Relaxation oscillations driven by the diocotron instability in trapped electron clouds

We report the first three-dimensional kinetic simulations that self-consistently reproduce relaxation oscillations of high-density, strongly rotating electron clouds confined in Penning-like potential wells. Using the finite element non-neutral electron clouds simulator code, we model the Trapped Electrons eXperiment (T-REX), a dedicated setup designed to study electron cloud dynamics relevant to gyrotron electron guns. The simulations are in quantitative agreement with experiment and show that the observed periodic current bursts originate from the diocotron instability developing within the trapped cloud, driving rapid cross field transport and electron loss. The presence of the diocotron mode is further confirmed by spatiotemporal current measurements from dedicated probes in T-REX.