Absolute value measurement of ion-scale turbulence by two-dimensional phase contrast imaging in Large Helical Device

TL;DR

This paper demonstrates the first quantitative measurement of ion-scale turbulence amplitudes using two-dimensional phase contrast imaging in a large helical device, and compares results with nonlinear gyrokinetic simulations.

Contribution

It applies a previously proposed absolute measurement method to 2D-PCI data in a large helical device, enabling direct quantification of turbulence amplitudes.

Findings

Localized turbulence amplitude is approximately 3.5×10^{15} m^{-3} at specified plasma conditions.

Measured turbulence spectrum is consistent with nonlinear gyrokinetic simulation within error margins.

First quantitative comparison of 2D-PCI turbulence measurements with simulation results.

Abstract

Absolute value measurements of turbulence amplitude in magnetically confined high-temperature plasmas can effectively explain turbulence-driven transport characteristics and their role in plasma confinements. Two-dimensional phase contrast imaging (2D-PCI) is a technique to evaluate the space-time spectrum of ion-scale electron density fluctuation. However, absolute value measurement of turbulence amplitude has not been conducted owing to the nonlinearity of the detector. In this study, the absolute measurement method proposed in the previous study is applied to turbulence measurement results in the large helical device. As a result, the localized turbulence amplitude at $n_e=1.5\times 10^{19}$m$^{-3}$ is approximately $3.5\times 10^{15}$m$^{-3}$, which is 0.02\% of the electron density. In addition, the evaluated poloidal wavenumber spectrum is almost consistent, within a certain error range, the spectrum being calculated using a nonlinear gyrokinetic simulation. This result is the first to the best of our knowledge to quantitatively evaluate turbulence amplitudes measured by 2D-PCI and compare with simulations.