Preliminary Test of Superconducting RF Cavities for PLS-II

The main part of installation for PLS-II upgrade was finished at June 2012 and is on the way of user operation through elaborate commissioning. Up to now, the achievement is 150mA beam current at 3.GeV with multibunch mode with 5 normal conducting cavities which served in the PLS before. With 2 SRF cavities, installed during summer maintenance of 2012, PLS-II will be 300 mA beam current with 20 IDs. One additional SRF cavity will be operated in October 2014, and beam current will rise to 400 mA. The two SRF modules are at the final stage to be installed at storage ring in August 2012. The two main subsystems such as SRF cavities and ceramic windows were tested independently to confirm their performances. Each cavity recorded its accelerating voltage as 3.27 MV and 3.24 MV at 4.2K, respectively. Two RF windows also passed its specification, 300 kW CW travelling wave and 150 kW CW standing wave. One cryomodule was confirmed through the after-fabrication test (FAT) and delivered to PAL site at present, under preparation of horizontal test. The other one is just at last step for the factory acceptance test. INTRODUCTION The PLS-II is the upgrade machine from the old PLS, which was a 3rd generation synchrotron light source with 2.5 GeV energy and 190 mA beam current and provided users beam since 1996. The energy and beam current of PLS-II are 3 GeV and 400 mA, respectively. The number of insertion devices at PLS-II is 20, compared to 10 of PLS by modifying TBA to DBA magnetic lattice. It can be possible by shortening bending radius of dipole magnet with same ring circumference of PLS. The expected power loss from 24 BMs and 20 IDs including broad band losses in RF cavities, vacuum chambers, BPMs, gate valves and etc is estimated as high as 500 kW. The RF related parameters of PLS-II are shown Table 1. The design requirements for PLS-II upgrade are low higherorder modes (HOM), high reliability and high power capability. Even though top-up mode operation at PLS-II, long beam lifetime as long as 10 hours is also important requirement. For required lifetime and sufficient RF bucket height at injection system the energy be assured by 4.5 MV RF voltage with the over voltage factor of 2.71. With deliberate design and investigation based on the requirements above, the superconducting RF technology was chosen. By selecting SRF cavities with low impedances from cavity HOMs, more stable beam operation at high beam current is expected with low beam emittance. The relative short MTBF (Mean Time Between Failure) compared to NC RF system should be accepted at the initial phase of SRF operation, but with deliberate efforts such as sufficient aging SRF cavities with beam operation and more experiences with training the MTBF at PLS-II is expected to be approached to reasonable standard. Tow SRF modules will be operated from October 2012 and other one will be October 2014. The 1st two modules are the last phase of fabrication. Table 1: Parameters of PLS-II Storage Ring