Design and first tests of the trapped electrons experiment T-REX

Gyrotrons are essential for electron cyclotron resonance heating in fusion reactors, making efficient operation crucial for advancing 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 [Pagonakis et al., Phys. Plasmas 23, 023105 (2016)]. We present the initial findings of the trapped electrons experiment developed at the Swiss Plasma Center, designed to understand the physics of electron clouds in gyrotron MIGs. T-REX replicates MIG geometries, as well as their typical electric and magnetic fields, and it is supported by 2D particle-in-cell simulations with the FENNECS code [Le Bars et al., Phys. Plasmas 29, 082105 (2022); Le Bars, Ph.D. thesis, EPFL, Lausanne, 2023]. The setup includes two coaxial electrodes in a vacuum chamber atop a superconducting magnet, with a central electrode biased to negative DC voltages and an outer one at the ground, creating a radial electric field (1-2 MV/m) and an axial magnetic field (B < 0.4 T). This setup mimics Penning-Malmberg traps. We present the experimental device and first findings on current distribution and also a qualitative comparison with FENNECS simulations [Le Bars et al., Comput. Phys. Commun. 303, 109268 (2024)]. Planned diagnostics include optical emission spectroscopy, phosphor screen imaging, streak camera imaging, and potentially electric field distribution via the Stark effect. This research aims to enhance gyrotron performance and reliability in fusion energy systems.