Both \({\rm e}^{+}{\rm e}^{-}\) Higgs factories discussed in Section \ref{sec:Higgs} (TLEP and ILC) have high-energy upgrade options. In the case of TLEP, the centre-of-mass energy can be increased to \(\sqrt{s}=500\) GeV by tripling the RF length from 600 to 1700 m, thereby increasing the total RF voltage from 12 to 35 GV to compensate for the 31 GeV lost per turn by synchrotron radiation in the 100 km ring. In the case of the ILC, its length can be doubled to reach a centre-of-mass energy of 500 GeV.

With a 2.5% momentum acceptance at each interaction point, TLEP-500 would have a one-minute beam lifetime, which would allow for an average luminosity of 90% of the peak luminosity with the baseline TLEP top-off injection scheme. With these parameters, a luminosity of \(0.5\times 10^{34}~{}{\rm cm}^{-2}{\rm s}^{-1}\) would be delivered at each interaction point with a beam-beam tune shift of 0.1, for a total luminosity of \(2\times 10^{34}~{}{\rm cm}^{-2}{\rm s}^{-1}\) when summed over the four IPs, as displayed in Fig. \ref{fig:TLEPUpgrade}. Although not included in the TLEP baseline programme at this time, the design study will investigate the feasibility of such an option and define the maximum reachable centre-of-mass energy under reasonable assumptions.