A mode coupling theory for the electromagnetic accordion

Summary form only given. A new scheme, referred to as an electromagnetic accordion, which directly converts a static electric field into coherent microwave radiation by a laser-produced ionization front (i.e., a moving plasma/gas boundary) in a gas-filled capacitor array is investigated theoretically. The output radiation has a frequency of /spl omega//sub p//sup 2//2k/sub 0/c, where k/sub 0/ is the wave number of the static field, and an amplitude approximately equal to the static field intensity. This scheme has the potential of producing high-power, ultra-short microwave pulses in a broad frequency range. A proof-of-principle experiment by USC/UCLA recently verified the frequency scaling law of this scheme. However, the theoretical model developed in previous work is based on a simplified planar approximation. This approximation is only valid near the axis of the front's propagation when large upshift occurs. The existing model can only predict a single-frequency output radiation and therefore loses the information of frequency and mode structure of the output radiation. Therefore, we are developing a mode coupling theory to model this scheme. We consider waveguide effects and include all the modes that can be excited behind the ionization front. We calculate the frequency as well as the transmission coefficient of each mode, and from this the output frequency spectrum is obtained. We also consider the use of other geometry, one such example will be a 2-D stripline structure. Preliminary results show multi-mode excitation and spectrum broadening which are ignored by previous model. This new model helps us to understand the mechanism in detail and is useful for the design of future experiments.