  A low loss (LL) type 500 MHz 5-cell superconducting niobium prototype cavity
with large beam aperture has been developed successfully including the
optimization, the deep drawing and electron beam welding, the surface treatment
and the vertical testing. The performance of the fundamental mode was optimized
and the higher order modes were damped by adopting an enlarged beam pipe for
propagation. Surface preparation or treatment including mechanical polishing,
buffered chemical polishing and high pressure rinsing with ultra-pure water and
so on was carried out carefully to promise a perfect inner surface condition.
The vertical testing results show that the accelerating voltage higher than 7.5
MV was obtained while the quality factor was better than 1E9 at 4.2 K. No
obvious multipacting or field emission was found during the test. However, a
quench happened while increasing the field a little higher than 7.5 MV that at
present limited the cavity performance.
