Collective scattering of CO2‐laser light from ion‐acoustic turbulence

The spectral density function S(k,w) associated with spontaneous turbulence in a current‐driven plasma of medium density (n e = 1019−10220 m −3) is measured with the scattering of the radiation of a cw low‐power CO2‐ laser (P i ≃2 W, l i = 10.6 mm). Optical homodyne detection has been used. A remarkable result is that the level of fluctuations with wave vectors k∥B is close to thermal; B is the magnetic field for plasma confinement. The level of fluctuations with k⊥B is several orders of magnitude larger than the thermal level. The phase velocity of these fluctuations is equal to the ion‐acoustic velocity. A power law spectrum of the form n̄ e /n e ∼w−2 has been found. It is consistent with the spectrum below 1 MHz as measured by optical probes.