For heating plasma in fusion devices, researchers unravel how electrons respond to neutral beam injection

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(Left) Measured and simulated electron temperature curves during an NBI pulse showing the outcome of the competing effects between neutral particle-derived fast ions and cold electrons. (Right) One of DIII-D's four neutral beams. Credit: Image courtesy of the DIII-D National Fusion Facility.

Heating a plasma for fusion research requires megawatts of power. One approach that research tokamaks use to achieve the necessary power input is neutral beam injection (NBI). With NBI, fast neutral particles are generated in a device called a beam source and then injected into the plasma.

Within the plasma, these particles can be ionized (given an electrical charge) at different locations along the injection path via a process called thermalization. The energetic ions then collide with existing plasma electrons and ions. This transfers most of the ions' energy to the electrons, heating the plasma.

As part of a graduate student project, researchers at the DIII-D National Fusion Facility monitored the change in electron temperature during the course of NBI and investigated the underlying physics. The work is published in the journal Physics of Plasmas.

Comparison with Monte Carlo simulations showed that the electron temperature curve trajectory was determined by the competing effects between the fast ions and cold electrons generated from injected neutral particles. Thus, the electron temperature curve can be used to deduce the neutral beam deposition profile, allowing experimental monitoring of neutral beam performance.

This will allow researchers to accurately monitor fusion reactions in devices using NBI. These devices include current research tokamaks and the large-scale ITER experiment now under construction. Additionally, electron transport affects plasma confinement and stability. The research will help scientists understand these effects to harness fusion for energy production.

More information: B. Zhao et al, Refined interpretation of electron temperature response to neutral beam injection at DIII-D, Physics of Plasmas (2023). DOI: 10.1063/5.0157020

Journal information: Physics of Plasmas

Provided by US Department of Energy