Excitation of ion-wave wakefield by the resonant absorption of a short pulsed microwave with plasma

Unmagnetized, inhomogeneous laboratory plasma irradiated by a high power (η=E²₀/4πn_ekT_e≈5.0×10^-2) short pulsed microwave with pulse length of the order of ion-plasma period (τ_pi≲2π/ω_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 (δn/n₀≈40%) and propagates with a velocity of the order of 10⁶ 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.