Real-time Charge Exchange Recombination Spectroscopy at DIII-D

At the DIII-D tokamak, the charge exchange recombination spectroscopy (CER) diagnostic routinely measures impurity temperature, rotation, and density. The ability to make these measurements in real-time within the Plasma Control System (PCS) has been expanded and improved upon in recent years. High speed network connections send raw data from 24 tangential CER views that cover the entire plasma radius to the PCS where spectral analysis is performed. Analysis is completed quickly by a simplified version of the least-squares fitting used for the highest quality CER analysis. Results are typically available within a millisecond of when the raw data becomes available and new raw data is typically sent to the PCS every 5 ms (for the most common CER sampling frequency). The results are shown to be quite close to more complex automatic analysis that becomes available minutes after a discharge is complete. Fully ionized carbon is the most common impurity used for analysis, and analysis can also be done for other impurities with relatively simple spectra such as neon and argon. The real-time analysis can be used for control in the PCS but it is most commonly used as a method for gaining very rapid assessment of ion conditions for the last discharge during an experiment.