Resistive wall wakefield in the LCLS undulator beam pipe

We have shown that, due to the ac resistive wall wakefield, the present LCLS undulator design, with its round copper beam pipe, will result in an unacceptably large energy variation induced within the bunch over the length of the undulator (0.6%). If, instead, we use a flat, aluminum chamber, the energy variation can be reduced to within acceptable limits (∼ 0.2%). Finally we have shown that the effect of the anomalous skin effect in the beam pipe wall is small, and can be ignored.