FREQUENCY DOWNSHIFTING AND TRAPPING OF AN ELECTROMAGNETIC-WAVE BY A RAPIDLY CREATED SPATIALLY PERIODIC PLASMA

Experimental and numerical results of the interaction of electromagnet ic waves with rapidly time varying spatially periodic plasmas are pres ented. It is shown that a number of Roquet modes, each with their own oscillation frequency, are created during the interaction. Included am ong these modes are downshifted waves which will not exist in the sing le slab case, and also waves with a larger upshifted frequency than on e can obtain with a single plasma layer of the same density. In additi on, the periodic structure is characterized by pass and stop bands tha t are different from those of a single plasma layer, and the frequenci es of the downshifted modes falling in the stop band of a single plasm a layer. Therefore these waves are trapped within the plasma structure until the plasma decays away. To show this phenomenon a chamber exper iment is conducted, with the periodic plasma being produced by a capac itive discharge. The power spectrum recorded for waves interacting wit h the plasma shows vastly improved efficiency in the downshift mechani sm, which the numerical calculations suggest is related to the trappin g of the wave within the plasma. Reproducible results are recorded whi ch are found to agree well with the numerical simulation.