Pulse-Shortening in a Relativistic Magnetron: The Role of Anode Block Axial Endcaps

A six-vane relativistic magnetron with a single radial output slot is studied by 3-D particle in cell simulations without the presence of plasma. We find that when the six radial slots in the anode block are closed at their axial ends by conducting endcaps, the power flow behaves in a way different from that when they are not. When there are no endcaps, microwave power pulse-shortening is the result of the magnetron impedance being undermatched to the pulsed-power generator, and it cancels when the magnetron impedance is increased sufficiently. The axial electron current flowing in the downstream cavity reduces to negligible values and plays no part in the power flow. When anode block endcaps are added, this axial leakage current is significant and the downstream cavity acts as a second load attached in parallel to the magnetron. For pulse-shortening to cancel, the magnetron impedance needs to increase, which can be achieved by reducing the downstream axial section impedance. We demonstrate this by increasing the axial current. Between the parameter regions where pulse-shortening exists and where it does not, there is a metastable transition region where there is mode competition resulting in a beating pattern separating two regions each characterized by a single mode.