Diagnosis of High-Temperature Magnetized Fusion Plasmas

Eiichirou Kawamori^1*, Zong-mau Lee¹, Tzu-Chi Liu¹, Yuan-Yao Chang¹, Xuan-Tong Wang²

¹Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan
²Academy of Innovative Semiconductor and Sustainable Manufacturing, National Cheng Kung University, Tainan, Taiwan

* Presenter:Eiichirou Kawamori, email:kawamori@isaps.ncku.edu.tw

This presentation outlines recent progress in the development of diagnostic systems for the first Taiwan spherical tokamak plasma experiment, the Formosa Integrated Research Spherical Tokamak (FIRST), together with an overview of approaches to diagnosing high-temperature fusion plasmas.

We have been developing an electron cyclotron emission (ECE) measurement system for evaluating the electron temperature and a D-band (110–160 GHz) interferometer system for measuring the electron density. These diagnostic systems are essential for the FIRST experiment, as achieving an electron temperature of 100 eV represents a key milestone in the initial phase of the project. In this talk, I will introduce our ongoing activities in these developments, along with an advanced diagnostic technique we have recently established [3].

Experimental attempts have previously been made to diagnose the electron velocity distribution function (EVDF), f(v), from ECE spectra by utilizing relativistic frequency shifts [1, 2]. These methods allow the determination of the EVDF of mildly relativistic electrons from the second- and third-harmonic ECE intensities observed vertically in a tokamak, where the magnetic field strength remains constant along the line of sight. However, such approaches face limitations, including reliance on ad hoc assumptions about the pitch-angle distribution of the EVDF and applicability restricted to steady-state plasmas.

The MEM–HT (Maximum Entropy Method with the Hankel Transform) is a novel approach developed to reconstruct the fluctuation components of the EVDF, f(v), and the associated electron entropy from ECE harmonic spectra in optically thin plasmas [3]. It enables diagnosis of relativistic electron populations except in cases involving harmonic overlap or optically thick conditions. Notably, the method avoids ambiguous assumptions and does not require radiometer calibration for EVDF reconstruction.

Furthermore, measurements of relativistic frequency shifts can be incorporated with the MEM–HT method to derive the dependence of δf(u) on the relativistic parallel velocity amplitude |u∥|. Accurate knowledge of the local electron cyclotron frequency, Ωce, is crucial for the success of this reconstruction. A particularly effective configuration involves vertical observation of the tokamak plasma from the top of the device, where the magnetic field is uniform along the viewing path—similar to the Hutchinson–Kato method [1, 2]. In this configuration, the reconstructed EVDF represents a line-integrated quantity along the line of sight.

We plan to apply the MEM–HT method in the FIRST experiment, whose initial experimental campaign is scheduled to begin in mid-2026. Details of the ECE-based measurement plan for the relativistic EVDF in the FIRST experiment will be presented in this talk.

References
[1] Hutchinson I H and Kato K 1986 Nuclear Fusion 26 179
[2] Kato K and Hutchinson I H 1986 Phys. Rev. Lett. 56 340
[3] Kawamori E 2025 Nuclear Fusion 65 026024

Keywords: Electron cyclotron emission, Interferometry, Magnetized fusion plasma, Relativistic electrons