Gyrotron: from the concept to the physics and technology issues related to megawatt sources for fusion reactor

Gyrotrons are millimeter wave oscillators based on the Electron _Cyclotron Maser (ECM) instability. The free energy is the rotational energy of relativistic electrons (1 < y 5_ 1.2) and, from a single particle analysis, a net transfer of energy from the electrons to the electromagnetic field in the resonant structure occurs due to the phase bunching when the wave frequency is larger than the relativistic electron cyclotron frequency K2 or its harmonics. Many types of millimeter wave devices are based on the ECM instability, such as gyro-klystron, gyro-TWA or gyro- BWO. In this review talk, we shall limit ourselves to the gyrotrons.