Anomalous‐Energy Runaway Electrons Originating in Fast Ionisation Wave During Pulsed Gas Breakdown

ABSTRACT Anomalous‐energy runaway electrons (RAEs), whose energy exceeds the maximum potential difference across the discharge gap, are widely observed in various plasma phenomena. This study investigates the origination of anomalous‐energy RAEs in fast ionisation waves (FIWs) formed during pulsed gas breakdown. Synchronised observation of RAEs and their induced bremsstrahlung X‐ray at different locations of the FIW is performed, which is combined with measurement of spatial‐temporal evolution of FIW potential. It is revealed that anomalous‐energy RAEs, preceding regular RAEs, have an ultrashort beam with a picosecond pulse width, and RAEs tend to maintain their energy during FIW propagation before being deposited into the anode. Particle‐in‐cell Monte Carlo collision (PIC‐MCC) simulation illustrates that the ‘pace‐matching’ between initial electron acceleration through cathode potential drop and potential lift‐up during FIW inception opens a narrow window for forming anomalous‐energy RAEs.