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Description
Failures in the electro-magnetic separator (EMS) system of the Future Circular Collider electron–positron collider (FCC-ee) can induce fast trajectory excursions with potentially critical impact on beam dynamics and machine components. This work investigates EMS failure scenarios and their consequences on beam trajectory and synchrotron radiation. Two main failure scenarios are considered: high-voltage breakdown leading to a complete loss of the electric field in an EMS unit, and partial magnetic field reduction due to power supply failure. Both cases result in a local mismatch of fields and generate transverse trajectory distortions. The EMS are installed before and after the RF cavities. Failures occurring upstream of the RF cavities can introduce a non-zero beam angle, leading to non-rectilinear trajectories within the cavities. This induces synchrotron radiation emission inside RF structures, potentially causing localized heat loads, radiation damage and inducing a trip of one or more cavities. Failures occurring downstream of the RF cavities can reduce the nominal deflection, compromising beam separation and propagating trajectory errors into the arcs. The resulting orbit distortions can lead to increased synchrotron radiation in bending regions, beam losses on aperture restrictions, and can create transverse instabilities. Mitigation strategies to minimise effects are discussed, including segmentation of the EMS into independent units and the use of high-voltage decoupling resistors to localize failures. These measures limit the impact of single events but do not avoid fast trajectory excursions, which must be addressed by dedicated machine protection systems. This study highlights the critical role of EMS reliability and failure mitigation in ensuring safe and stable FCC-ee operation.
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