Fusion is the holy grail of energy production, as it is cleaner, more efficient, and uses a virtually inexhaustible fuel source. However, since it was first envisioned in the 1930’s, it has been considered infeasible or even impossible, due to the difficulty of creating and containing temperatures and pressures comparable to those in the center of the sun.

Despite this, research has continued steadily, and recent research has brought us within reach of actually powering the electrical grid with nuclear fusion. In fact, a flurry of new research has been published just this year demonstrating solutions to an array of problems once thought to be insurmountable.

What is Nuclear Fusion?

With enough temperature and pressure, the nuclei of hydrogen isotopes can overcome their natural repulsion and fuse together into helium, releasing incredible amounts of energy. The two atoms of hydrogen weigh more than the resulting helium, with the missing mass being converted into energy, as dictated by Einstein’s E = mc2. That is, even a tiny amount of mass (m) becomes a crazy amount of energy (E) when multiplied by the speed of light squared (c2), which is about 9 x 1016 m2 / s2 .

In fact, this is how the sun works. In its center, the temperature is around 100 billion degrees Kelvin and the pressure is billions of times what we experience on the Earth’s surface. At these extreme conditions, 600 million metric tons of hydrogen are fused together every second into helium, producing 3.846×1026 Watts.

More massive stars have higher temperatures and pressures, so they can fuse even heavier elements together.

What are the Benefits of Nuclear Fusion?

Power plants run by nuclear fusion would be similar to our current nuclear power plants, in that they heat water into steam, which turns turbines. However, the nuclear power plants we use today rely on fission, which is the splitting of atoms. This requires large atoms, mainly uranium and plutonium, both of which are hard to mine, expensive to refine, and, of course, are radioactive.

Fusion, on the other hand, requires small atoms, namely deuterium and tritium, which are two isotopes of hydrogen. Hydrogen and its isotopes are virtually unlimited, need much less preparation to use, and are safe to handle.

Furthermore, both fission and fusion produce radioactive waste, although the waste produced by fusion is much less harmful, as it is short-lived. The waste from fission can remain highly radioactive for several thousand years.

Fusion also produces far more stable reactions than fission. That is, fission produces a chain reaction, which has the potential to run away and lead to a nuclear melt down, whereas fusion is an isolated event. Therefore, there is no risk of a future Chernobyl or Fukushima type disaster.

Moreover, fusing lighter atoms together produces more energy than splitting heavier atoms. In fact, fusion produces four times as much energy as fission and four million times as much as burning hydrocarbons.

The Lawson Criterion and Tokamaks

In a classified paper published in 1955, John Lawson laid the criteria for creating a successful fusion reaction, in that it produces more energy than went into it. Lawson found that there needs to be a delicate balance between temperature, density, and confinement time. That is, the hydrogen isotopes need to be hot enough, within a narrow window of density, and be confined long enough to fuse.

Confinement is achieved usually through magnetic fields, although it may be possible with laser or ion beams. When a powerful magnetic field is created in a tokamak, which is donut shaped reactor, the hydrogen isotopes are squeezed together, ideally stopping the fuel from escaping. Temperature is achieved by heating the fuel with electricity, microwaves, and neutral particle beams, which turns the hydrogen isotopes into plasma, as the electrons wander away from the nucleus. The density of the fuel can be a mixture of deuterium and tritium, with a 50/50 split being thought to be the most efficient.

With the above setup some teams of researchers have produced net energy, although not enough to be feasible for commercial use. However, researchers are rapidly solving problems, making nuclear fusion far more efficient.

Speedy Ions

One problem with the current setup of fusion reactors is containing supercharged particles known as speedy ions. These are the source of much of the necessary heat, but they can easily escape the magnetic containment fields. Therefore, figuring out as much as possible about these particles has been a priority for fusion researchers. The problem, though, is that conditions in the center of a tokamak are rough enough to destroy fragile sensors.

A team of researchers from the DIII-D National Fusion Facility got around this problem by looking at the magnetic field instead of the particles. With a simple sensor and powerful computers they were able to measure tiny fluctuations in the magnetic field as the particles pass through it, providing a wealth of useful data.

According to the team’s press release, this data is already being used to guide the development of “real-time control of the conditions that affect fast ions.”

