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

Abstract

Growth rates and saturation of stimulated Brillouin scattering in a C0 2 -laser-irradiated plasma are investigated by picosecond Thomson scattering of ruby laser light and backscatter spectroscopy. The temporal growth rates are found to be consistent with theoretical predictions. The ion acoustic waves saturate at a fluctuation amplitude of <20% and subsequently show a modulated temporal behavior, as well as sidebands in the backscatter spectra consistent with saturation by strong ion trapping.PACS numbers: 52.35.Qz, 52.25.Rv, 52.35.Nx, 52.70.Kz Stimulated Brillouin scattering (SBS) is believed to be a potentially dangerous loss mechanism in proposed laser fusion schemes. In recent C0 2 -laser experiments, optical-probe-laser Thomson scattering has been used to investigate the temporal and spatial behavior of SBS. 1 " 3 Other experiments at 1.06 /xm (Ref. 4) and 10.6 ^m (Refs. 5 and 6) have examined the temporal development of SBS backscattered radiation. Theory 7 predicts that at least for small damping of the electromagnetic (incident and scattered) and ion waves, the ion fluctuation amplitude initially should grow absolutely. Measurements of absolute growth have not been reported previously either because of limited temporal resolution or because heavy damping limited the growth to the convective regime. In this Letter we report first measurements of the absolute growth of SBS ion waves in C0 2 -laser-plasma interactions using probe ruby-laser Thomson scattering. The results support previous theoretical predictions. 7 In addition, we would like to report on experimental evidence for dominant saturation by ion trapping obtained consistently from Thomson-scattering measurements and measurements of the backscatter spectra.The experiments were performed with a C0 2 -laser that produces 2-ns FWHM, 1.2-ns rise-time pulses of ^ 12 J focused to a measured Gaussian l/e radius of ~-50 /xm. The target consists of a pulsed supersonic N 2 jet, -1.5 mm thick, which flows from a twodimensional Laval nozzle into a 5-Torr He background. 8 The plasma evolution has been investigated previously. 9 Single-shot backscatter spectra were obtained with a 0.5-m spectrometer-image-dissector 10 combination and a Cu:Ge detector. The combined detector-gigahertz-oscilloscope rise time permitted some temporal resolution of the spectra.The Thomson-scattering arrangement is shown in Fig. 1. A cylindrical lens focuses a 6-ns ruby pulse to a line ( -1 mm high, -5 mm long) in the interaction region. Scattered ruby light is imaged onto the slit of a picosecond streak camera (Hamamatsu Temporal Disperser C1370-01/III Analyzer System) by means of two 30-cm focal-length lenses. The streak records are digitized and stored on tape for computer analysis. Two sets of scattering experiments were performed. In the arrangement of Fig. 1, a strip just in front of the first lens is imaged onto the slit in order to study the k spectrum of the SBS ion waves. Detected are waves in the approximate range k 0 ^ k ^ 4k 0 (k 0 = k C o 2 ), which is limited by the ...

Observation of the Parametric Two-Ion Decay Instability with Thomson Scattering

“…The SBS instability results from the resonant coupling of a high intensity laser pulse with an ion-acoustic wave to produce an electromagnetic wave scattered back from the plasma. It has been shown that SBS saturates for high laser intensities [1][2][3][4]. The two-ion decay, where a large amplitude ion-acoustic wave decays into two daughter waves, has been postulated as a saturation mechanism for SBS, particularly in medium-or high-Z plasmas [5][6][7][8].…”

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

“…SBS has been a concern in inertial confinement fusion (ICF) applications as a potential cause of decreased laser-target couphng efficiency. The instability continues to be of interest in a scientific sense because the seeding and saturation mechanisms are not well understood.Although Thomson scattering [6,7] has been used extensively to identify ion waves in interactions between plasmas and 10.6 p, m wavelength lasers [8,9], the plasma and laser parameters for ICF targets will be substantially different. SBS in short-wavelength laserplasma interactions has been studied via scattered light spectra near the incident laser wavelength, and, although there has recently been evidence of seeding of SBS by the two-plasmon decay (TPD) instability at n, /4 [10],the interpretation of spectra is confused by the role of the (unknown) expansion velocity profile.…”

“…The measured scattered signal can be related to the density fluctuation level n. If the scattering takes place from a coherent ion acoustic wave, then the fluctuation level is proportional to the square root of the scattered power level [8,14]:…”

Fluid nonlinear frequency shift of nonlinear ion acoustic waves in multi-ion species plasmas in the small wave number region

“…[2,3] Many methods have been taken to reduce the SBS scattering level, [4][5][6][7][8] including frequency detuning due to particle-trapping, [9][10][11][12][13][14] increasing linear Landau damping induced by kinetic ion heating, [15,16] nonlinear damping due to wave-breaking and trapping [17][18][19][20] or coupling with higher harmonics [21,22].…”