The waveguide to cavity coupling coefficient β for two types of accelerating structures: a Plane Wave Transformer (PWT) linac and a 1.6 cell photocathode gun have been tuned to obtain critical coupling in both. Analytical calculations of the dimensions of the slot required for critical coupling have been done using Gao's formulation based on the Bethe's theory for hole coupling. While the PWT linac structure, with high intercell coupling, shows good agreement between measured and predicted slot dimensions for different values of β, the agreement is not so good in the photocathode gun on account of poor inter-cell coupling and a new tuning method has been devised to tune the gun. This paper discusses details of the analytical calculation of slot dimensions for the two structures, their comparison with experimentally measured results, and the procedure adopted for tuning the two structures to critical coupling.
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