Accumulation of the space charge in the adiabatic trap of centimeter and millimeter wavelength gyrotrons

In gyrotron some electrons are captured into adiabatic trap between the cathode and cavity and then make worse the beam quality and can initiate beam instability in some regimes. The results of numerical simulation of Helical Electron Beams (HEBs) formed by the magnetron injection guns (MIGs) for gyrotrons of centimeter and millimeter wavelengths are presented. Numerical procedure is based on the PIC approach and takes into account influence of the beam space charge, secondary emission from the cathode, thermal velocities of electrons and emitter roughness. It is shown that the secondary emission is the most important factor for the accumulation of the particles in the trap and further development of the space charge oscillations. MIGs with laminar HEBs are more preferable to decrease value of the locked charge and obtain stable electron beam.