions are achieved using photoconductive photodiodes with a novel frequency-compensated broadband preamplifier. The new BES system includes a next-generation preamplifier and upgraded optical coupling system. Notable features of the design are surface-mount components, minimized stray capacitance, a wide angular acceptance photodiode, a differential output line driver, reduced input capacitance, doubling of the frequency range, net reduced electronic noise, and elimination of the need for a cryogenic cooling system. The irreducible photon noise dominates the noise up to 800 kHz for a typical input power of 60 nW. This new assembly is being integrated into an upgraded multichannel optical detector assembly for a new BES system on the NSTX experiment.