Vertical Arc for ILC Low Emittance Transport

Abstract

The design and parameters of a vertical arc for the ILC Low Emittance Transport (LET) are reviewed. A 1 TeV CM ILC which relies upon 30 MV/m accelerating cavities with a packing fraction of 65% will require almost 48 km of main linac, which suggests that the total site length including BDS and bunch compressors will be on the order of 53 km. If built in a laser-straight tunnel with the low-energy ends near the surface, and assuming a perfectly spherical ''cue ball'' planetary surface with radius 6370 km, the collider halls will necessarily be 55 meters below grade, as shown in the top plot of Figure 1. Such depths would demand extensive use of deep tunneling, which would potentially drive up the cost and difficulty of ILC construction. An alternate solution is to use discrete vertical arcs at a few locations to allow a ''piecewise straight'' construction in which the depth of the tunnel below grade does not vary by more than a few meters. This approach is shown schematically in the bottom plot of Figure 1. In this Note we consider the issues for a design with one such vertical arc at the 250 GeV/c point (ie, midway down the linac for 1 TeV CM), and a second arc at the entrance to the BDS (ie, the entire BDS lies in one plane, with vertical arcs at each end).