A high efficiency, high power magnicon at 34.272 GHz has been designed and built as a microwave source to develop RF technology for a future multi‐TeV electron‐positron linear collider. To develop this technology, this new RF source is being perfected for necessary tests of accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. After preliminary RF conditioning of only about 2×105pulses at reduced beam voltage and pulse width, the magnicon produced an output power at 34.3 GHz of 10.5 MW in 0.25 &mgr;s pulses, with a gain of 54 dB. Slotted line measurements confirmed that the output was monochromatic to within a margin of at least 30 dB. © 2003 American Institute of Physics