Camera di reazione del reattore a fusione Tokamak

Tokamak, the promise of magnetic fusion

A device created to reproduce the power of the Sun and obtain a large amount of energy at zero emissions.

by Poornima Apte
17 November 2020
8 min read
by Poornima Apte
17 November 2020
8 min read

Carbon might have fueled much of industry as we know it, but the writing on the wall is clear. We need to decarbonise to push back on accelerating climate change. Among the smorgasbord of alternative energy sources on tap —magnetic fusion, a carbon-free source of power. Magnetic fusion is also a much more efficient energy source. Just one gram of hydrogen isotopes commonly used in the process delivers as much energy as 11 tonnes of coal.

The sun as inspiration

The sun is a hotbed of magnetic fusion as hydrogen atoms combine to form helium and release energy. The sun uses both high temperature and pressure for magnetic fusion to occur. It achieves these conditions because of its mass and strong gravitational fields. The high temperature —close to 100 million degrees centigrade— and the pressure create a plasma such that the hydrogen atom is unable to hold onto its electrons. That leaves the positively charged nucleus floating around. Ordinarily, two positively charged bodies will not fuse because of electrostatic forces of repulsion; however, the high-energy particles on the sun overcome this force and can fuse with their counterparts. The result: Even more heat. It is this kind of process that inspires scientists. The magnetic fusion process might work well for the sun, where gravitational forces provide the necessary fuel to reach dizzyingly high temperatures. However, reproducing similar conditions on earth requires some heavy-duty calisthenics. A tokamak, device invented in the Soviet Union in the 1950s, just might be the solution.

Built in southern France, the International Thermonuclear Experimental Reactor (ITER), the latest iteration of the tokamak, is a collaboration between 35 countries. It promises to be the first to test the scalability of magnetic fusion beyond the laboratory, into working power plants. The shopping list to deliver magnetic fusion as a long-term source of power is broad and lengthy. While the reactions in the sun result from temperature and pressure, the processes for magnetic fusion on earth are restricted mostly in reaching high temperatures to achieve the plasma state and confining the generated plasma in a tight space so fusion can occur; and ultimately, harvesting of the energy generated into electricity. Magnetic confinement fusion (MCF) is one of the more popular ways to heat the plasma up to magnetic fusion. Under the MCF umbrella, the tokamak has proven to be the most reliable delivery method. As the ITER project aims to demonstrate, it might also be the most promising route to controlled magnetic fusion.

A peek under the hood

A tokamak consists of a donut-shaped inner chamber (geometrically a toroid), where the hydrogen does its magic. The quirky name derives from a Russian acronym that translates to “toroidal chamber with magnetic coils." A donut shape of the container prevents particles leaking from the ends; they instead move around in continuous circles. A vacuum chamber envelops this inner layer to insulate the chamber end so that no extraneous particles can interfere in the process. Magnetic coils, made from superconductors because of their ability to generate strong magnetic fields with very little power input, surround the entire contraption.

They apply the forces needed to confine the 100 million-degree plasma and prevent it from leaking. The magnetic field provided all along the circumference of the toroid keeps stray electrons from impinging on the walls. Perpendicular magnetic forces further tighten the plasma, bringing the positively charged nuclei close enough for them to overcome electrostatic forces and actually fuse.

Fueling the reactions

A mixture of deuterium and tritium, isotopic variants of hydrogen, forms the basis for the magnetic fusion reactions in a tokamak as it is the combination that most efficiently reaches the optimal conditions needed. While deuterium is available in plenty, tritium is not. While initial runs will borrow tritium from the limited pool, The ITER tokamak plans on testing a self-sustaining model of producing tritium by wrapping the vessel facing the plasma with a series of blankets containing lithium. The D-T fusion creates helium and high-energy neutrons. These neutrons are absorbed by one of the lithium isotopes –the ⁶Li– in the blanket to create tritium and helium. The tritium formed in this manner can be pumped into the tokamak as fuel.

Achieving high temperatures

In the tokamak, the plasma reaches the high temperatures it needs through a combination of internal and external heating mechanisms. First the magnetic fields themselves produce heat, heating the plasma. In addition, a technique called neutral beam injection helps. Neutral beam injection transfers neutral particles into a medium, resulting in those particles reacting with plasma to produce heat. The ITER tokamak accelerates charged deuterium particles, passes them through a neutralizer to strip them of their charge, and injects them into the plasma. The neutral particles heat the plasma by transferring their energy. Eventually, the neutral particles also become a part of the plasma. Strong magnetic fields confine the plasma so the hydrogen fusion occurs. A coolant that surrounds the lithium blanket captures all generated heat and harvests it to produce electricity.

The pluses and minuses

Challenges are a part of the equation too. Superconducting materials that generate the desired magnetic fields only get to their superconducting state under very low temperatures. How then do engineers cool the coils in such a high-temperature environment? Tritium is radioactive and while its handling is not expected to be much of a headache, it still has to factor into the overall calculus while building a tokamak. The machine could leak plasma, which can damage the plant.

Alternative pathways to magnetic fusion

The MCF method is not the sole route to magnetic fusion. The Inertial Confinement Fusion (ICF) method uses a laser to pulse a fuel pellet into high densities. Shockwaves from these pulses heat the plasma. A combination of the two methods, magnetic and inertial, in which magnetic fields confine the plasma and laser or some other method heats it up, is called Magnetised Target Fusion (MTF)—yet another approach to fusion. Even under MCF, the stellarator, which looks much like the tokamak, is getting a second look. While there are subtle differences between the two, the primary one is the shape of the magnetic coils, which affects how the stellarator applies magnetic forces to the plasma. The stellarator's coils look like a crunched bangle. Because of this complicated shape, which in turn affects the effectiveness of the magnetic forces, the stellarator is more complicated to build than a tokamak. The Wendelstein 7-X MCF in Germany is a stellarator that seems poised for prime time.

In its current state, the tokamak does lead the charge as a darling in the field, specifically as research over the decades has supported its promise. The ITER project, for example, is set to deliver 10 times as much power (500 MW) as it will take in. The facility will test-drive the production of tritium using lithium blankets. The most exciting aspect of ITER is that it will tackle real-world problems, figuring out how the processes will scale to an industrial plant and ironing out any wrinkles before they are translated.

ITER has been subject to delays and increasing costs: In 2016, the project coordinators requested an additional 5 billion euros on top of its 18 billion euros tab, for staff and equipment. ITER also rescheduled its first plasma trial run from 2020 to 2025. When it finally does conduct the trial, the project might give us a glimpse into whether we can bring the promise of the sun back down to earth.

The author: Poornima Apte

Technology content writer. Trained engineer. Award-winning writer. Niches: robotics, smart manufacturing, IoT, cybersecurity, AR, VR, SaaS, artificial intelligence, machine learning, bioengineering, BIM, martech, 5G.