Emittance reduction with variable bending magnet strengths: Analytical optics considerations and application to the Compact Linear Collider damping ring design

Abstract

One of the main challenges of the lattice design of e + e − synchrotrons, used as light sources or damping rings, is the minimization of the emittance. The optimal configurations for achieving the absolute minimum emittance are the theoretical minimum emittance (TME) cells. This paper elaborates the optimization strategy in order to further reduce the betatron emittance of a TME cell by using dipoles whose magnetic field varies longitudinally. Based on the analytical results, the magnet design for the fabrication of variable bend with the optimal characteristics is discussed. In order to have a global understanding of all cell properties, the analytical approach for the theoretical minimum emittance (TME) cells with variable bends is presented. Numerical examples for the linear optimization of the Compact Linear Collider (CLIC) damping rings (DRs) are shown. The margin provided by this new design strategy on the output beam parameters after including the dominant effect of intrabeam scattering, enables the removal of a number of TME cells from the existing arcs while still keeping the requirements of the collider. The reduction of the circumference is further enhanced by the use of optimized high-field wigglers. The optimization strategy followed for the CLIC DRs is explained in detail and the output parameters of the new design are presented.

The CLIC project

“…However, the CLIC prototype is more challenging, as it is a tuneable permanent magnet combining dipole and quadrupole components, with a very high field of 2.3 T at its centre. A recent prototype has been built by CIEMAT as part of the EuCard2 program [69]. Magnetic measurements demonstrate the feasibility of these novel type of magnets and a new prototype is being design for its use on light sources under the I.Fast EU program.…”

Design and Principles of Linear Accelerators and Colliders

“…Most of the design challenges of the DRs are driven by the extremely high bunch density and the associated collective effects. In this respect, the DR parameters (Table 7.7) are carefully chosen and optimised in order to mitigate these effects [95,96]. In the case of CLIC, the steady state emittance is dominated by Intra-Beam Scattering (IBS).…”

“…In the case of CLIC, the steady state emittance is dominated by Intra-Beam Scattering (IBS). The ring energy [97] and lattice design [96], (racetrack shape with TME arc cells with variable field dipoles [98] and long straight sections filled with super-conducting wiggler [99] FODOs) is optimized for reducing IBS. The larger emittance specification of ILC, allows for higher ring energy, thus relaxing collective effects.…”

Mitigation of Collective Effects by Optics Optimisation

“…Even at this large detuning factor, the TMEs are preferable for their compactness, in particular for a ring in which radiation damping is dominated by wigglers. The use of variable bends with gradient in the TME cell was also studied using a similar approach, further reducing the IBS growth rates [25].…”

“…At the same time, as shown in Fig. 5 (right), by raising the field and using Nb3Sn wire technology, the reduction of the ring circumference can be also achieved, with beneficial impact to all type of collective effects, including to a potential impedance reduction [25,27]. These specifications were used for the superconducting wiggler prototype and short model developped for the CLIC DRs [26,27].…”