Bridge Coupler for APT

The Coupled Cavity Drift Tube Linac (CCDTL) used in the Accelerator for the Production of Tritium (APT) is fully described elsewhere [1]. The modules are composed of several machined and brazed segments that must account for the accumulation of dimensional tolerances in the build up of the stack. In addition, space requirements dictate that power fed to the accelerator cannot be through the accelerating cavities. As well, we would like to remove a single segment of the accelerator without removing additional segments. These requirements combined with phasing relationships of the design and space limitations have resulted in a different bridge coupling method used in the module comprising 3-gap segments. The coupling method, phasing relationships and other features that enhance the flexibility of the design will be discussed. 1 BRIDGE COUPLER DESIGN The Bridge Coupler below addresses all of the above problems. A unique feature is the ability to take up tolerances, and employ the design over a wide range of particle velocities. The center cavity is bent into a U, where the distance between the legs may vary, while maintaining a total constant length. This accommodates different spacing between Accelerator Cavities (AC). The bridge coupler consists of an odd number of cavities in order to preserve the π/2 operating mode of the RF structure. Figure 1 shows the basic bridge coupler design. The concept uses TM01 mode pillbox cavities, with axes parallel to the accelerator center-line. A unique aspect of this structure is that due to phasing requirements, the center cavity is no longer a pill box cavity, but has been replaced with a waveguide operating in a TE013 mode. Figure 2 defines the relative phase differences, shown with arrows, for the segments being coupled. The two Coupling Cavities (CC) (see Fig. 1) are unexcited in the π/2 mode, while the center cavity is excited to a power level determined by the relative sizes of the coupling slots shown. Since the center cavity is excited, RF power can be fed directly into it through a slot coupled to a waveguide. *Work supported under contract DE-AC04-96AL89607 Slot Cavity (5) Accelerating Coupling Vacuum Flange Coupling Cavity (4) Cavity (3) Center Location Stem Accelerating Cavity (1) Cavity (2) Coupling Noses for Tuning Power Feed Slot Center Cavity Figure 1. Drawing of Bridge Coupler Cross Section.