Abstract
The high-speed rotation diagnostic developed for Columbia’s HBT-EP tokamak requires a high quantum efficiency, very low drift detector/amplifier combination. An updated version of the circuit developed originally for the beam emission spectroscopy experiment on TFTR is being used. A low dark current (2nA at 15V bias), low input source capacitance (2pF) FFD-040 N-type Si photodiode is operated in photoconductive mode. It has a quantum efficiency of 40% at the 468.6nm (HeII line that is being observed). A low-noise field-effect transistor (InterFET IFN152 with eNa=1.2nV∕√Hz) is used to reduce the noise in the transimpedance preamplifier (A250 AMPTEK op-amp) and a very high speed (unity-gain bandwidth=200MHz) voltage feedback amplifier (LM7171) is used to restore the frequency response up to 100kHz. This type of detector/amplifier is photon-noise limited at this bandwidth for incident light with a power of >∼2nW. The circuit has been optimized using SIMETRIX 4.0 SPICE software and a prototype circuit has been tested successfully. Though photomultipliers and avalanche photodiodes can detect much lower light levels, for light levels >2nW and a 10kHz bandwidth, this detector/amplifier combination is more sensitive because of the absence of excess (internally generated) noise.
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