Estimation of orbit change and emittance growth due to random misalignment in long linacs

In linear accelerators, the transverse beam orbit is induced by tilts of accelerating cavities (deviation of accelerating field direction from designed beam direction) and transverse offset of quadrupole magnets. Estimating induced orbit and emittance growth due to such random errors is important for evaluation of performance of linacs, especially where stable and low emittance beam is required, such as linear colliders. Usually, such estimations are performed using tracking simulations, including the Monte Carlo method, which tends to take a long time. Here, a much faster and simpler method of quantitative estimation of beam orbit and emittance growth is reported. This method is valid for very long linacs with many components, where statistical treatment of errors is justified. It is shown that the results from the method agree well with tracking simulations for the ILC (International Linear Collider) main linac.