  Gyrotrons are the only sources used for electron cyclotron resonance heating
(ECRH). Past gyrotron experiments have highlighted some instability issues,
leading to restricted operating ranges. One major cause is the possible
presence of trapped electrons within the gyrotron's magnetron injection gun
(MIG) region, resulting in undesired currents and subsequent operational
failures. Hereby, we present the design and initial experimental findings of
the TRapped Electrons eXperiment (T-REX), a novel and unique basic plasma
experiment constructed at the Swiss Plasma Center to investigate the underlying
physics of trapped electron clouds, therefore entering the realm of non-neutral
plasmas. The primary objective is to explore the formation and evolution of
electron clouds in gyrotron MIG designs. Typical MIG conditions are replicated
in T-REX in terms of geometries, electric and magnetic fields, and supported by
kinetic simulations via the 2D Particle-in-Cell (PIC) code FENNECS. T-REX is
characterized by two coaxial electrodes, placed in a vacuum chamber on top of a
superconducting magnet, with an applied external radial electric field (0.1 - 2
MV/m), and a nearly axial magnetic field (B<0.4T) inducing azimuthal drifts and
confining electrons with energies between 0.1-1 keV. The experimental
diagnostics include current, voltage, and Faraday probes, and in future plans
imaging via phosphor screen, optical emission spectroscopy, streak camera
imaging, and, potentially, electric field distribution via the Stark effect.
Moreover, T-REX is also designed to be capable of detecting potential growth of
diocotron modes via such diagnostics in combination with a segmented outer
electrode. Through T-REX, we aim to gain valuable insights into the trapping
and dynamics of electrons within MIG regions, paving the way for improved
gyrotron performance and reliability in magnetic fusion reactors.
