Stimulated Brillouin scattering (SBS) in a plasma is due to the interaction of an electromagnetic wave and an ion acoustic wave. SBS has been observed in laser-plasma interactions, identified by the characteristic frequency shift. Here we report the first observations of SBS in microwave interaction with plasma, finding not only the frequency downshift in the reflected wave, but directly measuring the wavelength of the ion acoustic fluctuation. An interesting feature of these experiments is that a small chamber reflectivity sets up ion fluctuations which then serve as an enhanced noise level for the initiation of the SBS. After an initial fast growth time, we find the SBS reflectivity growth rate to agree with the classical calculations.
Figure 1 shows representative frequency spectra of the incident and reflected waves. The red-shifted component of the reflected wave appears when the incident power exceeds threshold. For all ion species and pump wavelengths the measured red shift was found to be in excellent agreement with the appropriate acoustic frequency. Figure 2 shows the normalized density fluctuation level n/n0 observed at a fixed position as a function of time. The ponderomotive-force-driven fluctuations are clearly visible in the low power He case, where the pump power was below threshold for SBS. The measured growth time of the backscattered wave, after the initial ponderomotive-force-driven response, is shown in Fig.3 as a function of v0/vs, for a hydrogen plasma.