Visualizing the microscopic world of fast ions in fusion devices

Fast ion transport within DIII-D. (Top). Measured fast velocity-space flow images taken with the imaging neutral particle analyser. Fast ions are moving into the red areas from the blue regions. The fast ions move along the indicated paths due to Alfven waves. (Bottom). Simulation results replicate the behavior observed in the experiment. Credit: X. Credit: X.
The U.S. scientific world is currently developing the first nuclear-fusion power plants. This will change energy production. A fusion power plant, which is similar to the sun and stars, will create energy by fusing lighter elements like hydrogen into heavier ones like helium at temperatures greater than 25 million F. This process releases approximately 4 million times the energy of a chemical reaction such as the burning or oil of coal, gas, or oil. The fusion reaction does not release carbon dioxide into the atmosphere or produce radioactive biproducts. This makes it one the most promising methods to generate clean energy.

Plasma is a state in which atoms are decomposed into negatively charged electrons and positively charged nuclei at high temperatures. The plasma is held in a vessel called a tokamak by strong magnetic fields. These magnetic fields can control the path of charged particles. To generate fusion energy efficiently, plasma must be kept in a container long enough to allow the ions to combine and fuse.

However, these plasmas can be home to electromagnetic oscillations known as Alfven wave, which cause the magnetic field to vibrate like a guitar string being pulled. These vibrations can transport energetic ions from the plasma's hot core before fusion takes place, similar to a surfer riding on a wave. Understanding the paths and destinations of these ions is key to understanding how they travel in a fusion reactor and how to control them. Unresolved mysteries have surrounded the individual migration paths of the ions for a long time.

Scientists at San Diego's DIII-D National Fusion Facility have recently developed an imaging neutral particle analyzer. It works like this. The plasma core is where the fast-ion flow takes place. First, a beam containing neutral (uncharged), particles is injected. The electrons exchange between the particles when a neutral particle is injected and a fast ion in the core. The charge-exchange process sees fast ions accepting electrons and becoming fast neutrals. Because they lack charge, these fast neutrals are able to escape the core plasma. The imaging neutral particle analyzer captures them as they flee, and can reconstruct the trajectory, energy, and orientation of the original fastions.

This information combined with the data revealed for the first-time details about the rapid ion flow caused by the Alfven wave. Researchers found that some ions traveled toward colder regions of plasma while others were drawn towards hotter core regions by the Alfven waves.

Particle migration is similar to birds. It follows specific routes. The environment in which the particles are moving, including wave frequency and wave number, determines the route. These paths, which are induced from multiple waves, intersect with one another. This suggests that fast ions may experience large-scale migration through transitions among multiple routes. Some ions will escape the core while others return to it. State-of-the art plasma simulations also confirmed the observed migration routes.

Important to remember is that strong fast ion flows can be destructive to the operation and stability of fusion plasmas. If the particles leave the plasma too quickly to transmit their energy, it will reduce the temperature at which the process can continue. Scientists are looking for ways to decrease this transport and reduce their intersections. They also want to eliminate the "last mile" route that fast ions use to escape the plasma.

Current acceptance of the relevant paper has been made and will be published in Phys. Rev. Lett. (2021).

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