Influence of Spent Electrons on BWO Operation

Summary form only given. One of the important problems for high power microwave sources is to provide removal of spent electrons to avoid overheating and erosion of the output channel, part of which is used as the electron dump. An effective solution of this problem is achieved by spreading the electrons along the surface of the output channel. However, it is noted that changing the location of the electron dump leads to a change in the output characteristics of the BWO (such as radiation power P and pulse duration). Evidently, spent electrons can exert an influence on BWO operation owing to electrons reflected from an electron dump or a virtual cathode. The appearance of a virtual cathode is associated with a large spread of electron energies after passing through the interaction space. Computer simulations show that the distribution of spent electrons over energies becomes almost symmetrical with respect to their average energy gamma macr with maximal values of current I on the ends gamma_{min, max} ap gamma macrplusmnDeltagamma of the distribution I(gamma), where Deltagamma ap gamma macr. Thus, there is a large fraction of electrons with very low energy, and even an insignificant deceleration of the spent electron beam in the output channel can lead to the appearance of a virtual cathode. Two factors can lead to the degradation of output characteristics owing to reflected electrons. One of them is an increase of space charge near a cathode suppressing its electron emission, which leads to degradation of radiation when the BWO operates in a regime of short pulses or modulation of radiated power. Another reason is the influence of cyclotron absorption. Simultaneously with double (Cherenkov and cyclotron) interaction of initial monoenergetic electron beam with the operating wave, the same double interaction of reflected electrons with the reflected operating wave takes place, which significantly extends the zone of cyclotron absorption due to the - wide energy distribution. As a result, a very strong guide magnetic field is required to achieve the maximum radiated power. Computer simulations of the experimental relativistic BWO confirming the aforementioned factors are presented.