  In this paper the single and multibunch emittance growths in the main linac
of a linear collider are analytically treated in analogy to the Brownian motion
of a molecule, and the analytical formulae for the emittance growth due to
accelerating structure misalignment errors are obtained by solving Langevin
equation. The same set of formulae is derived also by solving directly
Fokker-Planck equation. Comparisons with numerical simulation results are made
and the agreement is quite well.
