Simulation of an Industrial Magnetron Using Cathode Modulation

Abstract

Results of a simulation study of the L3 Electron Devices Inc. CWM75KW industrial strapped magnetron will be presented. This study is part of a larger project which studies the feasibility of the achieving phase control and faster startup in the magnetron via controlled electron injection by using gated field emission arrays (GFEAs). The device was simulated by using the 3-D PIC code VSim at its typical operating conditions (18kV, 5A, 1900G, 896-929MHz). The simulated geometry of the device is a one-to-one reconstruction of the physical device based on drawings provided by L3 Electron Devices Inc. The startup behavior was examined with 1) no priming of any kind, 2) RF Priming, and 3) cathode modulation. With no priming or modulation of any kind, the simulated device failed to oscillate in a simulation time of 300 ns; this result was expected since the actual device startup may take milliseconds. For the RF priming study, a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pi$</tex> -mode RF field at the magnetron's operating power (75kW) was defined in the magnetron's interaction space; this driving field was then shut off after 50 ns. Upon removal of the RF driving field, the RF field strength in the interaction space begins to decay; however, after another 100 ns (150 ns from the beginning of the simulation), the RF field strength begins to regrow, and the device starts to oscillate. With cathode modulation, electrons are injected in-phase to form electron spokes; the simulated device was able to reach full oscillation within 130ns of startup by continuous modulated electron injection. Analysis of the cavity voltage, after full oscillation was reached, indicates an oscillation frequency of 898.07-914.49 MHz.