Generation of large amplitude low frequency waves by the resonant absorption of a short microwave pulse

Abstract

Summary form only given. Interaction of an electromagnetic wave with a non-uniform plasma is relevant to different applications such as laser-based accelerators, inertial fusion, radio wave propagation in the ionosphere etc. In the present experiment, resonant absorption of a short microwave pulse in an unmagnetized laboratory plasma is investigated. The pulse duration (/spl tau/ = 100 ns) was chosen to be of the order of an ion plasma period (/spl tau/ = 2/spl pi///spl omega//sub pi/, where /spl omega//sub pi/ is ion plasma frequency) to study the nonlinear plasma responses in the ion acoustic regime. When such short microwave pulse interacts with the laboratory plasma, generation of the large amplitude density perturbations in the resonant absorption region has been observed. The characteristic frequency of these perturbations is below the ion plasma frequency, i.e. in the ion-acoustic frequency range. The density perturbations propagate into the under-dense plasma as a wave packet, which, however, does not satisfy the dispersion relation for the ion-acoustic mode calculated by the linear theory. In particular, the wave phase velocity is much higher than the ion sound velocity and depends on the amplitude of the density perturbations. It has also been shown that the generation of the density perturbations correlates with the propagation of a high-energy ion bunch created near the resonance layer. A possible mechanism of the wave generation can include the excitation of large amplitude wakefields by injecting a short time microwave pulse in a resonance absorption region. Indeed, the plasma wave excited in the resonance region can accelerate ions to produce a bunch of high-energy ions, which propagates down the plasma density gradient with the velocity much greater than the ion acoustic velocity. This ion bunch, with its duration of the order of ion wave period, can excite the wakefields, which have been observed in our experiment as the large-amplitude low-frequency plasma density perturbations.