||For the Research and Development of
the crab cavity for KEKB, we have fabricated two prototype cavities.
The crab cavity is a beam-deflecting cavity excited with the 508 MHz,
TM110-like mode. To extract unwanted parasitic modes, the cavity has a
squashed cell shape and a coaxial coupler along the beam pipe. The
prototype cavities were tested in a vertical cryostat with/without the
coaxial coupler. We observed multipacting at the first RF excitation.
To study this event in detail, we measured temperatures of the coupler
and cavity during RF tests. Temperature measurement showed that
multipacting occurred at the tip of coaxial coupler at low RF fields,
and then along the coaxial line. These events were processed in an
hour. We observed another type of temperature rise around the iris.
Temperature rise began at low fields, continued to rise up to a surface
peak field of 18 MV/m, and then, disappeared above 20 MV/m. The
excitation mode of the crab cavity has a maximum magnetic field and
zero electric field on the same iris position. Temperature rise near
this region suggests the multipacting events. A model simulation shows
a stable two-point multipacting can exist at this position. We will
present temperature measurement results and discuss multipacting near