The suppression of stray output of Multi-Beam broadband Klystron

The purity of output spectrum of high power microwave vacuum electronic devices is very important for the microwave electronic system. The stray outputs with frequency in or near the operating frequency band have been observed in some Multi-Beam Klystron (MBK). The exits of the stray outputs have serious influence on the normal operation of the microwave electronic systems. The variation of stray outputs with different operating frequency observed in an S band MBK was described, and the reasons for generating the stray outputs were analyzed by using 1D klystron code in this paper. The reflecting electrons and secondary electrons caused by the interception of electron beam on gap head of the cavity are responsible for the stray outputs. They will be accelerated by RF electric field on the cavity gap in reverse direction, and return back to previous cavity. The retuning electrons provide the feedback way between successive two cavities, and generate oscillation, which will be amplified by following cavities and form stray outputs. The stray outputs can be reduced or suppressed by decreasing the number of retuning electrons. It can be realized by making small slots on the surface of the gap head. The new MBK with slots on the gap head was made and tested, the power level of the stray outputs are less than -70dBc.