Study on beam wave interaction and mode competition in a fusion gyrotron using 3-D electromagnetic particle-in-cell simulation*

3-D time-domain modeling of beam wave interaction in a fusion gyrotron is very challenging due to the open-end features of cavity structure and higher-order mode excitation employed to achieve high efficiency. In a previous project, namely GyroPIC, funded by DOE, several required capabilities or algorithms had been developed in a 3-D particle-in-cell (PIC) simulation code in order to simulate the beam wave interaction in a high-order mode gyrotron. In past decades, it has been considered as an impossible mission to simulate a fusion gyrotron in 3-D using a PIC based method. The preliminary results showed that the 3-D electromagnetic PIC method could provide an alternative modeling tool for studying the gyrotron cavity interaction and mode competition for gaining more physics insight and further improving the efficiency. Upon finishing the code development, a research effort has been continued in recent years to study the beam wave interaction and mode competition using the well-developed 3-D electromagnetic PIC simulation.