INTERACTION OF AN ELECTROMAGNETIC-WAVE WITH A RAPIDLY CREATED SPATIALLY PERIODIC PLASMA

The interaction of electromagnetic waves with rapidly created time-var ying spatially periodic plasmas is studied. The numerical results of t he collisionless case show that both frequency upshifted and frequency downshifted waves are generated. Moreover, the frequency downshifted waves are trapped by the plasma when the plasma frequency is larger th an the wave frequency. The trapping has the effect of dramatically enh ancing the efficiency of the frequency downshift conversion process, b y accumulating incident wave energy during the plasma transition perio d. A theory based on the wave impedance of each Floquet mode of the pe riodic structure is formulated, incorporating with the collisional dam ping of the plasma. Such a theory explains the recent experimental obs ervations [Faith, Kuo, and Huang, Phys. Rev. E 55, 1843 (1997)] where the frequency downshifted signals were detected repetitively with cons iderably enhanced spectral intensities while the frequency upshifted s ignals were missing.