The detailed spectral content of strong Langmuir turbulence generated
by an electron beam is investigated experimentally in an unmagnetized
plasma. It is found that the power spectra of the electrostatic waves
follows a consistent, reproducible pattern in which the Langmuir energ
y is concentrated in high frequencies (410 MHz) immediately following
a collapse event, transitions to lower frequencies between events, wit
h collapse taking place at the lowest frequencies (350 MHz). Also, aft
er a collapse event, the electrostatic wave intensity falls by about t
wo orders of magnitude in a microsecond, but then continues to fall ov
er the next approximately 10 mu s by an additional factor of 10 before
beginning to build toward the next collapse. The spectral width and e
lectron saturation current also exhibit reproducible patterns. Measure
ments are also performed to determine the low-frequency, ion acoustic
spectral content. It is found that the peak of the ion acoustic spectr
um scales inversely with the average time between collapse. (C) 1997 A
merican Institute of Physics.