3-D EM PIC simulation study on low-frequency oscillation in a fusion gyrotron

In a previous study, it was found that not only magnetic compression profile but initial thermal velocities of electrons play an important role in causing a low frequency oscillation (LFO) in the operation of a magnetron injection gun (MIG) employed in an MIT fusion gyrotron. An unphysical particle boundary condition, i.e., large initial thermal velocities of electrons, had to be assumed to produce the LFO in the 3-D electromagnetic (EM) particle-in-cell (PIC) simulation. In this work, we have included the gyrotron cavity along with the MIG as well as a large vacuum envelope representing the vacuum chamber similar to that employed in the MIT experiments. In the EM PIC simulation, it is found that the momentum of electrons is suppressed by the space charge due to the vacuum envelope. For the first time, without unphysical approximations, the LFO could be reproduced in the \pmb3D time domain EM PIC simulations. The initial velocity spread at the cathode temperature is assumed in this simulation in contrast to an unphysical larger velocity spread formerly used.