Vircators and the magnetic field

Summary form only given, as follows. It is well known that vircators differ from many other microwave tubes, because they stop oscillating when immersed in an axial magnetic field. This was a bit of mystery, which we have been able to resolve the problem first of all analytically and then more directly by modelling the Texas Tech vircator with the help of a powerful computer code called MAGIC. Algebraically we have decided to analyse a system of two plane electrodes with a magnetic field parallel to them and a strong electron beam with a reasonably high initial velocity injected at right angle to the electrodes and the magnetic field. It is well known that in the absence of the magnetic field, depending on the intensity of the electron beam, we have three consecutive regions as the intensity of the electron beam increases: stable, unstable and finally a turbulent region when a virtual cathode in succession appears and disappears between the electrodes. As we gradually introduce the magnetic field the turbulent region first shrinks and then quickly disappears altogether, irrespective of the intensity of the injection current. Since the existence of a virtual cathode is essential to the operation of a vircator, we would suggest that the above analysis explains why a vircator usually stops oscillating when immersed in an axial magnetic field. Having obtained encouraging analytical results we have decided to test them in the model of a real tube. We have chosen for that purpose the vircator being currently developed in Prof. M. Christiansen´s research group at the Texas Tech in Lubbock. Using MAGIC as a suitable computer code we have been able to show that even a relatively small axial magnetic field would completely destroy the build up of a virtual cathode in the important interaction region.