Nonlinear subharmonic generation in nonuniform plasmas

Direct subharmonic wave generation in a nonuniform plasma is considered. That mechanism exists only when leaking surface waves can be parametrically excited. An expression for the instability growth rate, which includes collisions, resonance absorption and leaking losses is derived. A possibility of generating subharmonics at omega 0 /4, 3 omega 0 /4 and 5 omega 0 /4, where omega 0 is the pump wave frequency is pointed out, and the corresponding field intensities are estimated. The conditions for total energy absorption are discussed and the pump wave intensity, which produces complete absorption, is obtained for a plasma with a steep density gradient.