Observation of ion wave streamers and low frequency sheath instability by the resonant absorption due to nonlinear interaction of microwave-plasma

Abstract

Unmagnetized, inhomogeneous laboratory plasma irradiated by an oblique p-polarized microwave with pulse length 0.2–1.5 μs and power P=1–2 kW is studied. The incident electromagnetic wave is linearly converted into an electrostatic plasma wave when the incident wave frequency ω0 is equal to the local plasma frequency ωp. The localized linear enhancement of the driven oscillating field can lead to nonlinear phenomena driven by the ponderomotive force, which expels electrons from the resonance region, and the resulting ambipolar electrostatic fields also expel the ions, creating density cavities at the resonance region. Expelled ions tend to form an ion bunch and accelerate up to energies greater than 10 kTe. After all these processes are achieved, it has been observed in the experiment that the density cavity develops as ion wave streamers and propagate both up and down the density gradient from the resonant layer. It is observed that the downward streamer velocity Vdown and upward streamer velocity Vup have the relation as Vdown>Cs>Vup. Another physical phenomenon, called the low frequency sheath instability, in the plasma sheath area created by the accelerated ion bunch near the resonant region, is also observed in the experiment.