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Physical principles of operation of the orotton, a Smith-Purcell free-electron laser in the millimeter and submillimeter wavelength regions, are described. Electron bunching in this device occurs when the electron beam interacts with an evanescent component of the electromagnetic wave that propagates along the metallic diffraction grating of the device with a phase velocity neatly equal to the electron velocity; this bunching produces coherent radiation. The equations of motion are linearized, and the starting current and electronic tuning characteristics of the device are calculated in closed form. Numerical calculations of the electron efficiency are described.