By Rama Calaga (CERN)

The first RF dipole crab cavity cryomodule, which arrived with two major non-conformities, has now been successfully repaired and commissioned.

This cryomodule was completed under a joint collaboration between CERN and UK-STFC and shipped to CERN in October 2023. During the acceptance tests in the SM18 facility, two major non-conformities were detected on the fundamental power couplers (FPCs) leading to a postponement of its installation into the SPS machine. This consisted of a vacuum leak on cavity 2 in the FPC line and a short circuit on the FPC of cavity 1.  The origin of both non-conformities is believed to be due to an alignment issue during the cryomodule assembly.

A crash program was initiated to execute a repair without opening the beam vacuum on each of the cavities in two steps, followed by a cool-down and RF test.

A special supporting system was designed and implemented on the cryomodule to support the cavity string during this repair. The vacuum leak was successfully closed after sufficient access to the FPC line was made available with the special supporting system. This was followed by cool-down to 2K and comprehensive RF testing of this cavity in the second quarter of 2024.

The short circuit on cavity 1 was repaired in a second step up by applying a pre-determined value of force to partially straighten the FPC line and recover coaxiality.  This was followed by a cool-down and extensive testing of both cavities with high power similar to the machine configuration.

A four-week period of RF conditioning was necessary to bring the input RF power to nominal levels followed by several weeks of RF commissioning to nominal voltage of 3.4 MV or higher in pulsed mode. In continuous mode operation, cavity 1 was stably operated up to 2.1 MV and cavity 2 up to 3.4 MV. The RF operation was successfully terminated to prepare the module for its installation in the SPS and further RF commissioning will continue in the SPS machine in 2025.

Following these repairs, the two cavities were successfully commissioned and are currently being installed into the SPS (figure 2). 

Figure 2. Repaired RFD cryomodule prototype before movement to the SPS (left), and in the SPS (right). 

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