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The National Bureau of Standards’ (NBS) cw racetrack microtron (RTM) will be utilized as a driver for a free electron laser (FEL) oscillator. The NBS RTM possesses many exceptional properties of value for the FEL: (i) cw operation; (ii) energy from 20–185 MeV; (iii) small energy spread and emittance; (iv) excellent energy stability; and (v) high average power. The 1D FEL gain formula predicts that the FEL would oscillate at the fundamental approximately from 0.25–10 μm when upgrading the peak current to ≥2 A. In this paper, we present 3D self‐consistent numerical results including several realistic effects, such as emittance, betatron oscillations, diffraction, and refraction. The results indicate that the design value of the transverse emittance is small enough that it does not degrade the FEL performance for intermediate to long wavelengths, and only slightly degrades the performance at the shortest wavelength under consideration. Due to the good emittance, the current density is high enough that focusing, or guiding, begins to manifest itself for wavelengths ≫2.0 μm.