A gyrotron is a microwave generator based on the relativistic electron cyclotron maser instability. It can be used for electron cyclotron resonance heating (ECRH) in fusion plasma heating. Gyrotron resonators are used for validating new absorbing boundary conditions or sponge layers.
The following picture shows the result for a 140 GHz gyrotron resonator operating in a mode at 50 ns. The simulation is performed with our 3D PIC code parallel on 512 cores with approx. 16 mio. DOF, p=4 (fifth-order), hexahedral elements and 250k electrons.