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When a non-uniform unmagnetized plasma is irradiated by a short microwave pulse with length of the order of an ion-wave period, the travelling wavepacket of the electron plasma wave, and nonlinear large-amplitude waves (δn/n 0 ≈ 40%) in an ion-wave regime have been observed. The observed large-amplitude waves typically have two frequencies. The higherfrequency wave is an ion wakefield excited by the travelling wavepacket of the electron plasma wave, while the lower-frequency mode, also called a density cavity streamer, can be interpreted as a pseudowave induced by the supersonic ion bunch with a typical energy of 25 eV produced at the resonance layer. The experimental results are in reasonable agreement with a theoretical model based on the Mach cone structure.