The slotted waveguide slow-wave stochastic cooling arrays are an integral part of the 4-8 GHz Debuncher Upgrade at FNAL. Unlike the standard array of stripline electrodes, these structures are designed to work when the beam pipe can support many microwave modes. The design theory and beam measurement results of this new type of pickup structure will be presented in this paper. In previous collider runs at Fermilab, all of the stochastic cooling pickup and kicker arrays consisted of stripline or planar loop electrodes. The signals from these electrodes are combined with a binary combiner tree formed by microstrip or stripline transmission lines. With a binary combining scheme, there must be no waveguide modes traveling down the beam pipe that would provide an alternate signal path in parallel to the binary combiner tree. The nominal Debuncher transverse aperture is 30 p-mm-mrad (95% un-normalized). To account for closed-orbit variations, the design aperture of the cooling arrays was set at 40 p-mm-mrad. With lattice beta functions on the order of 10 meters, the transverse dimensions of the beam pipe will be about 40 mm which will propagate waveguide modes above 4 GHz. The presence of travelling waveguide modes in the beam pipe will
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