Abstract: The Polarimetry Focusing Array(PFA)is one of the payloads in the enhance X-ray Timing and Polarimetry mission(eXTP),each telescope in the PFA contains a Gas Pixel Detector(GPD)which locates in the focal plane and will give the polarized degree of soft X-ray sources from celestial objects under extreme conditions.The GPD consists of a single Gas Electron Multiplier(GEM)foil with an effective area of （15.5×15.5）mm² and holes of double conical shape as the amplification stage.The GEM foil with a pitch of 50 μm,a hole diameter of 30 μm,and a cone degree of 10 ° is fabricated by laser-etched technique.The non-uniform intensity of the laser and overlap in some irradiation areas causes different gain values among the holes during the GEM fabricates.In this study,we constructed a simulation framework for the GPD-GEM based on the baseline design and obtain a consistent trend of the effective gain measured in the reference,giving the linear relationship between the cone angle of holes and the gain under a condition of the same voltage.The studies illustrate the cylindrical shape of the GPD-GEM is a better option and provide an optimal design of it.

Key words: GPD-GEM, Gain, Simulation