This thesis aimed to investigate the relativistic magnetron (RM), which is a high-power microwave (HPM) source. Since the RM can generate high-intensity microwave radiation, it can be used as a pulsed electromagnetic weapon to target electronic systems in different objects, such as drones, missiles, or vehicles. Other applications include electromagnetic compatibility (EMC) testing. In this thesis, a novel design of an RM with a transparent cathode configuration was investigated. This RM, referred to as the FOI-magnetron, was developed with the goal of generating the more advantageous TE11 mode of microwaves. This thesis starts with an in-depth theoretical exploration of the physics surrounding the RM, followed by a proof-of-concept study, where we compare our simulation results against published data. We then investigate the FOI-magnetron to determine if the transparent cathode configuration is more favourable than a solid cathode configuration. Particle-in-cell (PIC) simulations in MAGIC3D were used to study the RM, and extensive parameter studies were conducted for the FOI-magnetron to optimise its performance. The simulations revealed that the FOI-magnetron suffered from leakage currents. Moreover, parameter studies of the FOI-magnetron with transparent cathode demonstrated favourable TE11-mode emission of microwaves with a peak output power reaching 590 MW after 15 ns, having a frequency of 2.56 GHz, and an efficiency of 37%. Comparisons between thetransparent and solid cathode for the FOI-magnetron showed a slightly lower output power and efficiency for the transparent cathode, with minimal difference in the rise time of microwaves. Additionally, the transparent cathode exhibits a higher overall impedance and leakage currents. On the other hand, a lower back-current density on the transparent cathode and emitter was shown, resulting in less damage to the material. In this study, we found that we could reduce leakage currents by extending the interaction region without impacting the performance of the FOI-magnetron. Also, the frequency was shown to change with either a shorter emitter or a longer interaction region, allowing for frequency control. Lastly, a modified design of an RM with a semitransparent cathode showed a promisingly high efficiency of 46% with an output power of 600 MW. This design utilised endcaps, which are useful for significantly reducing leakage currents

A relativistic magnetron (RM) is a high-power microwave (HPM) source. The main objective of the RM is to generate directed electromagnetic pulses with high power, which can be used in e.g. HPM weapons and for electromagnetic compatibility testing. These pulses can disturb or damage electronic equipment. One of the main challenges when designing an RM is to generate the advantageous TE11 wave mode to the circular waveguide and antenna with high efficiency and peak power. This thesis investigates a new design of the RM, developed at the Swedish Defence Research Agency (FOI), referred to as the FOI magnetron. This design is based on the A6-magnetron and employs four large and two small cavities in the diffraction output of the RM, compared to the conventional design that has six identical cavities. The FOI magnetron has previously shown results that indicate the possibility of generating the TE11 wave mode. In this thesis, a literature study was performed to better understand the governing physical laws of the RM. This was followed by parametric studies using the ​​particle-in-cell code MAGIC3D for simulating the RM. To validate the simulation models, a model of a conventional RM was constructed and the results were compared against the published simulation results by Daimon and Jiang (2008).  Lastly, different geometrical properties, applied magnetic field, and applied voltage of the FOI magnetron were studied to see how they impacted the RM performance. Apart from the diffraction output, the geometry of the interaction region was studied to investigate the effect on frequency and power. The goal was to generate a clean TE11 mode in the waveguide of the RM with high efficiency. The validation yielded results that were in good agreement with the ones obtained by Daimon and Jiang (beam-to-microwave efficiencies of 37% and 36% respectively). The parameter studies of the FOI magnetron gave results that indicate a clean TE11 mode with a beam-to-microwave efficiency of ∼35% and peak powers up to 1 GW at frequencies of approximately 2.5 GHz. The studies on the interaction region showed that a shift of approximately 0.12 GHz was possible when making the rear part of the interaction region 4.5 cm longer. It was found that the length of the front of the interaction region can to some extent affect the output power. Lastly, it was found that a fraction of the output power (∼10−17%) that leaves the interaction region propagates back toward the input region and the voltage source.