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

TL;DR

This paper demonstrates that machine learning models trained on spectroscopic data can accurately measure electron temperature and detect plasma detachment in tokamaks, offering a faster and cost-effective diagnostic alternative.

Contribution

The study introduces a neural network approach for direct electron temperature measurement and detachment detection from spectroscopic data, achieving high accuracy and speed, and enabling low-cost diagnostics.

Findings

Neural network predicts electron temperature with less than 1 eV error at low temperatures.

Detachment detection classifier achieves 99% accuracy and 0.96 F1 score.

Spectroscopic diagnostics can be enhanced with machine learning for fusion plasma monitoring.

Abstract

A machine learning approach has been implemented to measure the electron temperature directly from the emission spectra of a tokamak plasma. This approach utilized a neural network (NN) trained on a dataset of 1865 time slices from operation of the DIII-D tokamak using extreme ultraviolet / vacuum ultraviolet (EUV/VUV) emission spectroscopy matched with high-accuracy divertor Thomson scattering measurements of the electron temperature, $T_e$. This NN is shown to be particularly good at predicting $T_e$ at low temperatures ($T_e < 10$ eV) where the NN demonstrated a mean average error of less than 1 eV. Trained to detect plasma detachment in the tokamak divertor, a NN classifier was able to correctly identify detached states ($T_e<5$ eV) with a 99% accuracy (F$_1$ score of 0.96) at an acquisition rate $10\times$ faster than the Thomson scattering measurement. The performance of the model is understood by examining a set of 4800 theoretical spectra generated using collisional radiative modeling that was also used to predict the performance of a low-cost spectrometer viewing nitrogen emission in the visible wavelengths. These results provide a proof-of-principle that low-cost spectrometers leveraged with machine learning can be used both to boost the performance of more expensive diagnostics on fusion devices, and be used independently as a fast and accurate $T_e$ measurement and detachment classifier.