Real-time charge exchange recombination spectroscopy at DIII-D

At the DIII-D tokamak, the Charge Exchange Recombination (CER) spectroscopy diagnostic routinely measures impurity temperature, rotation, and density. The ability to make these measurements in real time within DIII-D's Plasma Control System (PCS) has recently been expanded and improved. High-speed network connections send raw data from 24 tangential CER views to the PCS, so spectral analysis can be performed. These views make measurements on the outboard midplane of DIII-D from the magnetic axis to the scrape off layer. Spectral analysis is completed quickly by a simplified least-squares fitting routine. Results are typically available within a millisecond of when the raw data become available and new raw data for all 24 views are typically sent to the PCS every 5 ms (for the most common CER sampling frequency). Fully ionized carbon is the most common impurity used for analysis, and analysis can also be performed for other highly charged impurities with relatively simple spectra such as boron, neon, and argon. The large majority of the real-time temperature and rotation measurements are within 20% of the most accurate automatic analysis available. As DIII-D discharges are typically separated by 10-15 min, the more rapid availability of the real-time analysis can be a major aid for control room analysis during an experiment. The real-time analysis can also be used in the PCS for control purposes (e.g., plasma profile feedback).