Light pulse doesn't disappear, it goes on forever. I don't know much about the game, but if particle count goes high, it might put a limit on it, or it may bug out while trying to calculate the movement of previous pulses. Go to settings menu and click the X icon near the "Create Dust" option to remove the particles, and try again.
Also, if you're trying to pulse near a black whole, it won't even appear. Haven't tried, but if the mass of planet are high enough, it might cause a glitch or something.