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

Journal Article · Fri Sep 23 00:00:00 EDT 2016 · Review of Scientific Instruments

DOI: https://doi.org/10.1063/1.4963147 · OSTI ID:1371908

Truong, D. D. ^[1]; Fonck, R. J. ^[2]; McKee, G. R. ^[2]

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1. Univ. of Wisconsin, Madison, WI (United States); General Atomics
2. Univ. of Wisconsin, Madison, WI (United States)

The Ultra Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic is a highly specialized spectroscopic instrument with 2 spatial channels consisting of 8 spectral channels each and a resolution of ~0.25 nm deployed at DIII-D to measure turbulent ion temperature fluctuations. Charge exchange emissions are obtained between 528-530 nm with 1 μs time resolution to study plasma instabilities. A primary challenge of extracting fluctuation measurements from raw UF-CHERS signals is photon and electronic noise. In order to reduce dark current, the Avalanche Photodiode (APD) detectors are thermoelectrically cooled. State-of-the-art components are used for the signal amplifiers and conditioners to minimize electronic noise. Due to the low incident photon power (≤ 1 nW), APDs with a gain of up to 300 are used to optimize the signal to noise ratio. Maximizing the APDs’ gain while minimizing the excess noise factor (ENF) is essential since the total noise of the diagnostic sets a floor for the minimum level of detectable broadband fluctuations. The APDs’ gain should be high enough that photon noise dominates electronic noise, but not excessive so that the ENF overwhelms plasma fluctuations. A new generation of cooled APDs 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. Here, a gain of ~100 at 1700 V is found to be near optimal for most plasma conditions. Ion temperature and toroidal velocity fluctuations due to the Edge Harmonic Oscillation (EHO) in Quiescent H-mode (QH) plasmas are presented to demonstrate UF-CHERS’ capabilities.

• Research Organization: Univ. of Wisconsin System, Madison, WI (United States)
• Sponsoring Organization: USDOE
• Contributing Organization: Department of Engineering Physics, University of Wisconsin – Madison, Madison, WI 53706, USA
• Grant/Contract Number: FG02-89ER53296
• OSTI ID: 1371908
• Journal Information: Review of Scientific Instruments, Journal Name: Review of Scientific Instruments Journal Issue: 11 Vol. 87; ISSN 0034-6748; ISSN RSINAK
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English