The axial monotron as a high-power microwave tube

Summary form only given. Having been known since the 1930s, the monotron is a transit-time oscillator where the beam electrons cross the interaction space in a transit angle close to (4n+1)/spl pi//2, with n an integer. Under this synchronism condition, the beam can interact unstably with a cavity mode, causing the RF field to grow at the expense of the DC beam energy. Operating in the lowest order oscillation mode (n=1) gives the highest conversion efficiency on the order of 20%. The progress in pulsed high-voltage technology - in producing relativistic electron beams in the tens of kiloampere range - has rekindled interest in the monotron as a high-power microwave source. The present paper, however, addresses the monotron in the low-current regime by examining its capabilities regarding electronic tuning and high efficiency operation with three hollow concentric beams to produce hundreds of kilowatt power output. It is shown first (within the context of a one-dimensional analysis neglecting space-charge effects) that the discrete frequency spectrum of the monotron instability is determined by three parameters, namely, the interaction length, the injection beam energy, and the amplitude of the RF field. Then the paper discusses how the operating parameters should be selected to give maximum efficiency and uses such a methodology to design a triple-beam TM040, 6.7GHz monotron. The high efficiency operating regime predicted by one-dimensional analysis is verified through a 2.5-D particle-in-cell simulation, in which each beam is injected into the cavity (radius 8.4 cm, length 1.0 cm) at 80A current and 10keV energy. The calculated average output power yields 340kW, amounting to an overall efficiency of 14.0% in close agreement with typical 15% conversion efficiencies anticipated by one-dimensional theory. The last part of the paper reviews the present work and compares the monotron with the klystron and the gyrotron by indicating advantages and disadvantages.