Optical design of a novel near-infrared phase contrast imaging (NI-PCI) diagnostic on the HL-2A tokamak

The optical design of near-infrared phase contrast imaging (NI-PCI) diagnosis on HL-2A is introduced in this paper. This scheme benefits from the great progress of near-infrared laser technology and is a broadening of traditional phase contrast technology. This diagnostic can work as a keen tool to measure plasma wavenumber spectra by inferring string-integrated plasma density fluctuations. Design of both the front optical path which is the path before the laser transmitting into the tokamak plasma and the rear optics which is the path after the laser passing through the plasma is detailed. The 1550 nm laser is chosen as the probe beam and high-precision optical components are designed to fit the laser beam, in which a phase plate with a 194-nm-deep silver groove is the key. Compared with the conventional 10.6 \mum laser-based PCI system on HL-2A, NI-PCI significantly overcomes the unwanted phase scintillation effect and promotes the measurement capability of high-wavenumber turbulence with an increased maximal measurable wavenumber from 15 cm⁻¹ to 32.6 cm⁻¹.