TIME-RESOLVED MEASUREMENTS OF SECONDARY LANGMUIR-WAVES PRODUCED BY THE LANGMUIR DECAY INSTABILITY IN A LASER-PRODUCED PLASMA

Direct observations of secondary Langmuir waves produced by the parame tric decay instability of primary Langmuir waves are presented. The me asurements have been obtained using Thomson scattering of a short-wave length probe laser beam and are resolved in time, space, frequency, an d wave number. The primary Langmuir waves were driven by stimulated Ra man scattering (SRS) of a smoothed laser beam in a preformed plasma. M easurements of the amplitude of the density fluctuations associated wi th primary and secondary Langmuir waves show that the threshold of the Langmuir decay instability (LDI) is close to the threshold of the Ram an instability. This is in agreement with theoretical predictions. How ever, the ratio of amplitudes of the density fluctuations associated w ith both secondary and primary Langmuir waves does not agree with exis ting theories of SRS saturation due to LDI cascading and/or strong Lan gmuir turbulence in homogeneous plasmas. An explanation based on the i nteraction beam intensity distribution produced by the random phase pl ate in the plasma is discussed. (C) 1998 American Institute of Physics .