Forward cw CO2-laser scattering on a high density plasma column

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Abstract
Wave‐number and frequency spectra S(k,ω) associated with spontaneous electrostatic fluctuations in a high‐density (5×1021 m−3) current‐driven plasma are measured using forward CO2‐laser scattering with homodyne detection. Scattering measurements during the quiescent plateau phase of the discharge show fluctuation levels several orders of magnitude above the thermal level with k−4 and ω−2 spectral amplitude dependences. The fluctuations are found to be isotropic in a plane transverse to the magnetic axis with S(k∥,ω)≪S(k⊥,ω). The scattering apparatus is also shown to be a valuable diagnostic tool for basic studies of plasma current transport across magnetic field lines. In fact, a clear correlation is observed between the appearance of strong fluctuations and the rapid current penetration during fast plasma current reversal experiments. Finally, cw scattering results are used to find a set of experimental conditions suitable for the measurement of the ion temperature using a CO2 pulsed scattering system.