Real-Time Density Feedback Control Using FIReTIP

The Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) is under development to provide time-resolved, chord-integrated electron density and tangential B-field data on NSTX-U. The FIReTIP beam will enter on Bay G, reflect from an internal retroreflector placed within the Bay B port cover, and exit back through the Bay G entrance window. A heterodyne HeNe interferometer will provide real-time vibration compensation, with a field programmable gate array (FPGA) digitizing the voltage outputs of both FIReTIP and HeNe interferometer systems. The FPGA will analyze the data, correct the FIReTIP data for the mechanical motions observed on the HeNe interferometer, and then output a corrected signal in real time (except for a short time delay due to the amount of time required to perform the analysis) with a throughput of at least 5 kHz. The FIReTIP FPGA outputs will provide real-time density feedback control signals for the Real-time Control and Protection (RTCP) system of NSTX-U, and can serve as real-time constraints for EFIT equilibrium reconstructions. Details of the FIReTIP system, including ongoing FPGA development activities, will be presented.