Over the last several decades, nuclear fusion has been regarded as a potential alternative energy source in an effort to help solve our growing climate crisis. I addressed nuclear power in a previous blog post, discussing the current socioeconomic discourse surrounding the technology. This post will take a deeper dive into the science behind nuclear power, specifically in regards to nuclear fusion. Unlike fission, which involves using energetic neutrons to split a heavy, unstable nucleus into two lighter nuclei (e.g. the splitting of a uranium atom), fusion is the process of combining two light nuclei together to form a heavier nucleus. For example, a common fusion reaction is the fusion of hydrogen isotopes deuterium and tritium. When these two atoms are joined together, they form a helium atom and a neutron. Because the resulting helium atom weighs less than both the deuterium and tritium atoms combined, the leftover mass is released in the form of energy. One of the challenges with fusion energy is that deuterium-tritium reactions don’t just occur out of thin air. We need a very specific environment in order to make it happen. This environment is what is known as a plasma, the fourth state of matter that stars are made of. A plasma is a gas that becomes so hot that its electrons are stripped away from the atoms, forming an ionized gas, or a “spicy angry gas” as we like to call it in my cohort of graduate students.

So we have our plasma… Now what do we do with it? Scientists have developed a way to confine this plasma in devices known as tokamaks or stellarators. A tokamak is a device that uses a powerful magnetic field to confine the plasma in the shape of a symmetric torus (or a donut shape). A stellarator involves a similar configuration, however the magnetic field is asymmetric, leading to a more stable plasma. Even though the tokamak/stellarator configurations have proven to be the best methods for plasma confinement, there are still other roadblocks preventing us from being able to obtain a net energy gain from these fusion reactors. There are a number of these devices operational around the world, where scientists are running experiments to figure out how to address these challenges. One of the questions we are trying to answer is: how can we create a plasma that is “self-sustaining”? In other words, how can we achieve reactor conditions that allow the plasma to sustain itself without any external heating? To better understand this, we look at the fusion reactor’s performance, which is a measure of how well the magnetic field is maintaining the energy of the plasma over a period of time. This is determined by three important parameters: plasma temperature, plasma density, and the confinement time of the plasma. These are known as the “plasma parameters”. To illustrate just how difficult it is to create a self-sustaining plasma, an International Atomic Energy Agency (IAEA) article states that to create such a reaction you need “a temperature of about 100 million degrees Celsius, a density that is one million times less that of air, and [a] confinement time of just a few seconds”. A problem that fusion reactors currently face is that it is difficult to balance plasma temperature and density without breaching operational limits of the device. Pushing the boundaries of these limits creates a plasma instability, leading to termination of the plasma after only a few milliseconds.

Another important parameter related to the operation of fusion reactors is the “Q-value”, or “fusion gain value”. The Q-value is a ratio of the power required to fuel the fusion reaction versus the power generated from that reaction. If the amount of power going into the reaction equals the amount of power coming out, then we have Q=1, which is known as “breakeven”. Until very recently, the concept of breakeven was only theoretically possible. In late 2022, the National Ignition Facility at Lawrence Livermore National Lab was able to achieve a fusion gain of 1.5 using 192 lasers to deliver a huge amount of energy to a hydrogen fuel pellet. While this achievement is a monumental step, we will need a Q of greater than 5 in order to create a self-heating plasma, and above 10 to even consider fusion as economically viable. We also must achieve these Q-values in a tokamak or stellarator, if the ultimate goal is to use these devices to deliver energy to the grid.

The development of fusion as an energy source is a very complicated process with a lot of moving parts. Because the end goal is to put fusion on the grid, it not only requires the involvement of scientists, but also policymakers, economists, and risk evaluators. It is one of the most challenging projects humanity has ever undertaken. Fusion has already faced many challenges from all angles. I am fortunate enough to be working at the forefront of these challenges, tackling major scientific questions. Even though we have a long way to go before fusion becomes a reality, I am optimistic that one day fusion will power our cars, home appliances, and city lights.