The first phase (500 GeV centre-of-mass) will be constructed using a tunnel long enough to support a gradient of 31.5 MV/m operational gradient (~2x10km assumning a 0.75 fill factor). A second phase (phase 2) upgrade to 1 TeV centre-of-mass will then require extending the tunnel (away from the IR) an additional ~2×9.3km assuming cavities capable of 36 MV/m operational gradient.
The Beam Delivery System and main dump systems will be configured for the 1 TeV option from the start.
Injectors will be configured for phase 1 so that there is a minimum impact on them when upgrading to phase 2. Prospective sites must be chosen with the TeV phase 2 machine in mind; specifically the availability of both the total required land and power.