Low-noise photodiode detector for optical fluctuation diagnostics

Abstract

The beam emission spectroscopy optical fluctuation diagnostic requires the highest possible quantum efficiency detector at 656 nm to minimize the photon statistical baseline limit to the detectable fluctuation level. A photoconductive photodiode detector with an extremely low-noise preamplifier and a reactive feedback circuit provides quantum efficiencies up to 70%–80% for a useful frequency range of at least 0–150 kHz with incident powers of ∼10 nW. The diodes are chosen for negligible leakage current and hence do not require active cooling. These detectors have provided increase in the sensitivity to plasma fluctuation amplitude by a factor of ∼14 over photomultipliers and a factor of 4 over large area avalanche photodiodes.

The beam emission spectroscopy diagnostic on the DIII-D tokamak

“…Fast preamplifiers and signal conditioning circuits amplify this signal to of order 1 V for digitization while retaining a frequency response of over 0.5 MHz. The circuit design is similar to those utilized on the TFTR BES system, 8 though they have been extended from a frequency response of up to 150 kHz to approximately 750-800 KHz. The frequency response has been increased using lower impedance circuitry and faster components.…”

Plasma Turbulence Imaging via Beam Emission Spectroscopy in the Core of the DIII-D Tokamak

“…The interference filter is a custom-designed, high transmission filter that transmits light in the spectral range λ = 652-655.5 nm, cutting off near D α,o . Specialized ultra low noise cryogenically-cooled transimpedance preamplifiers [18] convert the photodiode current to a voltage signal. Signal conditioning electronics then frequency filter and further amplify the signal.…”

Turbulence imaging and applications using beam emission spectroscopy on DIII-D (invited)

“…Each spectrometer consists of an f /2 collimating lens, a ⌬ Ϸ3 nm interference filter and an aspheric f /1 lens to image the light onto a photodiode detector. Photodiode detectors and preamplifier electronics 14 are cryogenically cooled to reduce dark current and electronic noise. Signals are passed through antialiasing circuits ͑cutoff at f Ϸ450 kHz) and digitized at 1 MHz typically with a 0.5 s time record.…”