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

Lower Hybrid Current Drive (LHCD) systems are well suited to drive current in the outer third of the minor radius in tokamak reactors, as well as drive off-axis current, which is necessary to sustain both the hybrid and steady-state plasmas. Lower Hybrid waves are coupled to plasmas using rf waveguide array antennas (launcher) operating at low frequencies (2-5 GHz). Although LHCD systems have shown good coupling performance with a power reflection coefficient of ~3%, the power handling capability is limited by the peak electric fields in the antenna. Further increase of the plasma density can significantly lower the peak electric fields at the antenna mouth improving the power handling capability, however, it results in increased microwave power reflections making the system less efficient. Therefore, a method to increase power handling capability in LHCD launchers without power reflections is highly desirable and will offer an effective coupling of the lower hybrid waves with plasma, leading to a more efficient plasma current drive. In this project, we propose to develop a lower hybrid current drive 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 a high-Q resonator to ensure that the resonator fully traps all impinging energy without reflection; this approach can be applied to open systems such as waveguides and antennas. The system will include a LHCD launcher module based on the multijunction design which will be driven from a high-power microwave amplifier (klystron), and a low-level microwave (LLRF) system that will modulate the shape of excitation signal to achieve the virtual critical coupling regime. In Phase I of the project, we plan to conduct numerical simulations to study the effects of a complex frequency excitation signal used in the VCC concept on the coupling of lower hybrid waves to plasma. We will perform the rf design of an LHCD antenna module with VCC and estimate its efficiency. Finally, we will perform an experiment with a 5 MW-power S-band klystron, available at RadiaBeam, to demonstrate the VCC effect in a high-power environment. In Phase II, we will focus on the development of the LHCD system and experimentally test its performance to prove stable and efficient plasma current drive using virtual critical coupling of microwaves. This project will enable RadiaBeam to deliver a LHCD system free of power reflection using virtual critical coupling with improved power handling which could be used in numerous nuclear fusion research facilities and experimental fusion devices, such as tokamaks, stellarators, in the U.S. and around the world. Additionally, the technology developed in this project could also be applied in high-power VCC couplers which can become a revolutionary tool for high-power accelerators, such as electron-positron colliders or novel proton radiotherapy linear accelerators that require operation at ultra-high gradients and power levels of >20 MW.