CERN Accelerating science

by Rama Calaga and Ofelia Capatina (CERN)

The crab cavities for HL-LHC and their validation tests in the SPS are well into the fabrication phase. The compact superconducting cavities are made from 4mm high purity (RRR) Niobium sheets which are operated to at 2 K to sustain the very high surface electromagnetic fields at the required performance. The cavities are immersed in bath of super-fluid Helium contained in specially designed Titanium tank. This vessel also provides necessary stiffness to the cavity body to withstand the strong external forces applied to the cavity in its lifetime.

To minimize undesired stresses during assembly to the RF cavity, a novel concept of a bolted helium vessel is used. Superficial welds are applied to ensure vacuum integrity and minimize the stress induced during welding. To validate this approach, a full scale prototype was built and thoroughly tested. The prototype was machined from six thick Titanium plates, mounted and welded using the same procedure envisioned for the real system. The dimensions of the tank were monitored using 12 different position sensors between the different steps of assembly and welding which showed only minor deformation after the full assembly. A pressure test of up to 2.6 bar and a several cool-down tests performed by immersing the tank in a bath of liquid nitrogen.  Leak tightness was verified during these tests. The prototype showed no degradation, plasticity, leaks or unloading of the screws during the assembly and testing steps.

The minor deformations during the welding steps are being analyzed to further compensate them for the real system. Extensive simulations of the steps showed close agreement with the measurements which was also an important validation step.

An internal magnetic shield was proposed as a solution to suppress the residual trapped flux during cool-down. This became essential due to the numerous interfaces of the cavity which cannot be easily shielded by only an internal shield and therefore compromise the cavity performance. The shields are made of Cryophy which after special thermal cycle, exhibits excellent shielding properties. However, their properties can easily be compromised due to shock impact and handling. These shields were developed in collaboration with UK-STFC and UK-industry which were successfully fabricated and assembled. Final shielding measurements show perfect agreement with the expected shielding from simulations and therefore qualified for their assembly into the Helium vessel.