Spectral content of strong Langmuir turbulence in the beam plasma interaction

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 energy is concentrated in high frequencies (410 MHz) immediately following a collapse event, transitions to lower frequencies between events, with collapse taking place at the lowest frequencies (350 MHz). Also, after a collapse event, the electrostatic wave intensity falls by about two orders of magnitude in a microsecond, but then continues to fall over the next approximately 10 mu s by an additional factor of 10 before beginning to build toward the next collapse. The spectral width and electron saturation current also exhibit reproducible patterns. Measurements are also performed to determine the low-frequency, ion acoustic spectral content. It is found that the peak of the ion acoustic spectrum scales inversely with the average time between collapse. (C) 1997 American Institute of Physics.