Estimates of Dispersive Effects in a Bent NLC Mail Linac

Each arm of the NLC Main Linac is almost 11 km in length; including the final focus region, the total complex extends over 30 km. Were it to be constructed using straight-line tunneling, its sagitta relative to a gravitational equipotential surface would be about 18 meters. While that seems like a small number, its impact on pumping requirements is significant. Further, if the tunnel is dug using ``cut-and-fill'' methods, the increased cost of moving the extra dirt must also be taken into account. An alternative being considered is not to tunnel in a straight line but to bend the Main Linac into an arc so as to follow an equipotential. We begin here an examination of the effects that this would have on vertical dispersion, with its attendant consequences on synchrotron radiation and emittance growth. Two scenarios are studied: (a) gently bending the beam ``continuously'' to follow an equipotential surface, and (b) introducing sharper bends at a few sites in the linac so as to reduce the maximum sagitta produced. In both cases, and throughout this paper, we will adopt a minimalist approach, always making the smallest possible changes to the already existing NLC design. We are striving here only for some order of magnitude calculations, not a serious design study. Fermilab FERMILAB-FN-690 May 2000 Estimates of dispersive effects in a bent NLC Main Linac Leo Michelotti and Michael Syphers