[magv]

LEP was annihilating electrons with positrons; LHC does not, as it only accelerates protons. FCC will have the option to do both: the plan includes two separate pipes in the same tunnel, one for protons, one for electrons.

This however does not increase the collision energy significantly: a single electron-positron pair has the (rest) energy of ~1MeV -- this is what is released during the annihilation, but the energies we want to go to at the next collider go up to 10TeV, so 10^7 times larger.

[magicalhippo]

LHC smashes protons together, and for comparison the rest mass (energy) of a proton[1] is just shy of 1GeV.

So that's still 4 orders of magnitude below the collision energy of the LHC.

[1]: http://pdglive.lbl.gov/DataBlock.action?node=S016M&home=BXXX...