# W-Band Modular Antenna/Detector Array for the Electron Cyclotron Emission Imaging System in KSTAR
With an even higher magnetic field (\(B_{0}=3\) T), the O-mode radiation occurs in 75-120 GHz, and the X-mode radiation occurs in 120-140 GHz [14]. The ECE imaging system measures 2D images of the electron temperature fluctuations according to radial and vertical points of the tokamak plasma. Additionally, in KSTAR, the antenna/detector array was designed with various RF components (e.g., antenna, diode, etc.) especially optimized to measure W-band (75 to 110 GHz) signals out of the wide spectrum of ECE radiated frequencies. A conventional antenna/detector array for the ECE imaging system is shown in Figure 1. The antenna/detector array consisted of 12 channels to receive the vertical ECE signals from the tokamak. Each channel was comprised of an elliptical mini-lens and an antenna/detector. The 12 sets of antennas/detectors were printed conjointly on a single board. Additionally, the 12 elliptical mini-lenses were fabricated in a single block structure. Therefore, in case of failure or degraded performance even with a single channel, the entire array module had to be removed for the repair service. This caused the maintenance process to be costly and time-consuming with the conventional array. While various conventional ECE imaging systems have been actively utilized in tokamaks around the world, there are strong needs for improved systems for more effective diagnostic operations [15; 16; 17; 18]. Recently, a new heterodyne W-Band ECE imaging system on DIII-D, which could dramatically improve signal-to-noise ratio, was presented in [19]. However, the system showed a non-flat conversion gain (33-47 dB), meaning that the frequency range of the system would be limited. The heterodyne ECE receiver requires ultra-wideband MMICs and microwave/mm-wave components to cover the full frequency band of ECE measurements (~10s of GHz). When commercial MMICs for the required band are not available, customized MMICs should be developed. Additionally, the uniform  
Figure 1: Conventional antenna/detector array for the KSTAR ECE imaging system: (**a**) top view, (**b**) bottom view. and precise assembly for the whole receiver array may be very expensive. Therefore, a conventional-type ECE imaging system, which uses an array of a dielectric lens integrated with an antenna and a Schottky diode, may still serve as a practical ECE imaging system for most tokamak devices as long as the intensity of the plasma-radiated ECE signal is not so weak for the measurements. Additionally, continued efforts are expected to improve the performance and flexibility of the ECE imaging system. In this paper, a new design of a modular antenna/detector array, based on a unit antenna/detector module, for the KSTAR ECE imaging system is presented for efficient maintenance and performance improvement. Each unit antenna/detector module, which is mounted in an array frame, is composed of an elliptical mini-lens, a dual-dipole antenna, a Schottky diode, an ultra-wideband balun, a low-noise amplifier (LNA), and a unit frame. The dual-dipole antenna and elliptical mini-lens are optimized for W-band ECE signals [20]. Wideband, high-gain performance can be achieved by combining the antenna with the elliptical dielectric lens since the dielectric lens helps in focusing the ECE signals to the center of the dual-dipole antenna, where a Schottky diode is placed. The W-band ECE signal is mixed with a W-band local oscillator (LO) signal by the Schottky diode, generating an intermediate frequency (IF) signal. The IF signal propagates through an ultra-wideband balun consisting of a coplanar strip (CPS) taper and a CPS-to-microstrip line (MSL) transition. Additionally, an LNA is integrated to amplify the weak IF signal from the ECE radiation. For efficient maintenance of the ECE imaging system, each unit antenna/detector module can be easily and independently removed and replaced on the array frame. Moreover, the bias lines for the Schottky diode and amplifier are connected from the bias PCBs to each unit module using a connector socket.