Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2012 / S19 / 3 / c

Both machines are proposed electron–positron linear colliders, using sources, damping rings, bunch compressors, two opposing main linacs, and final-focus and beam-delivery sections that bring very small bunches to an interaction point. The spent beams go to extraction lines and beam dumps rather than making repeated collision turns.

In the exam-era International Linear Collider reference design, polarized electrons and an undulator-based positron source feed damping rings and two superconducting main linacs; the full $500$ GeV centre-of-mass facility is about $31$ km long. The Compact Linear Collider adds high-current drive-beam generation and deceleration lines parallel to its main linacs; its $3$ TeV concept is roughly $48$ km long. The linear layout avoids the repeated synchrotron-radiation losses that would be severe for multi-hundred-GeV electrons in a storage ring. ^{[ref] [ref]}

For a comparison appropriate to this 2012 examination, use the reference-design and conceptual-design targets rather than later revised starting stages:The ILC reference programme included scanning approximately $200$–$500$ GeV and a possible $1$ TeV upgrade. The CLIC 2012 studies included lower-energy construction stages as well as the ultimate multi-TeV machine. These are design goals, not achieved operating luminosities.

Collider luminosity sets the event rate for a process of cross-section $\sigma$ through $dN/dt=\mathcal{L}\sigma$; it is not radiated optical power. ^{[ref] [ref]}

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]}