Observations of Stimulated-Brillouin Scattering Initiated by Ponderomotive Force Density Fluctuations

Abstract

This Letter reports the first observations of stimulated Brillouin scattering in microwave interaction with a plasma, as verified by the satisfaction of the frequency-and wavelength-matching rules and growth rate. A small amount of chamber reflectivity causes ion fluctuations due to the standing-wave ponderomotive force, which then serve as an enhanced noise level for the initiation of the instability. PACS numbers: 52.25.Ps, 52.40.Mj Stimulated Brillouin scattering (SBS) in a plasma is due to the interaction of an electromagnetic wave and an ion acoustic wave. The resultant wave may be scattered at various angles, but the scattering maximizes in the backward direction, with a characteristic frequency downshift of the ion acoustic frequency, oo s = kc s , where c s is the ion sound speed and k is given by momentum considerations as 2k 0 , where k Q is the incident wave number. This process is important in laser fusion, since the scattered energy is not available to the compression process. SBS has been observed in laser-plasma interactions, 1 * 2 identified by the characteristic oo s 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 fluctuations. 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 stimulated Brillouin-scattered wave. After an initial fast growth time, we find the SBS reflectivity growth rate to agree with the classical calculations. 2The experiments were performed in an unmagnetized plasma of 75 cm diameter and 200 cm length, produced by a multifilament dc discharge with surface multidipole confinement. Typical operating parameters were gas filling pressure (1-3) xlO" 4 Torr, electron density n e = 10 10 -10 n cm" 3 , electron temperature T e -2 eV and electron-ion temperature ratio T e /Ti= 10-12. Various gases were used including hydrogen, helium, neon, and argon. Experiments were performed at a number of microwave pump frequencies in the range 3-16 GHz, with peak powers up to 1 MW and pulse widths, T P , from 0.1-20 p,s. The wave was launched along the chamber axis by a high-gain (-20 dB) gridded horn, the radiation pattern of which gave an effective interaction length of 70-120 cm. In order to avoid possible complications introduced by effects other than SBS, e.g., the parametric decay instability, the electron density was adjusted to 0.1 of the critical density, n c1 for all experiments.The incident and backscattered electromagnetic waves were separated for detection with use of either a circulator or directional coupler, and fed into a square law detector for power measurements or a spectrum analyzer for frequencyshift measurements. Ion waves were detected with movable Langmuir probes, after the microwave pump is off, thus avoiding the problem of rf pickup.Since T e /T i »l, Landau damping is rath...

Measurement of absolute growth rates and saturation phenomena for stimulated Brillouin scattering in aCO2-laser–irradiated plasma

“…For intensities well above threshold, y 0 depends very weakly on T e /T h and so higher ion temperature should not strongly affect y 0 -13 Kroll 12 predicts that between times // V_ and // V ia (as in the present experiment), the instability should exhibit a transient behavior with the amplitudes increasing exponentially with the square root of time. Such behavior was not seen either in the present experiment or in a previous microwave experiment 14 where good agreement was observed between the measured growth rate and that predicted by Eq. (1).…”

Coupling between electron plasma waves in laser–plasma interactions

“…In the experiment, where the fast and slow waves are driven by collinear optical mixing and SRS, respectively, the amplitudes of the daughter modes will be modified by coupling to both the laser and possibly stimulated Brillouin scattering ͑SBS͒. 7,8 In addition, the modes near zero frequency may be ion or electron Landau damped. Thus, we expect to reproduce in the experiment the frequency and wave number spectra of the coupled modes only in a qualitative sense.…”

Spatial Profiles of Stimulated Brillouin Scattering Ion Waves in a Laser-Produced Plasma

“…To relate the calculated gain to the measured n/n we need two things: (1) a relation between n/n and the scattered light intensity, and (2) an estimate of the initial noise level from which the instability grows. %e use the ion acoustic wave equation in one dimension to relate n/n to the light energy [15] EL, and setting tt = E,/EL gives a' n/at'c, a' n/ax' = 4koc, 'n, u(vo/v, )'cos 2kox, (5) which has the solution n/n, = a(vo/v, ) cos(2kox)(1cos toot).…”