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

The novel CO₂ laser based tangential imaging diagnostic has been completely calibrated and the system has been tested on CDX-U plasmas. It has been shown that localized, two-dimensional images of the plasma electron density fluctuations in the tokamak core can be obtained from a tangential imaging beam.1 A variation of the Zernike phase mirror is used. Test measurements made on soundwaves verify that the system will image phase variations placed at the object plane. An absolute sensitivity of ≈1×10⁷ cm⁻³/$Hz$ has been determined. A series of plasma measurements made on CDX-U are described. Results show that the plasma fluctuations are 2–3 orders of magnitude above the noise floor of the diagnostic. It is also verified that a high-pass cutoff wave number (from ≈1–8 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.