Evaluation of diffraction effects of phase contrast imaging on HL-2A and HL-2M tokamak

Diffraction effects of diagnostic windows and a phase plate are evaluated in sets of phase contrast imaging (PCI) systems on HL-2A tokamak. Results show that wave field of an incident laser with shorter wavelength is less effected by diffraction. The diameter of viewing windows for PCI is asked to be larger than 50mm to eliminate diffraction effect, implying the defection of the traditional PCI diagnostic with 10.6 mu m CO2 laser based on the viewing port with an inner diameter of 35 mm. This investigation promises a remarkable anti-diffraction performance for PCI diagnostics on the constructing HL-2M tokamak, which reserves a pair of diagnostic windows with an inner diameter of 80 mm. Evaluation of response to the phase plate indicates that the signal response and the laser beam width are restricted by each other. A suitable choice of laser beam width depends on the specific plasma physics.