For Heating Plasma in Fusion Devices, Researchers Unravel How Electrons Respond to Neutral Beam Injection

The Science

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. Researchers at the DIII-D National Fusion Facility recently studied the variation in electron temperature during NBI and used the data to experimentally determine the neutral beam deposition profile.

The Impact

This work provides a way to monitor NBI performance using readings of electron temperature. 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.

Summary

Fusion reactions require heating plasma to temperatures 10 times hotter than the core of the Sun. NBI is one approach used to maintain the necessary plasma temperature in a tokamak. The fast neutral particles that are injected undergo a thermalization process that produces ions capable of transferring energy to the plasma via collisions. However, the thermalization process also distributes cool electrons derived from the neutral particles, which can cool plasma electrons. These heating and cooling processes related to NBI can occur on notably different time scales, producing an evolution in overall electron temperature that researchers can monitor across the plasma profile.

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. 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 provide additional validations or constraints for modeling of the transport of plasma electrons and fast ions.

Funding

This work was supported by the Department of Energy Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.