Modeling Photodetector Circuits for BES Systems*

Beam Emission Spectroscopy (BES) systems can measure localized plasma density in a fusion reactor. BES systems use photodiodes to detect the emission intensity from collisionally-excited neutral beam particles. It follows that low-noise photodiode amplifier circuits are integral to achieving accurate BES measurements. In this study, we used LTspice to simulate the photodetector circuits used in NSTX to determine what circuit parameters have the strongest effect on photodetector response and noise. Ultimately, we will use this information to aid in designing a BES system that uses a 2D photodiode array in place of individual photodiodes distributed among multiple PCBs. Examples of parameters we analyzed include parasitic capacitances and inductances of the photodiode and the first amplifier stage’s JFET. We performed the analysis by modeling the component of interest (eg. photodiode) as a network of discrete components and studying circuit behavior as we varied the component parameters.