Nuclear fusion promises a green and infinitely renewable supply of energy—if we can harness it. Fusion happens all the time inside the sun. But to recreate the process on Earth, we must control incredibly hot, chaotic matter in an exceedingly dense state.
Prototypes of several different fusion-reactor designs are being tested around the world. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, for example, uses lasers to spark fusion in a small pellet of fuel. Tokamaks, such as the International Thermonuclear Experimental Reactor (ITER) in France, use electromagnetic fields to confine plasma and heat it to the temperatures and densities necessary to ignite fusion. And stellarators, such as the Wendelstein 7-X experiment in Germany, add a twist to the magnetic field concept of tokamaks.
It’s too soon to say whether any of these technologies can overcome their challenges to become a reliable energy source. But the motivation to make that happen is clear. “Necessity is the mother of invention,” says Laura Berzak Hopkins, associate laboratory director at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL). “We have increasing energy demands and a changing climate, and fusion is the way we can address both those needs.”
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WHAT IS FUSION?
Nuclear fusion is the process by which two atoms combine to form a larger atom (minus a bit of mass) plus energy.
To achieve sustained fusion, the atoms must reach a certain temperature and density, and they must stay in these states for an extended period. There are three general ways to meet these conditions.
The goal is to get more sustained energy out of the system than goes in.
Experiments in 2022 at NIF—the most famous inertial confinement facility—provided proof of concept. The project did release more fusion energy than its lasers used to create the reaction, but charging those lasers incurred an energy cost.
Recent experiments using magnetic confinement have also demonstrated progress. Two different concepts—a stellarator and a tokamak—have each held superheated plasma at the right temperatures and densities for nearly one minute, achieving new records. Why is this significant? Containing the fuel for sustained times is a huge challenge. To understand why, let’s dive into an example.
MAGNETIC CONFINEMENT
Tokamak reactors—such as the massive ITER project, which is still under construction—use a doughnut-shaped container. Here’s how they work:
1 • Remove all gas from the vacuum chamber, then charge the magnetic system around the vessel.
2 • Inject a small amount of deuterium and tritium gas into the vacuum.
3 • Switch on the coil of wire called a solenoid at the center of the tokamak to start up the magnetic field that will keep the gas contained. Run a powerful electric current through the vessel. This current strips electrons off the gas particles, which collide with other particles to kick off more electrons. The atoms become an ionized gas called a plasma, in which charged particles follow magnetic field lines.
4 • Heat the plasma to thermonuclear temperatures (150 million degrees Celsius) by injecting electromagnetic radiation and beams of high-energy neutral atoms.
5 • As the temperature rises, the density and energy within the plasma increase, causing particles to collide and initiate fusion. Some of the energy released from each reaction is used to heat additional incoming fuel, perpetuating fusion. The goal is to then transfer most of the heat out of the reactor and use it to generate electricity via, for example, steam turbines.
WHAT’S THE PROBLEM?
The process seems straightforward. So why is it so difficult?
When left to its own devices, plasma is turbulent, with pockets of temperature variations that create convection currents. This turbulence also moves heat from the plasma core to the edge, dampening the fusion reactions.
Scientists want to encourage collisions between particles within the plasma to promote fusion, but they also need to avoid particle collisions with the reactor hardware itself. Powerful magnetic fields steer the plasma around the doughnut in a roughly circular path.
But a closer look reveals that the particle trajectories aren’t that simple. Different plasma shapes each have benefits and drawbacks in maximizing temperature and density. Within the suspended plasma inside of a tokamak, particles move in two general patterns: helical motion (called ion gyro motion) and a banana-shaped path.
Different reactor shapes and sizes result in different plasma trajectories and have different pros and cons.
All tokamaks confine the plasma using a central electric current that can make fusion reactions difficult to maintain. Traditional doughnut-shaped tokamaks have more space in the middle. This space makes room to shield a central electromagnet from the heat of the plasma.
Spherical tokamaks—such as PPPL’s National Spherical Torus Experiment-Upgrade—have narrower central areas than traditional tokamaks. They are more compact, can more efficiently confine plasma particles, and can be more economical to build. But the smaller central area requires a skinnier central electromagnet that can make the generation of the plasma current more difficult.
Stellarators, which take a twisted shape, don’t require a central current to keep plasma trajectories in check. Magnets along the winding tunnel wall do the trick. But getting up to temperature can be tricky.
Because our energy demands are high and getting higher, it’s likely that there is room for multiple models to succeed. “I’m confident that we need fusion,” PPPL’s Berzak Hopkins says, “so that makes me very confident that we will solve fusion.”
