Toward Integrated Microwave Diagnostics for Future Fusion Devices: Design of a Compact Waveguide Diplexer

Microwave diagnostics are essential for plasma control and physics studies in magnetic confinement fusion, providing measurements of electron density, electron temperature, and energetic-particle behavior. However, diagnostic access in future burning-plasma reactors will be severely constrained by breeding blankets, neutron shielding, and compact reactor geometries, requiring reduced blanket opening areas while maintaining diagnostic capability. This work presents a compact waveguide diplexer for front-end integration of multiple microwave diagnostics. The proposed diplexer combines a waveguide T-junction with two iris-coupled sixth-order Chebyshev bandpass filters, enabling microwave reflectometry and electron cyclotron emission (ECE) diagnostics to share a common transmission line while operating in separate frequency bands. As an initial study, the diplexer is designed for the 65-70 GHz reflectometry and 85-90 GHz ECE systems on the J-TEXT tokamak. Full-wave electromagnetic simulations demonstrate insertion losses below 1 dB and channel isolation exceeding 40 dB across both operating bands. The proposed methodology provides a compact and scalable approach to microwave diagnostic integration and establishes a practical basis for multi-diagnostic front-end systems in future fusion devices.