Ultimate Hadron Colliders: What is feasible? What is affordable? How to maximize reach for new gauge fields?

The potential for discovering new gauge fields of nature relies upon extending the collision energy of hadron colliding beams as far as possible beyond the present 14 TeV capability of LHC.  We must seek a balance of minimum cost/TeV for the ring of superconducting magnets, feasibility and cost of a tunnel to contain the ring, and balancing the luminosity against synchrotron radiation.  Balancing feasibility, technology, and cost is crucial if there is to be a high-energy frontier for discovery of new gauge fields. Three design cases exhibit the tricky balance among these parameters:

FCC-hh:                                100 TeV,              ~100 km tunnel around Geneva,               ~16 T magnets using Nb₃Sn

SuperCIC:            100 TeV,              270 km tunnel around Dallas,                      4.5 T magnets using NbTi

Collider-in-the-Sea:         500 TeV,              1900 km pipeline in the Gulf of Mexico,  3.5 T magnets using REBCO