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Dear Walter Wuensch, an invitation for the WebEx connection was forwarded to me by Bob Rimmer. Unfortunately the time would be in the middle of the night here at JLAB. I am not familiar yet with the TBL PETS plans and requirements on the HOM absorbers, but I have recently started to look into HOM absorber materials to aim for a simpler design of the present CEBAF rf load absorbers. These have to work at 2K. I have tested prototype absorbers made of AlN-based composites (with lossy carbon) down to 2K. These absorbers are placed in long standard WR340 waveguides (cutoff ~1.4GHz). The frequency range covered is limited by the coaxial-to-waveguide adapter, i.e. 1.7-2.6 GHz. For CEBAF cavities the investigation in this frequency range is sufficient as the first parasitic dipole passband modes are taken into account. Different waveguide sizes or better than the commercially available adapters would be needed to cover a wider frequency range. I am using the TDR capability of Vector Network Analyzers to gate out the unwanted signal from the adapters leaving just the DUT/absorbers. Attached a picture of the setup and the reflection response of the prototype absorber at 2K. The shape is not optimized but can by by analyzing the results in order to evaluate the complex material properties. Our plans are to test further material candidates. I will receive other samples (AlN composites) in February and I do have SiC absorbers originally foreseen for our proposal of a high current cryomodule where loads are placed at room temperature. If you are interested in our activities please let me know. With Best Regards Frank Marhauser
