Design of the ALPS II optical system

The Any Light Particle Search II (ALPS II) is an experiment currently being built at DESY in Hamburg, Germany, that will use a light-shining-through-a-wall (LSW) approach to search for axion-like particles. ALPS II represents a significant step forward for these types of experiments as it will use 24 superconducting dipole magnets, along with dual, high-finesse, 122 m long optical cavities. This paper gives the first comprehensive recipe for the realization of the idea, proposed over 30 years ago, to use optical cavities before and after the wall to increase the power of the regenerated photon signal. The experiment is designed to achieve a sensitivity to the coupling between axion-like particles and photons down to g${}_{\alpha \gamma \gamma }$ = 2 x 10${}^{-11}$ GeV${}^{-1}$ for masses below 0.1 meV, more than three orders of magnitude beyond the sensitivity of previous laboratory experiments. The layout and main components that define ALPS II are discussed along with plans for reaching design sensitivity. An accompanying paper (Hallal, et al) offers a more in-depth description of the heterodyne detection scheme, the first of two independent detection systems that will be implemented in ALPS II.