Two prototypes of RF dipole cavities which provide horizontal crabbing for the HL-LHC were successfully built at CERN. The frequency tuning setup before the final two welds is shown in the figure below. The frequencies of both cavities were within 100 kHz (negligible) showing the surgical precision in the manufacturing techniques for such a complicated RF structure. RF bead-pull measurements confirm the pole symmetry to within the specified tolerance of 300 microns.
After chemical etching of the internal RF surfaces to remove impurity layers (see Figure 11) and high temperature heat treatment at 650 degrees Celsius, the two cavities have been moved to the SM18 test facility and are being prepared in the ISO4 clean room prior to their high field validation at 2K. The two cavities will be tested to their nominal voltage of 3.4 MV and up to their quench limit.
Immediately after the RF validation, the cavities are first enclosed with a cold magnetic shield which is provided by the HL-LHC-UK2 collaboration as an in-kind contribution to the project. The cold magnetic shield on each cavity along with the cryomodule external magnetic shield will suppress the ambient magnetic field and thereby reduce the RF losses on the Niobium cavity surface. This assembly will then be enclosed in a special Titanium Helium vessel acting as the 2K Helium reservoir and as structural support to the cavity during its operation.
The technical design of the cryomodule in the final HL-LHC configuration is advancing jointly with HL-LHC-UK2 with the cryostating to begin in Q2 2021. The completed cryomodule is expected to be delivered back to CERN at the end of 2021 for validation in SM18 and beam tests in the SPS during the Run3 operation. Until this time, the DQW cavities in the SPS will remain in the tunnel for further beam tests in 2021 to push the voltage performance and operate with increased beam currents among other studies.
In parallel, the two US-LARP built DQW cavities, which have already undergone a campaign of RF tests both in the US and CERN, are being sent to the 2K RF testing facility at Uppsala University and KEK in Japan. The cavity sent to Uppsala will be the first SRF cavity in their newly built vertical cryostat which will also serve as a qualification test of the facility with crab cavities. The second cavity being sent to KEK will undergo an electro-polishing (EP) treatment developed jointly between KEK and HL-LHC for the RF surface. KEK plans to continue the R&D of Electro Polishing treatment of the crab cavities to improve the performance reach to beyond the already state-of-the-art surface fields demonstrated by a more conventional chemical etching, aka BCP.