The reflex triode is a type of vircator, which is one of the most interesting radiation sources for high power microwave (HPM) generation. To be able to develop an efficient radiation source, which is a key component in a HPM warhead, knowledge is needed about how changes in geometry and material affects the microwave generation since many interesting parameters for a radiation source of this kind are not possible to measure in practise, so the only alternative left is simulations. To be able to make simulations of vacuum sources like the vircator, a simulation program that calculates both fields and particles is needed. One such program is MAGIC. This report gives an introduction to the program, and describes some simulations made of the reflex triode at the Weapons and Protection division at FOI. The simulations have been validated by comparison with experiments. The results are promising and show the potential of the program, though there are details that are not modelled correctly. The main improvements that have to be done involve the models of the anode and cathode, where limitations in the program make it difficult to implement a good material model.