High Power S-Band Vacuum Load

S-Band vacuum loads at the SLAC linac are encountering operational problems now that they have to operate under the stringent requirements of the LCLS: 50 MW peak power, 6 kW average power, and extremely tight phase stability for the linac. Failure mechanisms have been studied which suggest an RF surface breakdown of the 200 m Kanthal layer. We propose a novel solution which incorporates mode conversion from TE10 in rectangular waveguide to TE01 in round waveguide. Lossy material will be placed in the round waveguide, and the selection of the TE01 mode minimizes the electric field normal to the surface of the lossy material. This lossy material in the TE01 round waveguide will be mechanically confined in compression (without brazing), in order to eliminate operationally induced tensile stresses in the lossy material. The wrap-around mode converter will be based upon the X-band mode converter invented at SLAC, and the TE01 waveguide load design will be based upon HOM load designs developed by Muons, Inc. In Phase I, a novel lossy ceramic material and a mechanical system for incorporating it into an S-band dry load were designed and tested. The dry load operates in the TE01 mode and incorporates mode converters designed by SLAC. The lossy ceramic components are cast into cylinders and other novel shapes from slurries composed of mixtures of SiC and porcelain and processed to full densification and vitrification. The microwave characteristics of the lossy ceramic cylinders were measured to determine the optimum mixture for various elements of the load. During Phase II, the manufacturing of components with the lossy ceramic material will be studied to find the optimum design for low-cost manufacturing of the complete load. This will include manufacturing processes for the mode converter, minimized post machining of the lossy ceramic material, compression techniques for various sections of the load, optimum configurations of the components to even out the heat load, high power testing and life tests. Commercial Applications and Other Benefits: This dry load system will have a wide range of application from beam line HOM loads to TE01 dry loads. Every SRF accelerator will benefit from the use of this material. The ability to cast the material into novel shapes allows for low cost solutions which would otherwise be prohibitive due to machining costs of more traditional lossy ceramics.