The design and operation of a field emission array (FEA) cathode and the subsequent demonstration of the first FEA gyrotron are presented. Up to 10 mA from 30 000 tips was achieved reproducibly from each of ten chips in a gyrotron environment, namely, a vacuum 1 3 10 28 mbar, 250 kV potential with multiple chip operation. The design parameters of the FEA gun were similar to those of a magnetron injection gun with an achievable electron beam current of 50 -100 mA and measured power 720 W cw. Coherent microwave radiation was detected in both TE 02 at 30.1 GHz and TE 03 at 43.6 GHz, with a starting current of 1 mA. [S0031-9007(96)