Microwave Fusion: Why Toast Plasma When You Can Zap It!

Scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), the private company Tokamak Energy, and Kyushu University in Japan have proposed a design for a compact, spherical fusion pilot plant that heats plasma using only microwaves. Typically, spherical tokamaks use a massive copper coil called a solenoid, located near the center of the vessel, to heat the plasma. Neutral beam injection, which involves applying beams of uncharged particles to the plasma, is also commonly used. However, similar to how a smaller kitchen is easier to design with fewer appliances, a compact tokamak would be simpler and more economical with fewer heating systems. This new approach eliminates ohmic heating, which is the same type of heating used in toasters and is standard in tokamaks. “A compact, spherical tokamak plasma resembles a cored apple with a relatively small core, so there isn’t space for an ohmic heating coil,” said Masayuki Ono, a principal research physicist at PPPL and lead author of the paper detailing the new research. “If we don’t need to include an ohmic heating coil, we can likely design a machine that is easier and cheaper to build.”

Identifying the Ideal Beam Angle and Heating Mode

Microwaves, a form of electromagnetic radiation, can be generated using a device known as a gyrotron. The gyrotrons would be positioned on the outside of the tokamak—metaphorically speaking, just outside the apple skin—aimed toward the core. As the gyrotrons emit powerful waves into the plasma, they generate a current by moving negatively charged particles known as electrons. This process, known as electron cyclotron current drive (ECCD), both drives a current and heats the plasma. However, the heating process is not as simple as just turning on some gyrotrons. The researchers need to model different scenarios and determine various details, such as the optimal angle to aim the gyrotrons so the microwaves penetrate the plasma effectively.

Using a computer code called TORAY coupled with one called TRANSP, the team scanned the aiming angles to determine what provided the highest efficiency. The goal is to use as little power as possible to drive the necessary current. “You also have to avoid any of the power you’re putting into the plasma from coming back out,” said Jack Berkery, a co-author on the paper and the deputy director of research for the National Spherical Torus Experiment-Upgrade (NSTX-U). This can happen when the microwaves are reflected off the plasma or when they enter the plasma but exit without changing the plasma’s current or temperature. “There were a lot of scans of different parameters to find the best solution,” Berkery said. The research team also determined which mode of ECCD would work best for each phase of the heating process. There are two modes: ordinary mode, known as O mode, and extraordinary mode, known as X mode. The researchers found that X mode is best for ramping up the temperature and current of the plasma, while O mode is ideal after the ramp-up, when the plasma temperature and current need to be maintained.

“O mode is good for a high-temperature, high-density plasma. But we found that O mode efficiency becomes very poor at lower temperatures, so you need something else to handle the low-temperature regime,” said Ono.

Considering the Impact of Impurities

The authors, including postdoctoral researcher Kajal Shah, also investigated how power would radiate away from the plasma. Such radiation could be significant in a plasma as large as one needed for commercial fusion. Luis Delgado-Aparicio, the Lab’s head of the Advanced Projects Department and a co-author on the paper, notes that it will be particularly important to minimize the number of impurities from elements with a high atomic number, also known as a Z number, in the periodic table. These are elements with many positively charged ions.

original source: https://www.sciencedaily.com/releases/2024/08/240806131216.htm