Design of the ALPS II Optical System

The Any Light Particle Search II (ALPS II) is an experiment currently being built at DESYin Hamburg, Germany, that will use a light-shining-through-a-wall (LSW) approach to searchfor axion-like particles. ALPS II represents a significant step forward for these types of experi-ments as it will use 24 superconducting dipole magnets, along with dual high-finesse, 122 m longoptical 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 powerof the regenerated photon signal. This concept will allow the experiment to achieve a sensitivityto the coupling between axion-like particles and photons down to g$_{αγγ}$ = 2 × 10$^{−11}$ GeV$^{−1}$ formasses below 0.1 meV, more than three orders of magnitude beyond the sensitivity of previouslaboratory experiments. The layout and main components that define ALPS II are discussedalong with plans for reaching design sensitivity. A set of top level requirements for the sub-systems is also provided for the first time and includes the requirements on the coherence andspatial mode matching of the cavity eigenmodes. An accompanying paper (Hallal, et al [1]) of-fers a more in-depth description of the heterodyne detection scheme, the first of two independentdetection systems that will be implemented in ALPS II.