The possibility of further increasing the centre-of-mass energy of the ILC by another factor of two to \(\sqrt{s}=\) 1 TeV has also been considered. The other linear collider project, CLIC \cite{cite:CLICDR}, could provide a higher-energy physics programme all the way to \(\sqrt{s}=\) 3 TeV. It would require, however, considerably more electrical power, estimated at \(\sim 600\) MW.

The ultimate energy-frontier option for TLEP is of a very different and more ambitious nature. It would consist of using the 80 to 100 km tunnel to house a very-high-energy large hadron collider, the VHE-LHC. If equipped with magnets of 15 T, pp collisions could be produced at a centre-of-mass energy of 80 to 100 TeV, giving access to the direct production of new particles with masses of 10 TeV or more. For completeness, we also note that pp collisions with a centre-of-mass energy of 33 TeV could be obtained by re-using the LHC tunnel for a pp collider using 20 T magnets, the high-energy large hadron collider, HE-LHC.

As mentioned in Section \ref{sec:ee500}, a centre-of-mass energy of 500 GeV cannot compete with the HL-LHC for the Htt coupling measurement. To reach an accuracy in \({\rm e}^{+}{\rm e}^{-}\) collisions similar to the HL-LHC (less than 5%), the upgrade of either ILC up to \(\sqrt{s}=1\) TeV or CLIC up to \(\sqrt{s}=3\) TeV is needed. A precision of 4% on the Htt coupling would be achieved with an integrated luminosity of 1 \({\rm ab}^{-1}\) (ILC-1000) or 2 \({\rm ab}^{-1}\) (CLIC). On the other hand, an integrated luminosity of 3 \({\rm ab}^{-1}\) with pp collisions either at the HE-LHC or the VHE-LHC would allow the precision on the Htt coupling to be significantly improved to a couple of per-cent or a fraction of a per-cent, respectively.

The measurement of the trilinear Higgs self-coupling \(\lambda_{\rm H}\) would benefit substantially from higher energy, because of the fast increase of the double-Higgs-boson production cross section, in both \({\rm e}^{+}{\rm e}^{-}\) and proton-proton collisions. Studies exist, albeit with different levels of maturity, for the sensitivity of the ILC \cite{ILC:Physics}, CLIC \cite{cite:CLICDR}, and HL-LHC \cite{cite:ATLAS,cite:CMS} to these couplings. From the HL-LHC estimates and from the known HH production cross-section increase at higher energies \cite{Mangano_Rojo_2012}, extrapolations for \(3~{}{\rm ab}^{-1}\) of pp collision data at the HE-LHC (\(\sqrt{s}=\) 33 TeV) and the VHE-LHC (\(\sqrt{s}=\) 100 TeV) can be inferred \cite{Snowmass}. An executive summary of the achievable precisions is displayed in Fig. \ref{fig:VHELHC}.