First Studies of the Halo Collimation Needs in the FCC-ee

The Future Circular Collider, FCC-ee, is an electron-positron collider that is currently being studied and is designed to be the highest-luminosity high-energy lepton collider to date. The total stored beam energy in the FCC-ee can reach values of up to 20 MJ, which can cause severe damage to accelerator components in the event of beam losses. Therefore a system of halo collimators is required to intercept stray beam particles and localise beam losses away from experiments and sensitive equipment. The purpose of this work was to study the halo collimation needs of the FCC-ee. This was done by creating an aperture model of the accelerator, which defines the shape and size of the physical openings of all the elements. Based on first estimates of the orbit and optics errors, this aperture model was used to calculate the available aperture around the ring, normalised to the transverse local beam size (i.e. in units of RMS beam size "sigma"), for on-momentum and off-momentum particles, in order to identify potential performance bottlenecks. The lowest horizontal and vertical on-momentum aperture bottlenecks were shown to be 14.88 σ and 10.26 σ respectively, allowing for the collimator jaw openings to be placed at 12 σ and 8 σ (to guarantee beam "lifetime") as a first minimum estimate of the betatron collimator cuts on the Gaussian tails of the particle beams. The off-momentum aperture studies showed bottlenecks much lower than 10 σ, motivating the need for a limit of 1.2 and 5.0 to be imposed on the residual beam size growth factor in the horizontal and vertical planes respectively, and for the fractional parasitic dispersion to be corrected to at least 0.02, which could be the goals for local optics corrections. The aperture was also studied in momentum space, and a momentum cut of 3% can be proposed to allow operational margins and guarantee aperture protection. Future iterations of this work should be performed using additional improved corrections of the optics.