OPTIMISATION OF THE CLIC DAMPING RINGS

The CLIC damping rings should produce the ultra-low emittance with high bunch charge necessary for the luminosity performance of the collider. This large bunch density triggers a number of beam dynamics and technical challenges. Lattice studies have been focused on low emittance cells with optics that reduce Intra-beam scattering. Collective effects such as electron cloud and fast ion instability can severely limit the performance and mitigation techniques have been identified and tested. The final beam emittance is reached with the help of superconducting damping wigglers. Results from recent simulations and prototype measurements are presented, including an absorption scheme design. Tolerances for alignment and technical system design such as kickers, RF cavities, magnets and vacuum have been finally established. DESIGN GOALS AND CHALLENGES The high luminosity of a linear collider at the lowest power depends strongly on the generation of ultra-low emittance high-intensity bunches, with remarkable stability. The required cooling mechanism is provided by the natural synchrotron radiation damping of the beam when circulating in rings. The performance challenges of these damping rings (DRs) are driven by the main parameters of the collider and the requirements of the upstream and downstream systems. The parameters guiding the design of the CLIC DRs are presented in Table 1, as compared to the ones of NLC [1]. The two lists resemble but the CLIC DRs have almost one order of magnitude smaller emittance requirements in all three dimensions (500nm horizontal, 5nm vertical and 6keV longitudinal). Although these emittances are unprecedented, modern light sources in operation or in the construction phase, are rapidly approaching these regimes [2]. Figure 1 presents the horizontal and vertical normalized emittance in a number of low emittance rings, including test facilities, DRs, B-factories and synchrotron light sources, under operation (red) or design (blue). The large input emittance especially coming from the positron source and the high repetition rate of 50Hz, requires that the beam damping is done in two stages, with a pre-damping ring (PDR) for each particle species. A careful lattice design and non-linear dynamics optimisation is necessary for providing a solid PDR design, with large dynamic and momentum aperture, enabling the efficient digestion of the incoming beam [3]. Most of the design challenges of the CLIC main DRs are driven from the Table 1: CLIC Versus NLC Parameters Driving the DRs Design Parameters [unit] NLC CLIC Bunch population [10] 7.5 4.1 Bunch spacing [ns] 1.4 0.5 Number of bunches/train 192 312 Number of trains 3 1 Repetition rate [Hz] 120 50 Horizontal norm. emittance [nm] 2400 500 Vertical norm. emittance [nm] 30 5 Longitudinal norm. emittances [keV.m] 11 6 Figure 1: Horizontal versus vertical normalized emittance for low emittance rings in operation (red) and under design (blue) Table 2: CLIC DRs Design Parameters DR parameters [unit] Value Energy [GeV] 2.86 Circumference [m] 420.6 Energy loss/turn [MeV] 4.2 RF voltage [MV] 4.9 Compaction factor 8× 10−5 Damping time x / s [ms] 1.9 / 0.96 Number of arc cells/wigglers 100/52 Dipole/wiggler field [T] 1.4/2.5 extremely high bunch density and the collective effects associated with it. In this respect, the DR parameters shown in Table 2 are carefully chosen and optimised in order to reduce these effects. In addition, these parameters drive the technology of a number of components such as wigglers, RF system, kickers and vacuum. WEPE089 Proceedings of IPAC’10, Kyoto, Japan 3554 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques