Numerical study of microwave propagation through a uniformly created fast growing plasma slab

Interaction between an electromagnetic wave and fast growing plasma in a bounded region can lead to the spectral change of the original wave. If the plasma density is changed suddenly and uniformly throughout a volume having its linear dimensions larger than several wavelengths of the wave, such an interaction can be characterized as an initial value problem which leads to the creation of new waves having frequencies different from that of the original wave. The results are realized by the fact that the new plasma medium is characterized by new dielectric property which imposes a different phase velocity to the propagating wave. Such an initial value problem has recently received considerable attention because of its potential to the development of a new technology for enhancing frequency tunable capability of existing sources microwave. In this work, the same physical mechanism is applied to convert a monochromatic source microwave into a frequency upshifted and chirped periodic pulse. It is done by generating a fast growing plasma slab repetitively in time and requiring the plasma density in the slab quickly to exceed the cutoff density of the incident wave. Therefore, the pulse length is determined by the transit time of the wave propagating through the plasma and the repetition rate of the plasma discharge defines the period of the pulse. By tapping the growth rate of the plasma, the frequency spectrum of the pulse can be made to be uniformly distributed around an up-shifted central frequency. A frequency up-shifted and chirped periodic pulse is, thus, generated. A theoretical model of the problem is developed for numerical simulation, which is performed to determine the required plasma growth rate profile in order to achieve the desired frequency up-shift and chirping. This study is intended to instruct how to up-grade the current experiment