R&D Status of C-Band Accelerating Section for SuperKEKB

For the future upgrade of KEKB injector linac for SuperKEKB to raise its positron beam energy from 3.5 to 8.0 GeV, we have been developing C-band accelerating section. Prototype study has begun based on a half-scale design of the present S-band section. The first prototype has suffered from frequent breakdowns around the couplers. Then we started improving the coupler structure. The second prototype has completed recently and two more sections with different types of couplers are under fabrication. We report on an R&D status of these four prototypes of the C-band accelerating section.