Description
We present the first three-dimensional kinetic simulations of the formation and disruption of high-density, strongly rotating electron clouds trapped in Penning-like potential wells inside coaxial cavities. The simulations are performed with the FENNECS code [1], a particle-in-cell tool that solves the Boltzmann–Poisson system, including electron–neutral collisions and realistic particle–wall interactions. We model the Trapped Electrons eXperiment (T-REX) [2], a dedicated device designed to investigate electron cloud dynamics in configurations directly relevant to gyrotron electron guns. In gyrotrons, such clouds are known to compromise efficiency and reliable operation [3]. In T-REX, relaxation-oscillation cycles characterized by a slow growth of the electron cloud followed by sudden disruption have been observed experimentally [4]. FENNECS reproduces this phenomenon for the first time, capturing the interplay between spontaneous cloud formation by avalanche ionization of the neutral gas and the subsequent development of the diocotron instability, which leads to rapid electron losses. Quantitative agreement is obtained with high-bandwidth current measurements and spatiotemporal probe diagnostics, providing experimental confirmation of the role of the diocotron instability. The simulations reproduce experimental observations over a wide range of operating conditions, demonstrating their predictive capability. We further show that similar relaxation-oscillation dynamics arise in simulations of a coaxial ITER-relevant gyrotron prototype, and can be directly associated with the disruptive phenomena observed during its experimental test campaign. Beyond gyrotron applications, FENNECS provides a versatile framework for investigating diocotron instabilities and related collective phenomena in non-neutral plasmas.
[1] G. Le Bars et al. (2024), Computer Physics Communications 303, 109268.
[2] F. Romano et al. (2024), Review of Scientific Instruments 95, 103511.
[3] I. G. Pagonakis et al. (2016), Physics of Plasmas 23, 023105.
[4] P. Giroud-Garampon et al. (2025), Physics of Plasmas 32, 053903.