Paul Greninger (General Atomics)
The bridge coupler design consists of two on axis cavities, two coupling cavities and one center cavity, which is a waveguide terminated with coupling slots at either end. This design meets certain phasing relationships in the low energy section of the APT Linac, which will be presented. The center cavity readily accepts one or two power feeds. While the length of the waveguide determines the resonant frequency, it is bent into a U, whose shape may be made shallower or wider, to accommodate distances between accelerating cavities over a large beta. Coupling slot sizes can be calculated from the Gao theory , with certain modification. The regular coupling equation from one cavity to the next is a function of the fields of both cavities, their stored energy, and elliptical integrals dependent upon the slot geometry. For the Bridge Coupled Cold Model, the fields and stored energy of the second cavity are replaced with the corresponding quantities from a waveguide resonant cavity operating in a TE013 mode. The resonant frequency of the on-axis cavity should equal 700 MHz. Its final frequency is calculated from the Superfish no slot frequency, less the frequency drop associated with two coupling slots, plus the action of the drift tube stems. Similarly, the coupling cavity frequency is the Superfish no slot frequency, less the action of two coupling slots, one leading into the accelerating cavity, and one leading into the center cavity. Relevant equations will be presented. Finally, the iris feed slot couples from a waveguide feed into the center cavity, which in turn couples to all on axis cavities of the segment in question. Requirements for coupling constant beta (derived from VSWR), will be discussed and an analytical expression presented.
 'Analytical formula for the resonant frequency changes due to opening apertures on the cavity walls,' J. Gao, Nuclear Instruments and Methods in Physics Research, A311 (1992) 437-443 North Holland
*Work supported by U.S. DOE contract DE-AC04-96AL89607.
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