Using Laser-Induced Rydberg Spectroscopy diagnostic for direct measurements of the local electric field in the edge region of NSTX/NSTX-U: Modeling

Abstract

We discuss a novel diagnostic allowing direct measurements of the local electric field in the edge region of NSTX/NSTX-U. This laser based diagnostic’s principle consists of depleting the naturally populated n = 3 level to a Rydberg state—sensitive to electric fields—that will result in a suppression of part of the Dα emission. We refer to this approach as Laser-Induced Rydberg Spectroscopy. It is shown that the local electric field can be measured through the Stark induced resonances observed as dips in the Dα emission. Using forward-modeling of simulated absorption spectra, we show precisions reaching ±2 kV m−1 in regions with a local electric field of 15 kV m−1.

Experimental evidence of lower hybrid wave scattering in Alcator C-Mod due to scrape off layer density fluctuations

“…This structure can be directly probed through a measurement of the spectral line profile. The result is a powerful diagnostic for electric fields in the fusion device, especially when laser-based spectroscopic techniques are implemented [22,23].…”