A Versatile Beam Loss Monitoring System for Clic

The design of a potential CLIC beam loss monitoring (BLM) system presents multiple challenges. To successfully cover the 48 km of beamline, ionisation chambers and optical fibre BLMs are under investigation. The former fulfils all CLIC requirements but would need more than 40000 monitors to protect the whole facility. For the latter, the capability of reconstructing the original loss position with a multi-bunch beam pulse and multiple loss locations still needs to be quantified. Two main sources of background for beam loss measurements are identified for CLIC. The two-beam accelerator scheme introduces so-called crosstalk, i.e. detection of losses originating in one beam line by the monitors protecting the other. Moreover, electrons emitted from the inner surface of RF cavities and boosted by the high RF gradients may produce signals in neighbouring BLMs, limiting their ability to detect real beam losses. This contribution presents the results of dedicated experiments performed in the CLIC Test Facility to quantify the position resolution of optical fibre BLMs in a multi-bunch, multi-loss scenario as well as the sensitivity limitations due to crosstalk and electron field emission. TOWARDS A CLIC BLM SYSTEM The BLM system is a key beam instrumentation element, protecting the machine from potentially dangerous instabilities while also providing beam diagnostics by localising and characterising the beam loss. The design of a BLM system capable of performing both functions for the Compact Linear Collider (CLIC) [1] is highly challenging due to the originality in the design of the machine. Its main constituent is the Two-BeamModule (TBM), a 2m long combination of accelerating structures (AS), quadrupoles and the Power Extraction and Transfer Structures (PETS) linking the parallel Main Beam (MB) and Drive Beam (DB) lines. The CLIC BLM system needs to cover 48 km of beam lines and more than 45000 quadrupoles. The idea of using optical fibre BLMs (OBLMs) has been introduced as a costeffective alternative to standard ionisation chambers. The main challenge is to achieve an adequate position resolution to distinguish losses occurring at consecutive quadrupole or cavity locations. Previous studies [2] for the case of singlelocation losses with long (1 μs) electron pulses have demonstrated that localisation of losses to within 2m is achievable, easily distinguishing between different quadrupoles. In the ∗ [email protected] present paper, the localisation of multi-location beam losses with multi-bunch beam is considered. Prior to the design of a CLIC BLM system, two factors that may limit the BLM sensitivity need to be thoroughly examined: the field emitted electrons from the high gradient RF cavities that can be accelerated, escape the cavity, and be detected by OBLMs; and the beam loss crosstalk between the DB and MB linacs. Both phenomena have been investigated, the field emission in a CLIC AS and the crosstalk in a prototype CLIC TBM.