Physicists argue that a spherical tokamak is the best candidate for a pilot nuclear fusion power plant.
Test cell of the National Spherical Torus Experiment-Upgrade with tokamak in the center. Image: Elle Starkman/PPPL
Fusion energy has long been heralded as the power-supply of the future, but the sad joke is, it always will be. The experimental energy source is perennially 30 years away from being viable on a mass-scale. Still, fusion energy could provide us with a low-cost, sustainable energy resource—if only physicists could figure out how to harness the power of the Sun on Earth.
This dream of a sustainable "star in a jar" was brought one step closer to reality this month by physicists at the Department of Energy's Princeton Plasma Physics Laboratory, who demonstrated how the design for a new type of "jar" could lead to the first commercially viable nuclear fusion power plant.
Fusion power is essentially the result of fusing the nuclei of two or more lighter atoms into one heavier nucleus, a process which releases massive quantities of energy and is perhaps best demonstrated by our Sun, the natural nuclear fusion reactor par excellence.
During the process of nuclear fusion, atoms' electrons are separated from their nuclei thereby creating a super hot cloud of electrons and ions (the nuclei minus their electrons) known as plasma.
The problem with this energy rich plasma is figuring out how to contain it, since it exists at extremely high temperatures (up to 150 million degrees Celsius, or ten times the temperature at the Sun's core). Any material you can find on Earth isn't going to make a very good jar.
To solve this problem, some Russian physicists back in the 1950s developed a device called a "tokamak," which uses magnetic fields to contain the plasma generated through nuclear fusion. Conventional tokamaks are shaped like a donut, but recent design improvements have led to the creation of spherical tokamaks, which are shaped more like a cored apple and are able to generate magnetic fields to produce high-pressure plasma in a more energy- and cost-effective manner.
The two most advanced spherical tokamaks on Earth are the UK's soon-to-be-completed Mega Ampere Spherical Tokamak (MAST) and the National Spherical Torus Experiment Upgrade at the Princeton Plasma Physics Lab (PPPL), which came online last year. As PPPL physicists demonstrated in their recent paper in Nuclear Fusion,the spherical tokamak design is a leading candidate for the creation of a fusion nuclear science facility (FNSF), which would bridge the gap between ITER, which will be the world's largest nuclear fusion experiment when it comes online in a few years, and a commercially viable nuclear fusion power plant.
Before a pilot FNSF could become viable as a commercial power plant, there are a number of design challenges that need to be solved, which are addressed in the new paper. For starters, the particles in the superhot plasma created in the tokamak are very turbulent, as a result of the magnetic field used to contain them. So figuring out a way to channel them around the tokamak more effectively is key. Physicists also must experiment with the materials used to build the walls of the tokamak to ensure the purity of the plasma particles which will inevitably interact with it.
Another key design consideration for a pilot FNSF would be replacing the large copper magnet coils used by conventional tokamaks by superconducting magnets which can generate higher magnetic fields while requiring less power to cool them.
While these design considerations look good on paper, the physicists conclude that the experiments conducted at MAST and the PPPL's spherical tokamak in the coming years will ultimately reveal the path to the compact, energy-efficient, commercially viable nuclear fusion plant of the future.