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Description
An experimental setup was designed and implemented for the study of the multipactor effect in a rectangular WR 284 waveguide geometry under high vacuum. Operated at S-Band frequencies the multipactoring electrons are directly detected via an Electron Multiplier Tube (EMT). The custom fabricated test section consists of a copper coated steel structure transitioning to standard WR 284 waveguide dimensions via varying tapers. An RF transmitter with 2.8 kW peak power and a center frequency of 2.85 GHz is used to inject an RF signal into a traveling-wave resonator, to which the test section is integrated, pushing the traveling wave power to > 20 kW.
Operating in the dominant TE$_{10}$ mode allowed the height of the waveguide to be changed without affecting the cutoff frequency, which was facilitated with an exponentially tapered impedance transformer. Multiple tapers have been constructed from OFHC copper with gap sizes conducive to developing first and third order multipactor (at 2 mm and 5.5 mm, respectively). With the reduction of the inner waveguide height from ~34 mm to 2 mm, an electric field greater than 2.7 kV/cm was achieved at 2.8 kW input power within the ring resonator. Simulations reveal a secondary electron yield of greater than one for copper for these conditions.
For detecting multipactoring electrons, the EMT is aligned with a 1 mm circular aperture in the broadside surface of the waveguide enabling electron multiplication through the tube dynodes. Initial electron seeding is accomplished via UV light below 280 nm injected into the waveguide structure. The onset of multipactor in graphene coated copper versus pure copper surfaces is discussed. Clearly observable is the early emergence of the direct electron signal over any visible light emission that could be detected with a photomultiplier tube.
