The Diocotron Instability in the Trapped Electrons Experiment T-REX and its Relevance to Electron Clouds in Gyrotron Guns

Abstract

Gyrotrons are essential for electron cyclotron resonance heating (ECRH) in fusion reactors, making their efficient operation crucial for fusion energy. Past experiments revealed instability issues due to trapped electrons in the magnetron injection gun (MIG) region, causing undesired currents and operational failures. To address this, tight manufacturing tolerances are required for the MIG geometry~\cite{pago2}. We present findings of the TRapped Electrons eXperiment (T-REX) at the Swiss Plasma Center, designed to understand electron cloud physics in gyrotron MIGs. T-REX replicates MIG geometries, electric and magnetic fields, and is supported by the 3D FENNECS code. The setup includes two coaxial electrodes in a vacuum chamber atop a superconducting magnet; a central electrode is biased to negative DC voltages and an outer one is grounded, creating a radial electric field up to 2 MV/m and an axial magnetic field B < 0.31 T. Initial discrepancies between experiments and simulations were linked to the diocotron instability, leading to FENNECS being upgraded to 3D and a dedicated set of diagnostics for T-REX. This instability causes the electron cloud to collapse and reform at a frequency depending on plasma conditions. Within this article, time-resolved current measurements on the outer electrode and top flange are presented. Further, a fast current probe array installed at the top flange is detailed. Measurements highlight rotating structures in the electron cloud resulting from the diocotron instability. Simulations show remarkable agreement with experiments, especially regarding the cloud's build-up/collapse frequency, and the rotation frequency and direction of the modes. These results improve our understanding of non-neutral plasmas in environments mimicking a real gyrotron MIG, paving the way for better gyrotron reliability.