The diocotron instability in the trapped electrons experiment T-REX and its relevance to electron clouds in gyrotron guns

Abstract Efficient operation of gyrotrons is crucial for electron cyclotron resonance heating (ECRH) in fusion reactors. Electron clouds trapped within the magnetron injection gun (MIG) region of high-power gyrotrons can induce disruptive leakage currents that degrade operational stability. While previous investigations utilizing the TRapped Electrons eXperiment (T-REX) established the foundational mechanics of cloud formation in the $\SI{}{\kilo\hertz}$ regime~\cite{Romano2024}, the high-frequency non-linear dynamics governing cloud saturation and cross-field transport remain uncharacterized. This work focuses on characterizing the underlying diocotron instability responsible for this transport and the resulting bursty plasma behavior within a Penning-Malmberg-like configuration. To resolve the high-frequency evolution, we present time-resolved current measurements on key experimental components—specifically the outer electrode and the top flange—complemented by a newly developed fast current probe array installed at the top flange to capture multi-mode azimuthal structures. To address early discrepancies between experiments and low-frequency models, the Particle-in-Cell (PIC) code FENNECS was upgraded to 3D~\cite{guilPoP,guilPoP2,guilth,FENNECS,Pierrick}, enabling the self-consistent simulation of the space-charge fields. The experimental measurements highlight rotating structures in the electron cloud and a periodic collapse and reformation driven by the instability. The 3D PIC simulations show remarkable agreement with these experimental findings, accurately reproducing the cloud build-up and collapse frequencies, as well as the rotation frequency and direction of the diocotron modes. These results significantly advance the understanding of non-neutral plasma dynamics in environments mimicking gyrotron MIGs, providing a predictive framework to mitigate parasitic currents and improve gyrotron reliability in fusion energy systems.