 Solution

The International Linear Collider uses externally powered superconducting niobium RF cavities at about $1.3$ GHz, with a reference average accelerating gradient of $31.5$ MV/m. Superconducting walls reduce RF dissipation, allowing relatively long RF pulses and bunch trains, at the cost of cryogenic infrastructure.

The Compact Linear Collider uses normal-conducting copper structures at about $12$ GHz and a main-linac gradient near $100$ MV/m in its multi-TeV design. A separate high-current, lower-energy electron drive beam is decelerated in power-extraction structures; the resulting RF power accelerates the high-energy, low-current main beams. This two-beam concept supplies short, high-power pulses without requiring a high-power klystron at every main-beam structure.

ILC: superconducting low-loss RF; CLIC: high-gradient normal-conducting two-beam RF. Both rely on damping and final focusing to obtain very small collision spots, so energy alone does not determine their luminosity. ^{[ref] [ref]}