Design and First Tests of the Trapped Electron Experiment T-REX*

Gyrotrons are the primary sources for electron cyclotron resonance heating (ECRH) systems within fusion reactors, therefore their efficient operation is crucial. Past gyrotron experiments highlighted instability issues, leading to restricted operating ranges. One major cause is the trapped electrons in the potential wells within the gyrotron's gun region, resulting in undesired currents and subsequent operational failures. To avoid such issues, the current approach is of tight manufacturing tolerances on the gun geometry in relation to equipotentials and magnetic field lines. Here, we present the design and initial experimental findings of the TRapped Electrons eXperiment (T-REX), a novel and unique experiment constructed at the Swiss Plasma Center. The objective is to explore the formation and evolution of electron clouds in gyrotron gun designs. A comprehensive understanding would enable, for example, the relaxation of the aforementioned stringent gyrotron manufacturing tolerances. T-REX replicates typical gyrotron gun geometries, electric and magnetic fields, background composition and pressure, all adjustables, and is supported by kinetic simulations via the 2D Particle-in-Cell (PIC) code FENNECS. T-REX bear resemblances to Penning traps, it is made of two coaxial electrodes, placed in a vacuum chamber on top of a superconducting magnet, with an external radial electric field (1-100 MV/m), and an axial magnetic field that depends on z (Bz(z) < 0.5 T) inducing azimuthal drift and confining electron energies between 0.1-1 keV. For such configurations, the Brillouin density ratio is close to 1. The diagnostics include current, voltage, and Faraday probes, imaging via phosphor screen, optical emission spectroscopy, streak camera, and, potentially, electric field distribution via the Stark effect. Moreover, the addition of a segmented outer electrode enables detection of potential growth of diocotron modes. Through T-REX, we aim to gain valuable insights into the trapping and dynamics of electrons within gyrotron gun regions, paving the way for improved gyrotron performance and reliability in fusion energy systems.