High gradient studies for ILC with single-cell re-entrant shape and elliptical shape cavities made of fine-grain and large-grain niobium

Based on the encouraging results of the first 1300 MHz 70 mm aperture single-cell re-entrant cavities, we continue the high gradient studies for ILC with new re-entrant cavities made of fine-grain as well as large-grain niobium. These new cavities have smaller apertures of 60 mm, providing a further reduced H
_pk
/E
_acc
or a further improved ultimate gradient. Four 1300 MHz 60 mm aperture reentrant cavities are made, two out of fine grain niobium and the other two out of large-grain niobium. In addition, two elliptical shape 1500 MHz cavities are also made out of large-grain niobium. A new record gradient of 59 MV/m was achieved in a 60 mm aperture 1300 MHz single-cell fine-grain niobium cavity.