Minimizing Emittance for the CLIC Damping Ring

The CLIC damping rings aim at unprecedented small normalized equilibrium emittances of 3.3 nm vertical and 550 nm horizontal, for a bunch charge of 2.6·109 particles and an energy of 2.4 GeV. In this parameter regime the dominant emittance growth mechanism is intra-beam scattering. Intense synchrotron radiation damping from wigglers is required to counteract its effect. Here the overall optimization of the wiggler parameters is described, taking into account state-of-the-art wiggler technologies, wiggler effects on dynamic aperture, and problems of wiggler radiation absorption. Two technical solutions, one based on superconducting magnet technology the other on permanent magnets are presented. Although dynamic aperture and tolerances of this ring design remain challenging, benefits are obtained from the strong damping. For optimized wigglers, only bunches for a single machine pulse may need to be stored, making injection/extraction particularly simple and limiting the synchrotron-radiation power. With a 365 m circumference the ring remains comparatively small.