The Staggered Double-Vane Traveling-Wave Tube(SDV-TWT) is attracting worldwide attention for its high power, wide bandwidth, ease of fabrication, and excellent heat dissipation. In this work, an ultra-wideband, high-power SDV interaction circuit driven by a sheet electron beam is designed for dual-mode operation in the G-band. Simulation results show that exploiting two synchronous modes can further extend the operating bandwidth of the SDV-TWT. By employing Bragg reflectors to isolate the signal and a segmented structure to suppress oscillations—both well-established techniques for stable traveling-wave amplification—the system generates coherent terahertz radiation with superior transmission, high stability, broad bandwidth, and high power, achieving an output power exceeding 100 W and an operating bandwidth greater than 60 GHz. Moreover, a planar three-layer high-frequency circuit fabricated with micro-/nano-scale processes is proposed, fully demonstrating the potential and feasibility of realizing a high-power, broadband, coherent terahertz source in practice.