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A sheet-beam traveling-wave amplifier has been proposed as a high-power generator of radio frequency (RF) from 95 to 300 GHz, using a microfabricated RF slow-wave structure (Carlsten, 2002). The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power, high-average power, and wide bandwidths. Simulations of nominal designs using a vane-loaded waveguide as the slow-wave structure have indicated gains in excess of 1 dB/mm, with extraction efficiencies greater than 20% at 95 GHz with a 120-kV, 20-A electron beam. We have identified stable sheet beam formation and transport as the key enabling technology for this type of device. Also, due to the high aspect ratio in the slow-wave structure, the RF coupling is complicated and requires multiple input and output couplers. The RF mode must be transversely flat over the width of the electron beam, which impacts both the vane design and the input and output coupling. We report on new insights on stable sheet-beam transport and RF mode control in the slow-wave structure.