Abstract
A 4-rod deflecting structure is proposed as a possible crab cavity design for the LHC high luminosity upgrade. Crab cavities are required for the LHC luminosity upgrade to provide a greater bunch overlap in the presence of a crossing angle, but must fit in the existing limited space. The structure has two parallel sections consisting of two longitudinally opposing quarter-wave rods, where each rod has the opposite charge from each of its nearest neighbors. The structure is transversely compact because the frequency is dependent on the rod lengths rather than the cavity radius. Simulations were undertaken to investigate the effect of rod shape on surface fields, higher order multipole terms and induced wakefields in order to obtain the optimal rod shape. The simulation results presented show that the addition of focus electrodes or by shaping the rods the sextupole contribution of the cavity voltage can be negated; the sextupole contribution is 321.57 mTm/m2, Epeak=27.7 MV/m and Bpeak=63.9 mT at the design voltage of 3 MV. The damping requirements for the LHC are critical and suitable couplers to damp all modes but the operating mode are presented. The results of various testing cycles of the first SRF 4 rod prototype cavity are presented and show that the cavity has reached the required transverse voltage of 3 MV.
Highlights
The LHC will be upgraded to increase the luminosity by a factor of 10 during the third long shutdown (LS3); which is planned for 2023-2025
III A and III B we show how focus electrodes and kidney-shaped rods can be designed to negate these sextupole fields in the cavity; kidney-shaped rods are selected for the final design as they allow for lower peak surface fields than wing-shaped rods and they are easier to machine
Four quarter-wave rods, aligned longitudinally, provide a deflecting field where transversely opposing rods have opposite polarity. Important cavity parameters, such as peak surface fields, lower order mode impedance and multipolar components in the cavity are all governed by the shape of the rods in the cavity
Summary
The LHC will be upgraded to increase the luminosity by a factor of 10 during the third long shutdown (LS3); which is planned for 2023-2025. Each rod has the opposite potential from its transverse and longitudinal neighbor for the crabbing mode, but the structure has four modes closely spaced in frequency for the four different combinations of rod polarity Such a cavity has been designed to meet the stringent HL-LHC spatial requirements initially modify the CEBAF design by increasing the beam pipe radius and rod size SRF operation in the LHC. Due to the lack of azimuthal symmetry the deflecting field is not a pure dipole and contains many higher order multipolar components This results in a transverse deflecting voltage that varies with particle transverse position which reduces the dynamic aperture of the LHC.
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