Excitation of nonlinear ionization waves caused by three-wave interaction inHe positive columns

The investigation of three-wave interactions among self-excited nonlinear ionization waves in a diffusion-controlled helium positive column showed that two self-excited waves generate the third one by resonance due to nonlinear interaction satisfying the selection rule. There is a mismatch in wave number in three-wave interaction but none in frequency. Strong phase coherence is detected in the nonlinear interaction closely related to the modulational instability.