Exploring the mysteries of trapped electrons: the T-REX Experiment

Fusion reactors, like the ones being developed for ITER and DEMO, rely on powerful microwave devices called gyrotrons. These devices generate the microwaves needed to heat plasma to temperatures hotter than the sun’s core. But gyrotrons have a tricky problem: under certain conditions, they can become unstable due to trapped electrons in a region known as the Magnetron Injection Gun (MIG). These trapped electrons can disrupt the operation of the gyrotron, leading to inefficiencies or even damage.

That’s where T-REX comes in. Francesco and his team have designed this experiment to simulate the conditions inside a gyrotron’s MIG and study the behaviour of these pesky trapped electrons. Unlike gyrotrons, which cannot be opened once built, making it impossible to directly observe what’s happening inside, T-REX allows researchers to recreate these conditions in a controlled environment. By understanding how these electron clouds form and evolve, the team aims to find ways to prevent them, thereby improving gyrotron performance and making fusion energy more reliable and efficient.

Building T-REX from the Ground Up

Francesco’s journey with T-REX began when he joined the Swiss Plasma Center as a postdoc. At the time, there was no machine—just some early simulation results and a concept. Over the course of two years, Francesco, with the support of a dedicated team of engineers and technicians, transformed those ideas into a fully operational experiment. T-REX saw its first plasma in 2023, a significant milestone that marked the beginning of a new phase of discovery.

The experiment itself is designed to replicate the conditions inside a gyrotron’s Magnetron Injection Gun (MIG) using a set of coaxial electrodes inside a vacuum chamber, all perched atop a superconducting magnet. The geometry of the electrodes shapes the electric field, while the superconducting magnet determines the magnetic field configuration. This combination of fields creates a “potential well,” acting as a reservoir where electrons can become trapped.

A large negative voltage applied to the central electrode generates a strong electric field, which, combined with the magnetic field, facilitates the trapping of electrons. These first free electrons can originate from mechanisms such as cosmic rays, field emission, or the finite ionization rate of the background gas. Once inside the potential well, these electrons undergo fast azimuthal rotation due to the large ExB drift, gaining significant kinetic energy. As they collide with neutral gas atoms, a chain reaction ensues, producing more electrons and forming an electron cloud confined within the potential well. The team uses various diagnostic tools to study this electron cloud in detail, exploring its dynamics and behaviours.

Francesco’s Story

But who is Francesco Romano? Born in Sassari, Italy, and raised in Udine, Francesco’s academic journey began in aerospace engineering at the University of Padova, Italy, where he completed both his bachelor's and master's degrees. He then pursued a PhD in Stuttgart, Germany, where he developed a helicon plasma thruster for satellites— in particular one that can use atmospheric particles as propellant for spacecrafts in very low Earth orbits.

In addition to his scientific pursuits, Francesco is an accomplished musician with a love for the piano and organ, having completed organ conservatory studies in Italy. When he’s not in the lab, you might find him cycling in the Alps, sailing on Lake Geneva, or indulging his passion for classic supercars and vinyl records.

T-REX and EUROfusion

T-REX isn’t just an isolated project. Earlier this year, it was recognized by EUROfusion as an indispensable facility, especially in conjunction with FALCON, another key testing facility at SPC. While the experiment is primarily funded by the Swiss National Science Foundation, its results are highly relevant to the broader fusion community and align with EUROfusion’s goals of advancing gyrotron technology for fusion reactors.

For Francesco, this recognition is a testament to the hard work and innovation that have gone into T-REX. As the experiment continues, it holds the promise of unlocking new knowledge that could help make fusion energy a reality.

What’s Next?

With T-REX now fully operational, Francesco and his team are focused on conducting a series of experiments to gather data and refine their models. Remarkably, the simulations performed by their current and previous PhD students already match closely with the experimental data—an impressive achievement that highlights the accuracy of their theoretical models. These findings could lead to more robust gyrotron designs and bring us one step closer to sustainable fusion energy.