Determination of absolute turbulence amplitude by CO2 laser phase contrast imaging

Phase contrast imaging (PCI) has the important advantages of good sensitivity and applicability for the measurements of fluctuation and turbulence in magnetically confined plasma. However, the determination of the absolute amplitude of fluctuation is not well established. In this article, we propose to determine the absolute amplitude of fluctuation with calibration. The fluctuation measured by CO₂ laser PCI (CO₂-PCI) was confirmed by simultaneous measurements by using a HeNe heterodyne interferometer (HeNe-HI). In the test experiments, CO₂-PCI and HeNe-HI measured sound fields, which are produced by the audio speaker (f=6~18 kHz, k=0.09~0.33 mm^-1) and the piezoelectric transducer (f=72 kHz, k=1.3 mm^-1). At f=13~18 kHz, k=0.24~0.33 mm^-1, the differences between CO₂-PCI and HeNe-HI were 20~30%. At f=72 kHz, k=1.3 mm^-1, the difference was less than ~10%. On the other hand, the difference between PCI and HI becomes larger at f<13 kHz, k<0.24 mm^-1. This is because in these regions some parts of the scattered and non-scattered radiation were not clearly separated inside and outside the groove of the phase plate. The procedure of the absolute value determination was applied to the detached plasma of LHD and compared with the measurement by using a CO₂ laser heterodyne interferometer (CO₂-HI). The signal of CO₂- HI at f>20 kHz is almost at the same as the level of noise. On the other hand, the 2D-PCI signal is clearly higher than the level of noise in all the entire frequency region. At f < 20 kHz, the difference between CO₂-HI and CO₂-PCI becomes large. This is likely to be due to the wavenumber cut-off of CO₂-PCI. Combining two diagnostics, it is possible to measure a wide range of fluctuations from the macroscopic to the microscopic scale.