A gun design criterion to limit the effects of the electron beam halo in gyrotrons

An important issue in gyrotron operation is the halo of the electron beam of the magnetron injection gun (MIG). It is formed by magnetically trapped electrons between the cathode and the cavity. These magnetically trapped electrons are generated by several sources, such as the roughness of the emitter ring, the secondary emission from other parts of the cathode surface, etc. The effects of the beam halo can be dramatic in gyrotron operation. As an example of such a dramatic influence of the electron beam halo on the operation of the gyrotron, the instabilities observed in the operation of the first industrial prototype EU coaxial cavity gyrotron will be presented. The positions of the damages found during the inspection of the tube indicate that the observed instabilities were absolutely correlated with the electron beam halo. A simple criterion for MIG design is proposed in order to limit the generation of the beam halo. This criterion has been already taken into account for the gun design of the refurbished 1^st prototype coaxial cavity gyrotron. The experiments on this tube showed no electron beam halo effects and instabilities. This important criterion has also been applied for the gun design of the new EU 1MW conventional cavity gyrotron for ITER.