Transition from a resonant to a nonresonant parametric decay process was observed by increasing pumping power for the frequency range ωLH≲ω0≲3ωLH (ω0: pump frequency, ωLH: lower-hybrid frequency). A resonant backward ion cyclotron wave was excited for 1.5≲ω0/ωLH≲3 and 1≲U/Cs≲5 (U: electron drift velocity, Cs: ion acoustic velocity). On the other hand, a nonresonant ion-quasimode was excited for 1≲ω0/ωLH≲2 and U/Cs≳2.5. These exciting regions can be also verified by measuring the radial distribution of the amplitudes of these decay waves.
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