Magneto-Inertial Fusion

While using magnetic fields is the most common, it is possible to create a fusion reaction with high powered jets, known as inertial confinement. The basic idea is that the jets are placed on the inside of a spherical container and fire super-heated plasma simultaneously towards the center, producing the necessary pressure and temperature to fuse hydrogen.

Researchers from Los Alamos National Laboratory combined magnetic and inertial confinement, as they believe this provides a number of benefits. In particular, they claim this method is simpler and cheaper, mainly because there is less potential damage to expensive equipment. That is, this setup allows for a greater distance between the fusion reaction and the jets.

Stellarators

Creating a powerful and consistent magnetic field is all about the the placement of the magnetic coils. Each coil emanates a magnetic field, which interacts with the fields produced by other coils. The donut shape of a tokamak allows for the coils to be placed in a configuration that produces an effective magnetic field, although not an ideal one.

Better configurations and even different designs of the coils exist, but modeling them becomes increasingly complex, making them difficult to analyze and employ in the real world. A stellarator is a specific type of fusion reactor with a twisted shape and specially designed coils, which provides for much better magnetic confinement. Research on them has been slow going due to the complexity.

However, scientists from the Princeton Plasma Physics Laboratory (PPPL) have recently found a way to drastically simplify the mathematics. In their paper, they describe a technique that uses a Hessian matrix, which is useful for determining the local high and low points of a function. They adapted this technique to analyze the weaknesses and strengths of each coil’s field as well as the overall magnetic field they produced.

They believe this will lead to more efficient coil design and better coil placement, allowing for the use of stellarators.

Rogue Waves

When the hydrogen isotope fuel is injected into the center of a fusion reactor, powerful waves might be created that can disturb the fuel, causing it to leak out of its confinement. These rogue waves can reduce the efficiency or even stop the reaction entirely.

Preventing this requires both predicting when they occur and counteracting them. To do this, another team of scientists from the Princeton Plasma Physics Laboratory developed mathematical models that can determine the likelihood of a rogue wave. These models also allow them to calculate the exact angle of a counter-wave to cancel out the rogue wave.

So far these methods have been shown to agree with experimental data from the fusion reactor at the PPPL. The researchers will soon be using data from other reactors to test their models.

Magnetic Islands

One of the biggest problems in magnetic containment fields is the presence of magnetic islands. These are anomalies where the magnetic field lines separate from the rest of the field and form a tube shape, allowing the fuel to escape. The presence of large magnetic islands can seriously hinder any attempts at a fusion reaction.

Scientists from the DIII-D National Fusion Facility in San Diego recently found a method to significantly shrink magnetic islands. In their paper, they show that injecting frozen pellets of deuterium into the fuel mixture caused just enough turbulence to disrupt the islands.

New Fuel Recipes

Preparing fuel for nuclear fusion is a delicate balance of electrical voltage, gas pressure, and magnetic field strength. The electrical voltage is applied to the gas to turn it into plasma, which happens within a few milliseconds. But, as this is happening, the electrical voltage and the magnetic field strength need to consistently adapt to the changing pressure of the gas. Finding the right combination of all three is exceedingly difficult, time consuming, and resource intensive.

To alleviate this difficulty, yet another team at the Princeton Plasma Physics Laboratory created a customization simulator. In their their paper, they claim this allows operators to experiment with ideal combinations based on the specifics of their reactor, “such as the geometry of the conductive elements and active coils, power supply specifications and coil heating and stress limits.”

The Holy Grail of Energy

All of the above gets a bit closer to having actual nuclear reactors powering the electrical grid, although a lot of work is still needed. Despite this, governments are setting goals and pouring money into the idea.

For example, the UK wants to have a fusion power plant operational by 2040 and they have committed nearly $300 million to make it happen. Likewise, the European investment bank has pledged $500 million to making it happen by 2050. According to a Reuters article, China has spent a few billion dollars trying to build a commercial reactor in the next decade.

The International Thermonuclear Experimental Reactor (ITER or “The Way” in Latin) group wants to do it by 2025.

Maybe this is overly ambitious. Maybe they can really do it. Either way, fusion is he way forward, as there are no other forms of energy that provide as many benefits.

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