Dual Pulse Enhancement of Laser Induced Plasma Discharge for Flow Control Applications

This study investigates the use of combined femtosecond and nanosecond laser pulses to enhance laser-induced plasma discharges at high repetition rates and elevated pressures. For this purpose, synchronized and delayed laser pulses of 0.5 - 2 mJ energy were focused into a pressurized air chamber with visual diagnostics conducted using high-speed Schlieren imaging. While femtosecond pulses alone produced steady, localized heating, the addition of ns pulses produced brighter breakdown events accompanied by strong shock waves. While the breakdown frequency was observed to increase with chamber pressure, bright sparks remain relatively rare at approximately one per hundred femtosecond pulses. Imaging reveals that nanosecond breakdown events are localized near the femtosecond laser beam axis, although avalanche breakdown occurred most often between 0.14 – 0.20 ms after femtosecond pulses, rather than coinciding with the pulses. This was further established by the observation of no significant difference between synchronized and unsynchronized pulse trains, despite the requirement of femtosecond laser excitation to observe avalanche breakdown. These findings suggest that while the initial fs laser ionization is insufficient to seed most nanosecond breakdown events, the interaction between pulses may play a crucial role in producing species, such as negative ions, that stimulate breakdown on subsequent nanosecond pulses.