## 1 Introduction  
Millimeter-wave (MMW) imaging has emerged as a promising technology in security screening, medical diagnostics, and industrial inspection owing to its ability to penetrate non-metallic materials and reveal concealed or internal structures with high resolution [1, 2]. However, traditional MMW imaging systems face significant barriers, including high costs, intricate configurations, and dependency on expensive detectors that often require cryogenic cooling. These challenges limit their widespread adoption and practical implementation. Addressing these limitations, this study introduces an innovative approach using Glow Discharge Detectors (GDDs), offering a low-cost, robust, and room-temperature-compatible solution that not only simplifies MMW imaging but also extends its accessibility to 3D imaging applications, paving the way for advanced capabilities in various fields. Traditional MMW detectors, including Schottky diodes, Golay cells, bolometers, and Zero-Bias Detectors (ZBDs), are effective but costly and come with notable limitations [3; 4; 5]. Schottky diodes and ZBDs, commonly used for MMW detection, are highly sensitive but prone to electrostatic discharge (ESD) damage, especially when exposed to high-power MMW signals, necessitating the use of attenuators in conjunction with horn antennas for protection. Additionally, the horn antennas required for these detectors need to be large enough to effectively receive MMW signals, which makes them unsuitable for designing compact focal plane arrays (FPAs) for imaging applications. Golay cells and pyroelectric detectors, while cost-effective, suffer from slower response times and narrower operational frequency ranges. Microbolometers, although highly sensitive, are hindered by slower modulation frequencies and higher costs. In contrast, Glow Discharge Detectors (GDDs), which operate based on weakly ionized plasma [6; 7; 8], present a promising alternative for MMW detection [9]. GDDs offer enhanced durability, reduced sensitivity to ESD, and high responsiveness to MMW radiation [10]. With a smaller cross-section, GDDs are well suited for use as pixel elements in FPAs, enabling the design of compact and scalable imaging systems. Furthermore, GDDs achieve a balance between sensitivity (NEP: 3.9 \(\times\) 10\({}^{-10}\) W/Hz\({}^{1}\)/2) and high-speed modulation (\(\leq\)10,000 Hz), surpassing Golay cells and pyroelectric detectors in performance. Operating across a wide frequency range (10 GHz to 300 GHz), GDDs provide versatility for imaging applications. Their lower cost and these performance advantages position GDDs as valuable candidates for scalable imaging systems, particularly in applications that demand large detector arrays or real-time imaging capabilities. A detailed comparison of GDDs with traditional detectors [3; 4; 5], as summarized in Table 1, is provided below. In this work, we introduce an MMW imaging system that leverages the GDD as a heterodyne receiver integrated with Frequency-Modulated Continuous Wave (FMCW) technology to extract 3D spatial information [11]. Our approach achieves significant advancement by enabling 3D MMW imaging using a GDD in electrical mode for the first time, which detects the variation in discharge current upon MMW incidence. The use of GDD technology allows for cost-effective, high-sensitivity MMW detection without the need for complex and costly traditional detectors, marking a breakthrough in the field. Operating in the W-band (100-110 GHz), our system employs a chirped FMCW signal split into object and reference paths via a waveguide splitter. MMW reflections from the