Quantifying noise sources in the KSTAR 2014 Thomson Scattering system from the measured variation on electron temperature

TL;DR

This paper analyzes the sources of noise affecting electron temperature measurements in the KSTAR Thomson scattering system, identifying photon noise as a significant contributor to measurement variability.

Contribution

It introduces a method to quantify noise sources in the KSTAR TS system by combining experimental data with synthetic data analysis to identify dominant noise contributions.

Findings

Photon noise significantly impacts temperature measurement variability.

Synthetic data helps estimate the photon noise level consistent with observed fluctuations.

Background noise alone cannot explain the 15% variation in electron temperature.

Abstract

With the Thomson scattering (TS) system in KSTAR, temporal evolution of electron temperature ($T_e$) is estimated using a weighted look-up table method with fast sampling ($1.25$ or $2.5$ GS/s) digitizers during the 2014 KSTAR campaign. Background noise level is used as a weighting parameter without considering the photon noise due to the absence of information on absolute photon counts detected by the TS system. Estimated electron temperature during a relatively quiescent discharge are scattered, i.e., $15$\% variation on $T_e$ with respect to its mean value. We find that this $15$\% variation on $T_e$ cannot be explained solely by the background noise level which leads us to include photon noise effects in our analysis. Using synthetic data, we have estimated the required photon noise level consistent with the observation and determined the dominant noise source in KSTAR TS system.