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.

I. Introduction

Radiation power pulse-shortening is found to be inherent to most high-power microwave devices based on power extraction from high-current electron beams. Considerable effort has been invested in attempts to alleviate this shortcoming in relativistic magnetrons. Relativistic magnetrons have first been studied in the 1970s [1], modelled by particle in cell (PIC) simulations [2]–[6], and experimentally investigated in various configurations ever since [7]–[18]. The accepted explanation of pulse-shortening is cathode plasma formation followed by plasma expansion toward the anode within the electromagnetic interaction region [19]. Although some modelling work has pointed out situations where pulse-shortening occurs without the presence of plasma [20], [21], the mechanism responsible for this has not yet been fully understood. In experiments performed in our laboratory, no plasma expansion was observed across the cathode–anode gap within the first ns [22], yet circumstances of pulse-shortening were detected.