Optimization and application of cooled avalanche photodiodes for spectroscopic fluctuation measurements with ultra-fast charge exchange recombination spectroscopy

The Ultra-FastCharge Exchange RecombinationSpectroscopy (UF-CHERS)diagnostic is a highly specializedspectroscopic instrument with 2 spatial channels consisting of 8 spectral channels each and a resolution of ∼0.25 nm deployed at DIII-D tomeasureturbulent iontemperaturefluctuations.Charge exchange emissions are obtained between 528 and 530 nm with 1μs time resolution to studyplasma instabilities. A primary challenge of extractingfluctuationmeasurements from raw UF-CHERS signals is photon and electronicnoise. In order to reduce dark current, theAvalanche Photodiode(APD) detectors are thermo-electrically cooled. State-of-the-art components are used for the signal amplifiers and conditioners to minimize electronicnoise. Due to the low incident photon power ( ≤1 nW),APDs with a gain of up to 300 are used to optimize the signal tonoise ratio. Maximizing the APDs ’ gain while minimizing the excessnoise factor (ENF) is essential since the totalnoise of thediagnostic sets a floor for the minimum level of detectable broadbandfluctuations. The APDs ’ gain should be high enough that photonnoise dominates electronicnoise, but not excessive so that the ENF overwhelmsplasmafluctuations. A new generation of cooledAPDs and optimized preamplifiers exhibits significantly enhanced signal-to-noise compared to a previous generation. Experiments at DIII-D have allowed for characterization and optimization of the ENF vs. gain. A gain of ∼100 at 1700 V is found to be near optimal for mostp...

Source: Review of Scientific Instruments - September 23, 2016 Category: Physics Authors: D. D. Truong, R. J. Fonck and G. R. McKee Source Type: research