Gyrotrons are essential devices for electron cyclotron resonance heating (ECRH) in magnetic fusion reactors, and need to deliver MW-level power continuously and reliably. However, experiments revealed that undesired trapped electrons in the magnetron injection gun (MIG), can cause disruptive events, leading to large currents that its power supplies cannot withstand. To prevent this, tight and costly manufacturing tolerances are required for the MIG geometry. Understanding physical principles behind such disruptive phenomena could allow for their relaxation. To address this, a novel and unique plasma experiment named "the TRapped Electrons eXperiment" (T-REX) has been built at the Swiss Plasma Center of EPFL. T-REX replicates the typical E and B fields and geometries of a MIG. It consists of two coaxial electrodes installed in a vacuum chamber on top of a magnet. The central electrode is biased negatively and the outer one is at ground, for a radial E up to 2 MV/m and an axial B < 0.4T. This setup mimics the trapping principle of Penning-Malmberg traps. The electron cloud forms spontaneously once a certain voltage is reached. Electrons are trapped in the potential well and rotate azimuthally very fast due to the E×B leading to even more electrons by ionizing the residual neutral gas. Currently, time-resolved (kHz) current measurements are performed on the main experiment components. Optical emission spectroscopy (OES) is employed for the local estimation of E and B via Stark and Zeeman effects. A system of 32 current probes has been installed at the top to measure radial and azimuthal electron distribution. T-REX is supported by 2D PIC simulations with the FENNECS code. Good agreement is observed between experiments and simulations in terms of the magnitude of the measured currents and the E and B threshold for the electron cloud formation. However, 2D simulations do not correctly reproduce the currents spatial distribution, nor their bursty dynamics. We identified that the observed discrepancies originate from diocotron instabilities. Therefore, FENNECS is extended to 3D to capture accurately this instability. The results of T-REX and FENNECS provide new understanding on nonneutral plasmas in conditions mimicking those of a real gyrotron MIG and prepare the way to enhance performance and reliability of future gyrotrons.
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Publication: [1] F. Romano, G. Le Bars, J. Loizu, M. Nöel, J.-P. Hogge, S. Alberti, J. Genoud, S. Antonioni, L. Naux, P. Giroud-Garampon, S. Couturier, T. Leresche, and D. Fasel, Review of Scientific Instruments 95 (2024), 10.1063/5.0212127.<br>[2] G. Le Bars, J.-P. Hogge, J. Loizu, S. Alberti, F. Romano, and A. Cerfon, Physics of Plasmas 29 (2022), 10.1063/5.0098567, 082105.<br>[3] G. M. Le Bars, Modelling of nonneutral plasmas trapped by electric and magnetic fields relevant to gyrotron electron guns, Ph.D. thesis, EPFL, 10.5075/epfl-thesis-10444, Lausanne (2023).<br>[4] G. Le Bars, J. Loizu, J.-P. Hogge, S. Alberti, F. Romano, J. Genoud, and I. G. Pagonakis, Physics of Plasmas 30, 030702 (2023).<br>[5] P. Giroud-Garampon, F. Romano, J. Loizu, J.-P. Hogge, G. Le Bars, S. Alberti, J. Genoud, F. Braunmüller, M. Podesta, and T. Goodman, Physics of Plasmas 32, 053903 (2025).
