3-D electromagnetic particle-in-cell simulation study on low-frequency oscillation in a fusion gyrotron

Abstract

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 although we were able 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 and measurement. In the EM PIC simulation, it is found that the momentum of electrons is suppressed by the space charge due to the vacuum envelope used in the simulations. It is suspected the space charge effect in the large vacuum envelope on the gyro-electron beam causes the LFOs. We have employed a larger scale simulation to verify this hypothesis. For the first time, without unphysical approximations, the LFO could be reproduced in the 3D 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. So, it is found that the LFOs observed in the MIT gyrotron experiments are possibly caused by the combing effects of initial thermal spread and space charge suppression the spent beam encountered after finishing the cavity interaction due to transition to the larger vacuum chamber used in the experiment.