Research and Development in a High-Power 6.7 GHz Monotron Joaquim J. Barroso, Pedro J. Castro, José O. Rossi, and José A. N. Gonçalves Associated Plasma Laboratory, National Institute for Space Research - INPE 12227-010 São José dos Campos, SP, Brazil Abstract: This article overviews recent activities carried out in the construction and tests of a 10 kW, 6.7 GHz monotron. In addition to the monotron components— injection electron gun, cavity resonator, and output waveguide—, a hard-tube pulser with three tretrodes in parallel and able to produce negative voltages up to 30kV/20A from a positive DC charging power supply is also discussed. Keywords: electron gun; microwave generation; monotron; high-voltage power supply. Introduction Consisting of a cylindrical cavity driven by a round, hollow electron beam, the monotron is the simplest microwave generator, whose principle of operation relies on transit- time effects experienced by the electron beam propagating across a resonant cavity. Here we describe the main components and operation parameters of a device designed to generate 10 kW output power microwaves at 6.7 GHz. Currently under development at our laboratory, this monotron [1] currently operates with a beam of about 3.0 A at 10 kV as discussed throughout the paper in connection with the operating characteristics of the electron gun Main Components The assembly (Fig.1) includes an injection electron gun, a resonant cavity coupled to the TM-mode output waveguide through a coupling hole. The cathode arrangement is devised to bend the equipotential surfaces so as to confine and focus the beam as the electrons move away from the emitting surface. The lower panel shows the electron beam bunched in the steady–sate regime, in which DC beam power is converted into electromagnetic power at a typical 20 percent conversion efficiency. Without divergence, the beam is injected into the cavity through an annular slot, as can be viewed in Fig.2, whereas the fully assembled device is shown in Fig. 3. Since the cathode should operate at temperatures close to 1000 o C, the choice of materials for its construction is limited. As for the requirements of mechanical strength and low vapor pressure at high temperature, materials of low emissivity and poor thermal properties are needed to reduce heating power losses. The most suitable materials with such properties are the refractory metals and their alloys. Of these, molybdenum was selected for the cavity’s entrance plate onto which, a pair of symmetric slots was drilled (Fig. 2). On the right and closing a ceramic pot, it is shown the circular cathode plate made from nickel on which the electron emission coating is applied. Housing the heater filament inside, the ceramic pot is held by a circular disk of titanium. The supporting disk stays fastened to three circular stainless steels rods directly attached to the vacuum flange. The heater is made of a 0.5-mm-diameter pure tungsten wire tightly wound, with the resulting helix bent to take the shape of a toroidal coil (Fig. 4) housed on the back of the cathode disk to heat the emitting region radiatively. Figure 1. Monotron particle-in-cell configuration Figure 3. Front plate of the cavity and gun assembly Authorized licensed use limited to: INSTITUTO NACIONAL DE PESQUISAS ESPACIAIS. Downloaded on December 9, 2009 at 12:03 from IEEE Xplore. Restrictions apply.