D-band Microwave Interferometer for Plasma Density Measurements in the Formosa Integrated Research Spherical Tokamak(FIRST)

Zongmau Lee^1*, Eiichirou Kawamori¹

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

* Presenter:Zongmau Lee, email:happyfat332@gmail.com

This research focuses on implementing a plasma density measurement system for the Formosa Integrated Research Spherical Tokamak (FIRST), Taiwan's first high-temperature plasma experimental device, crucial for advancing nuclear fusion development in the country.
A 160 GHz microwave interferometer (IF) was chosen due to its suitability for measuring plasma densities up to 10²⁰ m^(-3) , which matches the expected initial plasma density in the tokamak. The IF is currently being developed and calibrated at the NCKU Magnetized Plasma eXperiment (MPX) laboratory, where plasma densities range from 10¹⁶ m^(-3) to 10¹⁸ m^(-3). Reliable density measurements have been confirmed using a 35 GHz IF and a Langmuir probe, despite significant phase shifts during testing.
The IF system includes a frequency down-conversion circuit and IQ mixers to determine the phase difference between a 0.1 GHz reference signal and the plasma-processed signal. This method offers accuracy within 3 degrees of phase error but prevents direct observation of plasma density oscillations.
In addition, mirrors and an antenna array will be incorporated, using Abel inversion techniques to measure radial plasma density distributions. This approach will enhance the understanding of plasma behaviours. The poster will present the initial results from the 160 GHz interferometer in standard and microwave-heated plasma states.

Keywords: Spherical Tokamak , Interferometer , Formosa Integrated Research Spherical Tokamak