Novel accelerator structure couplers

Recent experience with X-band accelerator structure development has shown the rf input coupler to be the region most prone to rf breakdown and degradation, effectively limiting the operating gradient. A major factor in this appears to be high magnetic fields at the sharp edges of the coupling irises. As a first response to this problem, couplers with rounded and thickened iris horns have been employed, with improved performance. In addition, conceptually new coupler designs have been developed, in which power is coupled through the broadwall of the feed waveguide. A prototype "mode converter" coupler, which launches the TM/sub 01/ mode in circular waveguide before coupling through a matching cell into the main structure, has been tested with great success. With peak surface fields below those in the body of the structure, this coupler represents a break-through in the NLC structure program. The design of this coupler and of variations which use beamline space more efficiently are described here. The latter include a coupler in which power passes directly through an iris in the broad wall of the rectangular waveguide into a matching cell and one which makes the waveguide itself an accelerating cell. We also discuss techniques for matching such couplers.