Fig. 1.
Schematic (a) and photograph (b) of the 250 GHz tunable gyrotron oscillator. The electron beam is launched from the electron gun and is focused by the magnetic field gradient, which can be finely adjusted with the gun coil magnet. After depositing energy in the form of microwave power in the interaction cavity, the electron beam is safely collected in the water-cooled collector at the top of the vacuum tube.