Measurement of Long-Wavelength Turbulence within a Collisionless Shock by Scattering of Radiation from aCO2Laser

We describe measurements of the scattering of ${\mathrm{CO}}_2$ laser radiation by long-wavelength ($k{\lambda }_D\sim 0.1$) turbulent density fluctuations within a collisionless shock. The spectrum of the scattered radiation is shifted in wavelength from that of the incident laser beam, demonstrating genuine scattering rather than enhanced stray light. The observed level of fluctuations is a hundred times thermal but a thousand times lower than that to be expected from an extrapolation of the Kadomtsev spectrum observed previously for $k{\lambda }_D\sim 1$.