Numerical simulations of three-wave interactions among Langmuir, ion-acoustic, and electromagnetic waves by means of an efficient algorithm developed here for solving full Zakharov equations show that total electric field collapses if the electrostatic field collapses when plasma temperature is much less than 0.5 MeV and thus relativistic effects can be neglected, and that the electromagnetic wave energy generated in the high-temperature Langmuir collapse comes to no less than one-tenth of the Langmuir wave energy even if the mean intensity of the collapsing wave packet increases by a factor of less than 5.

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