  KEK-ATF is doing R&D establishing high-current and low-emittance electron
beam for the future linear colliders. In ATF, 15 ps length electron beam is
generated by combination of a thermionic gun, a couple of 357 MHz SHBs, and a
S-band TW buncher. In the linac, beam instability caused by the fluctuated
amplitude and phase of SHBs has been observed. The reason of the fluctuation is
considered to be the multi-pacting in the SHB cavity. In addition, Q value of
the cavity is almost half of the designed value, then SHBs are not operated in
an optimum condition. We have fabricated new SHBs with considerations avoiding
the multi-pacting and recovering Q value. A simulation shows that bunching
quality is improved with the new SHB cavity.
