EPJ D Highlight - Towards better neutral beam measurements for fusion reactors

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Published on 17 October 2025

New experiments show how a force probe can accurately track neutral particle beams, helping to improve fusion plasma stability

Neutral beam injection (NBI) is a valuable method for heating plasma inside a nuclear fusion reactor, and could be key to the success of upcoming projects including ITER. For the technique to work, however, the energy imparted by particle beams must be measured as accurately as possible. While ion beams can be tracked directly by their electric currents, chargeless neutral particles require more elaborate approaches – such as calorimetric profiling, spectroscopy, ionization-based methods.

In a new study published in EPJ D, researchers led by Thomas Trottenberg at Christian-Albrechts-University of Kiel, Germany, present a method to measure neutral beams using a device called a ‘force probe’, which directly measures the momentum transferred to a target. With initial measurements closely matching theoretical predictions, the demonstration could be an important step toward realizing nuclear fusion’s potential.

In NBI, neutral particles penetrate a fusion reactor’s plasma-confining magnetic field, and are then ionized by the plasma, transferring most of their energy to it. As the plasma heats up, fast-moving ions emerge which drive electrical currents in the plasma. In some types of reactor, such as the tokamak used in the ITER project, these currents can stabilize the magnetic confinement – helping to sustain the fusion reaction for longer.

To calculate the momentum imparted by the beams, the team’s force probe measures the elastic deformation of a ceramic cantilever holding a tungsten target. A mirror attached to the cantilever’s free-moving tip faces the flat end of an optical fibre. This forms a ‘Fabry–Pérot interferometer’: an optical cavity where light reflects back and forth, interfering with itself so that its intensity varies with the mirror separation.

When the particle beam strikes the target, the cantilever bends by a few microns, altering the interferometer’s light intensity. From this, Trottenberg’s team could calculate the beam’s momentum with high precision. They tested the approach using negative hydrogen ions generated by the Bavarian Test Machine for Negative Ions (BATMAN) facility, operated by the Max Planck Institute for Plasma Physics in Garching.

Firing the beam at the force probe from 1.4 metres away, the measurements closely matched theoretical calculations. Based on this by this success, the researchers now hope that future experiments can extend the method to neutral beams, potentially enabling more stable fusion experiments in the not-too-distant future.