Digital Lock-In Amplifier Based Ground Loop Monitoring System for Magnetically Confined Plasma Devices

The National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Lab (PPPL) currently uses an analog lock-in amplifier in its Ground Fault Monitor (GFM) system for ground loop detection. For the forthcoming National Compact Stellerator Experiment, a new GFM is currently under development. The current GFM for NSTX is a heritage system originally designed for the Tokamak Fusion Test Reactor and has since had significant enhancements of increased Loop Fault Sensitivity, improved operator interface, and data archiving. A digital GFM may further increase operational performance while adding new features. The digital system would run on a Personal Computer with National Instruments data acquisition hardware along with the tightly integrated LabVIEW software. LabVIEW's Lock-In Amplifier and Digital Signal Processing building blocks saved a significant amount of development time. The primary goal of the research was to determine the feasibility of a LabVIEW based GFM on a bench test setup and, time permitting, in-situ testing on NSTX. Secondary goals include: evaluation of performance of digital versus existing analog system, assessing the use of software implementations of filters, spectral analysis of received signals and data archiving.