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Use of a superconducting (SC) traveling-wave accelerating (STWA) structure with a small phase advance per cell, rather than a standing-wave structure, may provide a significant increase in the accelerating gradient in the ILC linac [1]. For the same surface electric and magnetic fields, the STWA achieves an accelerating gradient 1.2 larger than TESLA-like standing-wave cavities. In addition, the STWA allows longer acceleration cavities, reducing the number of gaps between them. However, the STWA structure requires a SC feedback waveguide to return the few hundreds of MW of circulating RF power from the structure output to the structure input. A test single-cell cavity with feedback was designed and manufactured to demonstrate the possibility of proper processing to achieve a high accelerating gradient. The first results of high-gradient tests of a prototype 1.3 GHz single-cell cavity with feedback waveguide will be presented. [ABSTRACT FROM AUTHOR]

Accelerating gradient is a key parameter of the accelerating structure in large linac facilities, like future Linear Collider. In room temperature accelerating structures the gradient is limited mostly by breakdown phenomena, caused by high surface electric fields or pulse surface heating. High power processing is a necessary procedure to clean surface and improve the gradient. In the best tested X-band structures the achieved gradient is exceed 100 MV/m in of ∼200 ns pulses for breakdown rate of ∼10-7. Gradient limit depends on number of factors and no one theory which can explain all sets of experimental results and predict gradient in new accelerating structure. In paper we briefly overview the recent experimental results of breakdown studies, progress in understanding of gradient limitations and scaling laws. Although superconducting rf technology has been adopted throughout the world for ILC, it has frequently been difficult to reach the predicted performance in these structures due to a number of factors: multipactoring, field emission, Q-slope, thermal breakdown. In paper we are discussing all these phenomena and the ways to increase accelerating gradient in SC cavity, which are a part of worldwide R & D program. [ABSTRACT FROM AUTHOR]

The most severe problem of the International Linear Collider is its high cost, resulting in part from the enormous length of the collider. This length is determined mainly by the achievable accelerating gradient in the RF system of the ILC. In the ILC project the required accelerating gradient is higher than 30 MeV/m. Further improvement of the coupling to the beam may be achieved by using a Traveling Wave SC structure [1]. We have demonstrated that an additional gradient increase of up to 46% may be possible if a π/2 TW SC structure is employed. 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. The test cavity with feedback is designed to demonstrate the possibility of achieving a significantly higher gradient than existing SC structures. The double-coupler powering excitation and tuning have been studied numerically and the corresponding model results are presented. The proposed double-coupler powering scheme significantly reduces the tuning requirements as long as any of the partial modes of given magnitude and phase are excited independently, providing a clear traveling wave regime of structure operation. [ABSTRACT FROM AUTHOR]