Experimental and numerical study of electromagnetic wave trapping in a time-varying periodic plasma

Abstract

Summary form only given. It is well known that as an electromagnetic wave propagates through a rapidly growing plasma it will have its frequency spectrum up-shifted. If the plasma additionally has a periodic spatial configuration, the incident wave can excite many Floquet modes of the structure. Many of these modes often elude experimental detection as the multiple scattering processes required to excite higher order modes does not provide for efficient wave-plasma interaction. A way to achieve more efficient interaction is to use a periodic plasma several free space wavelengths long to trap the incident electromagnetic wave. Considering such a structure, as the plasma density grows from zero, the incident wave initially sees a small plasma-free space discontinuity. This provides for a large transmission (and small reflection) coefficient into (from) the structure. In the time it takes the wave propagates to the far end of the structure the plasma continues to grow. At the boundary between the final plasma layer and free space, the plasma density has increased and the reflection coefficient at this boundary is greater than the one encountered at the beginning of the structure. Thus some of the wave energy is trapped within the structure, where it can effectively interact with the plasma, and alter its spectral content. The trapping process is expected to be more effective for the down-shifted Floquet modes. An experiment exhibiting this phenomena confirms that the amplitude of frequency altered pulses is vastly enhanced over the case of a single plasma slab. Experimental results, along with related numerical simulations, showing the large down-shifted lines are presented.