There are a few ways to do this. First, you can animate the Number graph and set a keyframe with a value of 0 at the frame you want the particles to stop emitting. That will kill any new particles from being born.
You can also just fade the particles out by setting a key with a 0 value on the Visibility graph. Particles will still be created, but they’ll be invisible.
If you just want to turn the emitter off entirely, you can set a key on the Active graph which acts like an On/Off switch. All the particles from that emitter will vanish at that point.
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Elvis Deane!
The Apprentice Magician’s Guide to particleIllusion
The particleIllusion FAQ