• J. Rose
  BNL, Upton, Long Island, New York
• A.E. Bogle, T.L. Grimm
  Niowave, Inc., Lansing, Michigan
• T. Yanagisawa
  MHI, Kobe

NSLS-II is a new ultra-bright 3GeV 3rd generation synchrotron radiation light source. The performance goals require operation with a beam current of 500mA and a bunch current of at least 0.5mA. Ion clearing gaps are required to suppress ion effects on the beam. The natural bunch length of 3mm is planned to be lengthened by means of a third harmonic cavity in order increase the Touschek limited lifetime. After an extensive investigation of different cavity geometries a passive, superconducting 2-cell cavity has been selected for prototyping. The cavity is HOM damped with ferrite absorbers on the beam-pipes. The 2-cell cavity simplifies the tuner design as compared to two independent cells. Tradeoffs between the damping of the higher order modes, thermal isolation associated with the large beam tubes and overall cavity length are described. A copper prototype has been constructed and measurements of fundamental and higher order modes will be compared to calculated values.