Microwave heating could be key to smaller, more efficient tokamaks

For decades, scientists have pursued the dream of harnessing fusion energy. The tokamak, a donut-shaped reactor, has been their best bet. But these machines are complex, expensive, and take time to build.

Scientists have now proposed that a smaller spherical tokamak could be more efficient and economical. But building a fusion reactor is like designing a tiny kitchen – every inch counts.

Princeton Plasma Physics Laboratory (PPPL) researchers are exploring ways to design a small fusion plant that relies solely on microwaves for plasma heating.

Use of microwaves

Tokamaks are huge devices designed to generate and contain incredible hot, charged gas—known as plasma—for nuclear fusion. This technology replicates the sun’s energy creation and has the potential to generate massive amounts of clean energy. However, these fusion devices are still in the experimental phase.

Most spherical tokamaks rely on a combination of methods for plasma heating, including a large solenoid coil and neutral particle beams. These systems take up valuable space. The team has proposed various techniques through which this smaller version would work.

The researchers are doing away with ohmic heating, a common tokamak heating method comparable to a toaster.

“A compact, spherical tokamak plasma looks like a cored apple with a relatively small core, so one does not have the space for an ohmic heating coil,” said Masayuki Ono, a principal research physicist at PPPL and lead author of the paper.

“If we don’t have to include an ohmic heating coil, we can probably design a machine that is easier and cheaper to build.”

Typically, microwave radiation is directed into the plasma using devices called gyrotrons. These waves create a current and heat the plasma simultaneously. It’s like cooking food in a microwave oven, but on a much larger scale.

The team recommends positioning the gyrotron around the tokamak to target the center plasma. Gyrotrons will produce powerful waves to push electrons within the plasma, creating a current and heating it.

Use of computer codes

To optimize this process, researchers are using complex computer simulations to determine the best angle and frequency for the microwaves to penetrate the plasma properly. The goal is to maximize efficiency and minimize energy loss.

For this, they used computer codes TORAY and TRANSP.

For maximum efficiency, the researchers also had to prevent cases in which microwaves were reflected by the plasma or passed through without transmitting energy. This might affect the plasma’s condition.

“There were a lot of scans of different parameters to find the best solution,” said  Jack Berkery, a co-author on the paper and the deputy director of research for the National Spherical Torus Experiment-Upgrade (NSTX-U).

The team identified the optimal ECCD mode for each heating stage. X-mode is best for initial heating, while O-mode is ideal for maintaining high temperature and current. O stands for “ordinary” mode whereas X is “extraordinary.”

“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 take care of the low-temperature regime,” said Ono in the press release.

Moreover, they add that impurities with high atomic numbers, or Z-numbers, must be minimized to ensure efficient fusion.

If successful, this approach could advance fusion energy, making it a more viable and affordable option.

The findings were published in the ​​journal Nuclear Fusion.