Spontaneous low-frequency oscillations have been observed in the circuit of a positively biased electrode when the ambient nonuniform plasma is irradiated by a microwave pulse of short duration, which is approximately equal to the ion-plasma period. The instability with its characteristic frequency below the ion-plasma frequency is driven by an accelerated ion component interacting with the sheath of the electrode. A qualitative model of the instability is suggested.

Unmagnetized, inhomogeneous laboratory plasma irradiated by a high power (eta=E(2)(0)/4pin(e)kT(e) approximately 5.0x10(-2)) short pulsed microwave with pulse length of the order of ion-plasma period (tau(pi) less, similar 2pi/omega(pi)) is studied. Large density perturbation traveling through the underdense plasma with a velocity much greater than the ion sound speed produced by the resonant absorption of the microwave pulse has been observed. In the beginning the density perturbation has large amplitude (deltan/n(0) approximately 40%) and propagates with a velocity of the order of 10(6) cm/s. But later its amplitude as well as the velocity decrease rapidly, and finally the velocity arrives with twice the ion sound speed. The oscillating incident electromagnetic waves enhance highly localized electric field by the resonant absorption process and develop time-averaged force field which pushes plasma electrons from the resonant layer. As the electrons are accelerated to be ejected, they pull plasma ions as a bunch with them by means of self-consistent Coulomb force. This suprathermal ion bunch can excite an ion-wave wakefield.

Comment on "Instability of the Shukla mode in a dusty plasma containing equilibrium density and magnetic field inhomogeneities" [Phys. Plasmas 11, 1732Plasmas 11, (2004] and "New resonance and cut-off for low-frequency electromagnetic waves in dusty magnetoplasmas" [Phys.

The dynamical behavior of ion density perturbations propagated at low-frequency wave nature is experimentally observed in microwave-plasma interaction. An unmagnetized, inhomogeneous laboratory plasma irradiated by an obliquely incident microwave with maximum power P = 10 kW and pulse width approximately ion plasma period ͑ pi Ϸ 2 / pi ͒ is studied. The p-polarized electric-field component of the interacted microwave of frequency 0 leads to a nonlinear phenomenon driven by the ponderomotive force by the process of resonance absorption at the critical layer where 0 = p is satisfied. The nonlinear ion density perturbations are created from the resonant layer and propagated to an underdense plasma as an electrostatic wave nature.

Unmagnetized, inhomogeneous laboratory plasma irradiated by an oblique p-polarized microwave with pulse length 0.2-1.5 s and power Pϭ1-2 kW is studied. The incident electromagnetic wave is linearly converted into an electrostatic plasma wave when the incident wave frequency 0 is equal to the local plasma frequency p. 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 kT e. 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 V down and upward streamer velocity V up have the relation as V down ϾC s ϾV up. 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.