Reflection-free microwave plasma current driver based on virtual critical coupling

Lower Hybrid Current Drive (LHCD) systems are key for advanced tokamak operation, enabling efficient off-axis current generation that enhances plasma stability and confinement in steady-state configurations. Although LHCD systems have shown good coupling performance with a power reflection coefficient of ~1-3%, impedance mismatch between plasma surface and launcher caused by electron density fluctuations can result in reflection of incident power from the plasma surface, inducing standing waves within the launcher structure that increase voltage in the vacuum region of the launcher. This places a fundamental limit on maximum power handling capability of a launching structure as peak voltage is limited by arcing induced by multipactor breakdown. In this project, we propose to develop a LHCD system incorporating the virtual critical coupling (VCC) technique to eliminate power reflections and therefore increase its power handling capabilities. The VCC concept consists of temporally shaping a complex frequency excitation signal in microwave resonators to ensure that the resonator fully traps all impinging energy without reflection. The system will include a LHCD launcher module based on the DIII-D tokamak design which will be driven from a high-power microwave amplifier (klystron), and a microwave circuit that will modulate the excitation signal to achieve the VCC regime. In phase I, we have optimized a VCC signal to eliminate power reflections in a 2856 MHz resonator. We developed and built a LL microwave circuit and experimentally demonstrated VCC at low and high power using a pulsed power S-band klystron. Additionally, we have modified the VCC model to account for the sudden appearance of cold plasma in a resonant system to study the compatibility of the VCC mechanism with a LHCD system and performed microwave simulations of the 4.6 GHz LHCD launcher module designed for DIII-D tokamak. Phase II work plan will extend the VCC model to optimize a complex excitation signal to achieve VCC specifically for the 4.6 GHz wave excitation required by the DIII-D LHCD launcher, plasma simulations will be performed in COMSOL. A VCC circuit compatible with the DIII-D LHCD system will be designed and built. We will fabricate a launcher module in GRCop-42 by Laser Powder Bed Fusion (L-PBF) Additive Manufacturing (AM). We will adapt surface post-processing techniques for the AM parts to address key requirements such as reducing porosity, improving surface finish, and ensuring ultra-high vacuum compatibility. Finally, the improved LHCD launcher will be tested with a simulated plasma load that emulates the cold plasma dispersion relation in a tokamak. This project aims to develop an LHCD system with VCC to eliminate power reflection and enhance power handling, making it suitable for fusion research facilities worldwide, including tokamaks and stellarators. The VCC approach could also benefit other microwave-based plasma heating and current drive systems, such as Electron Cyclotron Resonance Heating (ECRH) and Ion Cyclotron Resonance Heating (ICRH).