= 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.
\b[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. https://cds.cern.ch/record/1072192/files/ab-2007-078.pdf[ILC reference gradient and superconducting RF baseline]{ref} https://indico.cern.ch/event/216895/contributions/442123/attachments/347325/484269/lucaci_FCAL.pdf[CLIC CDR status, November 2012, slide 4: copper cavities, 12 GHz, 100 MV/m and drive-beam power generation]{ref}
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