The development of a superconducting traveling wave accelerating cavity with high gradient

The required accelerating gradient in the ILC project is over 30 MeV/m [1]. For current technology, the maximum accelerating gradient in superconducting (SC) structures is limited mainly by the value of the surface RF magnetic field. In order to increase the gradient, the RF magnetic field is distributed homogeneously over the cavity surface (low-loss structure), and coupling to the beam is improved by introducing aperture and cell shape (re-entrant structure). These features allow gradients in excess of 56 MeV/m to be obtained for a single-cell cavity. Further improvement of the coupling to the beam may be achieved by using a traveling wave (TW) SC structure with small phase advance per cell. Calculations show that an additional gradient increase by up to 46% is possible if a π/2 TW SC structure is used. However, a TW SC structure requires a SC feedback waveguide to return the few GW of circulating RF power from the structure output back to the structure input. We discuss variants of the superconducting traveling wave ring (STWR) with one and two feeding couplers.