A compact and reconfigurable millimeter-wave back-end for next-generation fusion reactors

Next-generation fusion reactors, such as DEMOnstration Power Plant (DEMO) [ https://doi.org/10.1088/0029-5515/56/2/026009 Nucl. Fusion 56 (2016) 026009], present significant challenges for plasma diagnostics due to limited in-vessel access and harsh operating conditions. Millimeter-wave (MMW) diagnostics and in particular Frequency Modulation of a Continuous Wave (FMCW) reflectometry offer a robust method for measuring electron density, a critical parameter for plasma position and shape control [ https://doi.org/10.1088/0029-5515/52/3/032003 Nucl. Fusion 52 (2012) 032003]. However, achieving comprehensive plasma coverage demands numerous probing channels with wide frequency bandwidths and compact, efficient back-ends [ https://doi.org/10.1016/j.fusengdes.2023.113512 Fusion Eng. Des. 190 (2023) 113512]. This paper details a redesigned MMW back-end system that overcomes limitations of a previous prototype. The system utilizes readily available Monolithic Microwave Integrated Circuits (MMICs) to establish a base frequency range of 10 GHz to 20 GHz, which is extended to 160 GHz through external frequency multipliers, enabling detailed plasma characterization across an ultra-wideband. To address component obsolescence, the system underwent a complete Printed Circuit Board (PCB) redesign. This work details the new back-end design and its subsequent performance evaluation.