Calibration and test of the tangential phase contrast imaging diagnostic on CDX-U

A novel CO₂ laser-based tangential imaging diagnostic was completely calibrated and the system was tested on CDX-U plasmas. It was shown that localized, two-dimensional images of the plasma electron density fluctuations in the tokamak core can be obtained from a tangential imaging beam [E. Lo, J. Wright, and R. Nazikian, Rev. Sci. Instrum. 66, 1180 (1995)]. A variation of the Zernike phase mirror is used. Test measurements made on sound waves verify that the system will image phase variations placed at the object plane. An absolute sensitivity of ≈1×10⁷ $cm−3/Hz$ was determined. A series of plasma measurements made on CDX-U is described. Results show that the plasma fluctuations are two to three orders of magnitude above the noise floor of the diagnostic. It is also verified that a high-pass cut-off wave number (from ≈1 to 8 cm⁻¹) for the fluctuations can be selected by translating the phase mirror. The density fluctuation k spectrum is measured and found to peak at ≈1–3 cm⁻¹.