Plasma Bullet Formation in Liquid-Damped Atmospheric Helium Flow

Plasma bullets are often observed in atmospheric He or He/O₂ plasma jets with a floating or solid downstream electrode. The fast propagating ionization wave associated with plasma bullets has been recently suggested as a way to deliver reactive plasma species into a diseased cavity in human such as lung cancer. In such scenarios, however, the plasma-forming gas is usually damped by a large quantity of water, and the plasma-treating sample is always covered by biological fluid. Does the presence of H₂O molecules in the gas flow and in the sample quench plasma bullet and compromise the delivery of reactive plasma species? We present an experimental study to show that plasma bullet persists up to 1200 ppm of H₂O in an atmospheric helium flow and travels at a speed of at least 5 km/s.