Abstract: An RF circuit for an electron beam device is formed with successive layers of fusible metal particles applied in layers and translated with respect to an electron beam. Segments of an electron device may be formed which include an open RF cavity at each end of the segment, which can be machined before the segments are brazed together. In one example, the electron beam device may have iron pole pieces assembled into the partially fused device to provide a magnetic circuit and support for subsequent layers of fused particles. The inner surfaces of the RF circuit structure may be smoothed using an etching or machining operation, and the segments may be brazed together to form a completed RF circuit structure with the magnetic pole pieces included in the monolithic structure segments.