Virtual Critical Coupling in High-Power Resonant Systems

Abstract: Exciting high-power resonators pose challenges such as managing power reflections, which can cause energy losses and damage system components. This is crucial for applications like Lower Hybrid Current Drive (LHCD) systems in tokamaks, where plasma stability and confinement depend on efficient energy transfer. In this work, we introduce the Virtual Critical Coupling mechanism to address reflection-related challenges in S-band resonators. We theoretically designed a complex frequency excitation signal tailored to the resonator's characteristics, facilitating efficient energy storage and minimizing reflections without mechanical modifications. Using a custom low-level RF system, we conducted experiments at 32 mW and 600 kW with a 5 MW S-band klystron, demonstrating a ninefold reduction in reflection coefficients compared to traditional monochromatic excitation in high-power tests. This approach enhances the efficiency and stability of high-power resonant systems, potentially advancing nuclear fusion energy production.