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Beam Break Up (BBU) thresholds have been studied as part of the Linac focusing scheme for the proposed 4GLS accelerator. A graded gradient focusing scheme, with a triplet of quadrupoles between each of the modules within the Linac has been chosen. These quadrupoles are set-up in a defocusing – focusing – defocusing format with strengths of -1/2k, k, -1/2k. This value of k was altered and the BBU thresholds for the machine calculated using various BBU codes. Alternate cavity designs have also been investigated using CST’s Microwave Studio to see how the effects of higher order modes (HOMs) can be minimised whilst maintaining fundamental field flatness across theaccelerating cells. The number of cells/unit length and cell-to-cell geometries have also been parameterised and the corresponding BBU thresholds presented as a function of cavity geometry, with the intention of providing an optimum solution for 4GLS.
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