Real-time radiative divertor feedback control development for the NSTX-U tokamak using a vacuum ultraviolet spectrometer

Abstract

A radiative divertor technique is planned for the NSTX-U tokamak to prevent excessive erosion and thermal damage of divertor plasma-facing components in H-mode plasma discharges with auxiliary heating up to 12 MW. In the radiative (partially detached) divertor, extrinsically seeded deuterium or impurity gases are used to increase plasma volumetric power and momentum losses. A real-time feedback control of the gas seeding rate is planned for discharges of up to 5 s duration. The outer divertor leg plasma electron temperature T estimated spectroscopically in real time will be used as a control parameter. A vacuum ultraviolet spectrometer McPherson Model 251 with a fast charged-coupled device detector is developed for temperature monitoring between 5 and 30 eV, based on the Δn = 0, 1 line intensity ratios of carbon, nitrogen, or neon ion lines in the spectral range 300-1600 Å. A collisional-radiative model-based line intensity ratio will be used for relative calibration. A real-time T-dependent signal within a characteristic divertor detachment equilibration time of ∼10-15 ms is expected.

Measuring the Electron Temperature and Identifying Plasma Detachment using Machine Learning and Spectroscopy

“…In some instances, the addition of impurity gases to the plasma is required, further complicating the experimental setup, and risking perturbation, although it does provide the advantage of measurement localization [3][4][5] . On the DIII-D tokamak and other carbon-walled machines, there has been significant effort to make use of carbon line ratios for T e measurement 6,7 . The measured temperature from line-ratio techniques is often a) samuell1@llnl.gov reported as an 'effective' temperature, denoting that the measurement is weighted by the location of the emitting species along the viewchord line of sight rather than the temperature at a particular location.…”