AVS 52nd International Symposium
    Vacuum Technology Tuesday Sessions
       Session VT-TuP

Paper VT-TuP2
Monte Carlo Simulation of Transmission Probability of Gas on Thin Orifice Considering Specular Reflection

Tuesday, November 1, 2005, 4:00 pm, Room Exhibit Hall C&D

Session: Vacuum Technology Poster Session
Presenter: M. Shiro, National Institute of Advanced Industrial Science and Technology (AIST), Japan
Authors: M. Shiro, National Institute of Advanced Industrial Science and Technology (AIST), Japan
M. Hirata, AIST, Japan
H. Akimichi, AIST, Japan
Correspondent: Click to Email
To estimate the conductance of an orifice, the influence of its thickness on the transmission probability of gas molecules should be considered. In general, the probability is calculated assuming that molecules hitting inner wall of the orifice reflect with cosine law. However, a specular reflection of gas molecules on the wall should be also considered on the calculation judging from that the accommodation coefficient of a spinning rotor gauge depends on gas species. In such case, it is difficult to calculate the probability analytically. In this study, the probability considering the specular reflection was calculated by the Monte Carlo simulation. The results are as follows, 1) the probability increases with the ratio of the specular reflection. 2) This increment is significant in thick orifice. Even if the thickness is small, it is necessary to consider the effect of the specular reflection. If the specular reflection elements are1%, 3% and 10%, this effect increases the probability 0.1% by the increase of the ratio L/r of thickness L to radius r of the orifice greater than 0.2, 0.07 and 0.02, respectively. In case of thin orifice, the probability can be expressed in a simple equation of the L/r and the ratio of specular reflection. The great portion of molecules passes through the orifice directly. Molecules reflecting in specular on the inner wall of the orifice pass through the orifice. 50% of remaining molecules hitting the wall also pass through the orifice. The deviation of this equation from the result of the simulation is less than 0.1% in the range of L/r<0.2, even if 10% of gas molecules reflected in specular.