Development of Electron Cyclotron Emission and Electron Bernstein Emission Radiometer for Measurement of Electron Temperature Profiles on the Taiwanese FIRST Spherical Tokamak

Tzu-Chi Liu^1*, Yuan-Yao Zhang¹, Eiichirou Kawamori¹

¹Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan City, Taiwan

* Presenter:Tzu-Chi Liu, email:aragorn0402@hotmail.com

To characterize the performance of fusion plasmas, electron temperature is one of the key parameters to be diagnosed during experiments. Measurement of electron cyclotron emission (ECE) [1] via heterodyne radiometers [2] is a well-established method that provides spatial profiles of the electron temperature with high temporal resolution. In toroidal devices, the one-to-one correspondence between the magnetic field strength and the radial position enables the deduction of the temperature profile from the frequency spectrum of ECE for optically thick plasmas. Such systems have been installed on many tokamaks and stellarators [2] to study MHD activities and transport phenomena of magnetized fusion plasmas.

For the Taiwanese FIRST spherical tokamak (major radius R = 45 cm, aspect ratio A = 1.41, B = 0.5 T) currently under design, ECE radiometer will be the main diagnostics for the electron temperature profile. However, with expected parameters of n_e ~ 10¹⁹ m^-3 and T_e ~ 100 eV , the first few harmonics of ECE is expected to encounter cutoff regions as it travels outwards, therefore mode conversion is required for outboard measurements. For this purpose, the electron Bernstein wave (EBW) [3], which propagates in the core region without any cutoffs, carries the temperature information up to the plasma edge and mode converts to electromagnetic waves that further travels to the vacuum region where the signal is collected. The EBW emission (EBE) will hence be utilized in overdense plasmas where the high density prohibits the propagation of conventional ECE.

Here we report about the planned setup of the 16-channel radiometer which covers the frequency range of 18-50 GHz (corresponding to 2nd-5th ECE harmonics for FIRST plasmas), with a radial resolution of ~ 5 cm at the outer plasma edge. Preliminary results of ECE/EBE measurements in a laboratory magnetic mirror device are shown, along with comparisons with Langmuir probes measurements. Design of quasi-optical lenses (for beam focusing) via FDTD simulations is also presented. By crosschecking the measured signal level with visible light spectroscopy, it is confirmed that ECE from electrons with energies larger than 50eV is measured.

[1] M. Bornatici et al., Nucl. Fusion 23, 1153 (1983)
[2] H. J. Hartfuss et al., Plasma Phys. Control. Fusion 39, 1693 (1997)
[3] H. P. Laqua, Plasma Phys. Control. Fusion 49, R1 (2007)

Keywords: Plasma Diagnostics, Fusion Plasmas, Spherical Tokamak, Heterodyne Radiometer