Ultimate Field Gradient in Metallic Structures

Significant progress has been made over the past decade by studies of normal-conducting linear colliders, NLC/JLC (Next/Japanese Linear Collider) and CLIC (Compact Linear Collider), to raise achievable accelerating gradient from the range of 20-30 MV/m up to 100-120 MV/m. The gain has come through a greatly increased understanding of high-power rf phenomena, development of quantitative high-gradient rf design methods, refinements in cavity fabrication techniques and through development of high peak rf power sources. Recently accelerating gradients in excess of 100 MV/m, at very low breakdown rates, have been successfully achieved with new techniques of conditioning in numerous prototypes at different laboratories. The talk will report on the impact of these new results on the understanding of the physics of breakdown and of conditioning, and on the ultimate gradients that can be expected in metallic RF structures.