Observation of abnormal discharge formation in microwave rocket and its quantitative impact on thrust performance

• Abnormal discharge events were visualized in non-conductive thrusters. • Carbon contamination in exhaust gases was identified as a possible cause. • A significant decrease in primary ionization front propagation velocity was observed. • Thrust reduction caused by abnormal discharges was quantitatively evaluated. Microwave Rocket is a beamed energy propulsion (BEP) concept in which thrust is generated by a millimeter-wave (MMW) beam transmitted from a remote power station. The thruster operates on a pulse detonation engine (PDE) cycle driven by millimeter-wave-supported detonation (MSD), where an ionization wave propagates supersonically inside the thrust tube and drives a shockwave. However, abnormal discharges that ignite away from the thrust wall during the PDE cycle can disrupt MSD formation, leading to severe thrust reduction. The initiation mechanisms of such abnormal discharges, as well as their quantitative impact on propulsion performance, remain poorly understood. In this study, we investigate four abnormal discharge events (occurrence rate ∼ 13%) that were randomly observed during double-pulse operation with a 550 kW MMW beam in dielectric-structured thrusters. Ionization-front dynamics were optically measured, and gauge-pressure histories at the thrust wall were recorded. Compared with normal operation, the occurrence of abnormal discharges reduced the propagation velocity of the primary ionization front by approximately 65%, indicating a comparably large loss of MMW energy due to scattering and/or absorption in the abnormal discharge region. The associated loss of plateau pressure at the thrust wall and the resulting thrust degradation were quantitatively confirmed. Furthermore, orange emission was observed near the abnormal discharge initiation region, suggesting that soot contamination in the exhaust plume may trigger abnormal discharge by locally enhancing electron density and/or lowering the breakdown threshold. These findings provide valuable insights not only for the design of high-performance, reliable Microwave Rocket systems, but also for the broader field of BEP systems employing high-intensity electromagnetic beams.