We studied the physics and properties of rf breakdowns in high gradient traveling wave accelerating structures at 100 GHz and 200 GHz. The structures are open, made of two haves with a gap in between. The rf fields were excited in the structure by an ultra-relativistic electron beam generated by the FACET facility at the SLAC National Accelerator Laboratory. We observed rf breakdowns generated in the presence of GV/m scale electric fields. We varied the rf fields excited by the FACET bunch by moving structure relative to the beam and by changing the gap between structure halves. Reliable breakdowns detectors allowed us to measure the rf breakdown rate at these different rf parameters. We measured radiated rf energy with a pyro-detector and characterized the rf spectrum with an interferometer. When the beam was off-axis, we observed beam deflection in the beam position monitors and on the screen of a magnetic spectrometer. The measurements of the deflection allowed us to verify our calculation of the accelerating gradient. This talk will report results of 200 GHz experiments.
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