  In order to study the background status in the ATF2 beam line and the
interaction point (IP), this paper developed an analytical method to give the
estimation of ATF beam halo distribution based on K. Hirata and K. Yokoya's
theory. The equilibrium particle distribution of beam tail in ATF damping ring,
when each electron is being affected by, in addition to the synchrotron
radiation damping effects, several stochastic processes, such as beam-gas
scattring, beam-gas bremsstrahlung and intra-beam scattering, was presented.
This method is common and can be applied on other electron rings.
