The influence of a diverging conical end on the beam's angular distribution at the outlet of the capillary

A cylindrical capillary with a diverging conical end was proposed to decrease the divergence of the beam at the capillary outlet in the free molecular regime. To evaluate the effect of the conical end on the angular distribution, test particle Monte Carlo simulations based on Cercignani-Lampis (CL) reflection were conducted. Firstly, the influence of the capillary's structural parameters on the azimuth integrated flux intensity was evaluated. The peak point of the azimuth integrated flux intensity shifted to small polar angle when decreasing the half angle of the conical end, and the magnitude of the peak decreased with the half angle when it was less than 20°, but the magnitude was still larger than that of cylindrical capillary. Meanwhile, the influence range of the conical end on the distribution increased with its height, and the structural parameters of the cylindrical part had little effect on the shape of angular distribution. Then, the effect of the conical end was studied with different CL accommodation coefficients, and the results showed that the shape of the distribution barely changed except for the transmission probability. Finally, capillary with two conical structures and the effect of conical end on an atomic hydrogen beam were discussed.