The Observation of Enhanced Thomson Scattering in Submicron-Laser Plasmas

Frequency up-scattered light produced by laser-plasma interaction was measured under three different plasma conditions. The corresponding down-scattered light was also observed. The measured wavelengths were, in all cases, in good agreement with the results of the enhanced Thomson scattcxing (ETS) theory. The upscattered band, shifted to a frequency higher than the laser frtquency wg, had not previously been systematically studied. The other was down-shifted in fnquency to between og and q j 2 and typically has been referred to as the Raman scattering band. Enhanced Thomson scattering is not an instability, but rather depends on the presence of nonlinear plasma processes (resonance absorption) and instabilities (stimulated Raman scattering or two plasmon decay) to produce fast and superfast electrons, respectively, that enhance background plasma waves to large amplitudes. Incoming laser light scatters from the enhanced density fluctuations into two "plasma-line" features, one on each side of the laser frequency. The position of these bands depends on the fast and superfast electron temperatures through the phase velocity of the plasma waves that arc enhanced The primary variation in the plasmas studied was in scale length. Variations were produced by changing laser parameters such as wavelength and energy. The fmt experiment was conducted using a 60-J, single-beam, 527-nm laser focused to a small spot, producing a scale length of about 80 pm. The presence of fast elections from resonance absorption was inferred. The second experiment was performed using a ten-beam, 20-kJ, 351-nm laser. Nine low-intensity beams were used to produce an ultra-long-scale-length plasma with a scale length of approximately 5,000 laser wavelengths. The tenth, or "interaction," beam, obliquely incident on a 15-pm-thick CH foil, was delayed 2.7 ns to simulate interaction with a reactor-type target The third experiment was conducted using a single-beam, 2-U, 351-nm, high-intensity laser which created a plasma with a 2Wpm scale length.