Experimental studies of Raman scattering from foam targets using a 0.35 μm laser beam

Abstract

Experimental studies of the forward scattered spectrum and stimulated Raman backscattering from foam targets are presented. An attempt has been made to isolate the effects of the presence of the quarter critical density on the Raman spectrum by creating plasmas with various peak plasma densities. The plasmas created had a length larger than 600 μm and a variable peak density between 0.11nc and slightly higher than 0.25nc. The total Raman reflectivity in the backward direction is of 0.3%, with 80% of its energy being scattered in a range of frequencies between 470 nm and 500 nm for all the targets used. The scattered intensity near the half-harmonic region shows a weak dependence on the presence of the nc/4 density layer. The forward scattered spectrum obtained from targets with average densities between 0.11nc and 0.22nc shows a broadband of frequencies similar to the backscattered spectrum indicating that the forward spectrum is probably being seeded by the beating of the plasma wave from stimulated Raman backscattering (SRS-B) with the ion acoustic wave from stimulated Brillouin scattering (SBS).

Time-dependent measurement of high-power laser light reflection by low-Z foam plasma

“…The onset of the parametric instabilities, including the two-plasmon decay (TPD), is known to occur when phase-matching conditions are fulfilled, and is enhanced when the characteristic length of the plasma density gradient is large [42,43] . Experiments about the irradiation of foams of subcritical density with small pores [32][33][34] reported a very low contribution to the scattered laser light in the focusing optics due to SRS, of about 1% or less of the incident energy. The SBS light scattered in the focusing optics was measured to be lower than 8% of the incident light [33,34] .…”

“…The experimental and theoretical analysis of light reflection is very important for understanding the physics of laser interaction with foams as well as the properties of the laser-produced plasma in foam targets. The reflected light from foam plasmas has been measured, so far, both through a time-integrated diagnostic [17,21,30,31] and through a timeresolved diagnostic, but in this case only devoted to the study of plasma instabilities in subcritical foam targets with a pore size of the order of a few micrometers, at most [32][33][34][35] .…”

Three-wave parametric instabilities in long-scale-length, somewhat planar, laser-produced plasmas

“…After becoming underdense, the target expands and the maximum density decays with time. To produce larger plasmas with a given laser energy, one can use one laser beam (or more) to preform the plasma and another to drive the instabilities (Figueroa et al 1984;Labaune et al 1985;Aboites et al 1986). Such targets have proven an effective medium for the study of instabilities at densities below /i c /4, such as SRS (Turner et al 1985(Turner et al , 1986Tarvin et al 1986;Drake et al 1988a;Drake 1989b;Drake et al 1989a or stimulated Compton scattering (Drake et al 1990), by allowing kJ lasers to produce large enough scale lengths (i.e., L n /\ 0 > 1,000 over significant volumes).…”

“…It is also possible to produce Iong-scale-length, planar plasmas by using low-density foam targets (Tanaka et al 1985;Figueroa et al 1987) or gas-jet targets (Mayer et al 1980;Meyer et al 1984) produced by supersonic laminar flow. We avoided these options because of the types of uncertainties they introduce.…”

Time-resolved x-ray imaging of high-power laser-irradiated underdense silica aerogels and agar foams

“…Figueroa et al [8] measured the forward-and back-scattered Raman spectrum from 0.11 -0.5 n, dextran foams using h = 0.35 pm, 850 ps laser irradiation at an average intensity of 1.5 x 1015 W/cmZ. They observed that the back-scattered spectrum was peaked between h = 0.47 pm and 0.50 pm, corresponding to scattering from regions of the plasma where the electron density is 0.05 -0.07 n , and observed that the spectral position of this peak did not vary with initial target density.…”