Non-ideal effects and spectral response of Phase Contrast Imaging

The response of Phase Contrast Imaging (PCI) [1] to long-wavelength fluctuations is enhanced by non-ideal effects, as shown in laboratory tests and measurements on the DIII-D tokamak. An optical system on a large plasma experiment is subject to misalignment caused by mechanical vibrations, j×B forces, and diffraction due to both the bulk plasma density and MHD instabilities and exacerbated by long or convoluted beam paths. PCI, which is sensitive to low-frequency deflections of 0.5 mrad, may require active beam steering to compensate [2]. Experimental tests now allow a quantitative calculation of the PCI effective response vs. wavenumber in the presence of small vibrations, showing potentially a large response to plasma fluctuations well below the PCI low-k cutoff (~1cm^-1 on the DIII-D PCI). Enhanced sensitivity also results from beam diffraction by MHD modes, which causes beam deflections of order 10 μrad, as confirmed by the recently-added PCI interferometer channel. These results provide criteria for vibration compensation in the design of PCI systems for large-scale plasma devices, especially for future PCI operating with shorter laser wavelengths.

[1] Davis et al, Rev. Sci. Instrum., 89 (2018) 10B106.

[2] S. Coda and M. Porkolab, Rev. Sci. Instrum., 66 (1995) 454.