Two basic designs have been developed for waveguide hybrids, or 3-dB couplers, capable of handling hundreds of megawatts at X-band. Coupling is provided by one or two connecting waveguides with h-plane junctions and matching elements. In the former case, the connecting waveguide supports two modes. Small apertures and field-enhancing ebends are avoided to reduce the risk of rf breakdown. The h-plane symmetry also allows the use of over-moded rectangular waveguide in which the height has been increased to reduce field amplitudes without affecting the scattering matrix. The theory and designs are presented, along with the results of prototype tests of functionality and power-handling capability. Such a device is integral to the rf pulse compression or power distribution system [2] of the Next Linear Collider (NLC) [1] for combining, splitting, and directing power. This work was motivated by the observation of rf breakdown at power levels above 200 MW in conventional and modified magic-T's.

In the course of developing new high peak power klystrons at SLAC, high electric fields in several regions of these devices have become an important source of vacuum breakdown phenomena. In addition, a. renewed interest in breakdown phenomena for nanosecond pulse, multi-megavoll per centimeter fields has been sparked by recent R&D work in the area of gigawalt RF sourcesThe most important regions of electrical breakdown are in the output c*vjiy gap area, the RF ceramic windows, and the gun ceramic insulator.The details of the observed breakdown in these regions, ex periments performed to understand the phenomena arid solu tions found to alleviate the problems will be discussed.Recently experiments have been performed on a new proto type R&D klystron. Peak electric fields across the output cavity gaps of this klystron exceed 2 MV/cm. The effect of peak field duration (i,e> pulse width) on the onset of breakdown havt-been measured. The pulse widths varied from tens of nanoseconds to microseconds. Results from these experiments will be presented.

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RF breakdown studies are presently being carried out at SLAC with klystron cavities in a traveling wave resonator (TWR). Different kinds of fabrication methods and several kinds of semiconducting and insulating coatings have been applied to XBand TMolo cavities. RF breakdown thresholds up to 250 MV/m have been obtained. Dark current levels were found to be depressed in TiN-coated and single-point diamond turned cavities. A new TMt,20 cavity with demountable electrodes has been designed and will be used to test a variety of materials, coatings, and processes. Recent tests of klystron output windows at 119 MW are also presented in this paper.

We describe the design, fabrication and bench-study of a mm-wave cavity employed as a relativistic klystron output structure. The OFE copper cavity was prepared by electrodischarge machining and diffusion bonding, cleaned, and tuned to 91.4 GHz. Measured cavity characteristics are presented and compared with theory, including quality factor, Q, coupling parameter ß, scattering matrix S 11 , and axial electric field profile E z . This work provides the basis for understanding of the cavity as a transfer structure.