High‐gradient linacs studies in the frequency range of CLIC

The present status of research and development for high‐gradient linacs required for future linear colliders is decribed. The interval of RF frequency considered here ranges from about 10 GHz to 30 GHz and many studies made for NLC (Next Linear Collider) proposed at SLAC, KEK, and IHEP as well as for CLIC (CERN LInear Collider) are covered. After a brief recall of the reason for operating at high frequency when high accelerating gradient is required, major issues are addressed. Effects of strong wake fields on the energy spread at the linac exit and on transverse beam stability, as well as possible compensations are summarized. Consequences for multibunch dynamics are also discussed, together with suggested remedies. The difficult question of the alignment tolerances is approached to underline the necessity of a very efficient scheme of trajectory correction, and the state of the art in building accelerating structure prototypes is given. Prospects for microwave klystrons as high‐power RF sources for normal‐conducting electron and positron linacs in NLC are briefly described. Finally, recent results concerning the CLIC scheme for generating the necessary power, based on a two‐stage accelerator, are presented.