Design and technology of high-power couplers, with a special view on   superconducting RF
  The high-power RF coupler is the connecting part between the RF transmission line and the RF cavity and provides the electromagnetic power to the cavity and the particle beam. In addition to this RF function it also has to provide the vacuum barrier for the beam vacuum. High-power couplers are one of the most critical parts of the RF cavity system in an accelerator. A good RF and mechanical design as well as a high quality fabrication are essential for efficient and reliable operation of an accelerator. 
W.-D. Moeller (DESY)
physics.acc-ph