Understand Fusion
Welcome to the August edition of Stanford University’s Understand Energy Learning Hub Energy Spotlight! Last month’s Spotlight covered fission, the source of all commercial nuclear energy production today. This month’s topic is fusion, a different type of nuclear reaction. Fusion energy is garnering lots of excitement and new investment. If you like what you see, please share widely and encourage others to subscribe. You can also check out all of our past issues!
What you need to know
Significance: Fusion offers the possibility of an abundant carbon-free energy resource with no long-lived radioactive waste. However, fusion energy is still in the research phase because we have not yet been able to make continuous and sustainable fusion reactions happen on Earth. In December 2022, Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) reached a significant milestone by producing more energy from fusion than was delivered to the fusion target.
The world spends billions of dollars every year on research to create commercially viable fusion energy, and progress is accelerating. Global investment in private fusion companies in the twelve months leading up to July 2025 was over $2.6 billion dollars, representing more than a quarter of the total private fusion investment to date, and the number of private fusion companies has more than doubled since 2021.
What is fusion? Nuclear fusion occurs when two or more atomic nuclei collide and combine to form a larger nucleus, releasing massive amounts of energy. Fusion reactions power the Sun and other stars. The most common fusion reactions are hydrogen isotopes into helium as shown below.
Fusion predominantly occurs in the plasma state–a state of matter in which gases become ionized as atoms shed their electrons, creating an electrically charged gas with free electrons and ions. Achieving a plasma state requires extremely high temperatures.
For fusion to occur, nuclei in a plasma state must overcome the Coulomb barrier, which arises because the nuclei are all positively charged and naturally repel each other. (Imagine trying to push the positive ends of two magnets together.) The high temperatures and extreme gravitational force in the center of the Sun and other stars provide the energy necessary for nuclei to overcome the electrostatic repulsion between them and fuse into a larger nucleus.
The Coulomb barrier
After fusion, the mass of the larger nucleus is slightly less than the combined mass of the original nuclei. The lost mass is converted to energy following Einstein’s equation E = mc². Even though m (mass) is very small, c is the speed of light (299,792,458 m/s), a huge number that gets squared. So a small amount of mass loss is converted to A LOT of energy.
Making fusion commercially viable for energy production
Three broad steps are necessary for commercially viable fusion energy:
- Make fusion happen on Earth (we’ve done this)
- Get more energy out than we put in (we’ve made significant progress)
- Achieve continuous and sustainable fusion to generate electricity in a thermal power plant (we haven’t done this)
Let’s look at each step in more detail below.
Step 1. Make fusion happen on Earth
What fuels do we use? The hydrogen isotopes deuterium and tritium (DT) are the fuels we use most commonly to make fusion happen on Earth because they release the highest energy at the “lowest” temperatures relative to other fuels.
Hydrogen Isotopes
DT fuel is extremely energy dense. By weight, it’s 10 million times more energy dense than coal, 6 million times more energy dense than natural gas, and 4 times more energy dense than fission. Deuterium is an abundant resource which can be found in seawater. Tritium is rare and very expensive. However, in the future, tritium could be bred from lithium continuously in a fusion power plant.
What’s required to make fusion happen? It’s hard to achieve the right conditions to create a fusion reaction on Earth. The Fusion Triple Product (related to the Lawson Criteria) is a convenient metric for how close a plasma is to energy breakeven once nuclei are in the plasma state:
temperature × confinement time × plasma density > 1021 keV s m-3
Temperature: In the Sun, fusion occurs at 10-15 million degrees Celsius. On Earth, we don’t have the immense gravitational pressures of the Sun, so fusion reactions typically require temperatures >100 million degrees Celsius. That’s more than 6 times the core temperature of the Sun!
Confinement and plasma density: No solid known material on Earth is capable of physically confining plasma at >100 million degrees. That means a confinement system that doesn’t allow the plasma to touch the walls must hold the plasma in place.
The two most common approaches to fusion are magnetic and inertial confinement.
- Magnetic confinement uses powerful magnetic fields to confine the plasma:
- Requires relatively low plasma density and long energy confinement times (seconds)
- The tokamak is the most researched magnetic confinement design and has the most global funding of all fusion designs
Tokamak
- Inertial confinement creates plasma and fusion with very quick bursts of energy imploding a fuel-filled target, typically using high-powered lasers:
- Requires high plasma density over a very short period (nanoseconds)
- An inertial confinement experiment holds the world record for fusion energy gain relative to energy delivered to the fuel
Indirect drive inertial confinement fusion
Step 2: Get more energy out than we put in
The variable Q represents the fusion energy gain factor, which is the ratio of produced energy to injected energy.
Q = produced energy/injected energy
We want Q > 1. Different levels of Q help us measure our progress toward commercial fusion. The levels are based on how we define the system.
$$Scientific\hspace{7px}Q\hspace{7px}(Q_{sci}) = {fusion\hspace{7px}energy \over energy\hspace{7px}delivered\hspace{7px}to\hspace{7px}the\hspace{5px}fusion\hspace{7px}target\hspace{7px} (e.g.,\hspace{4px}energy\hspace{7px}from\hspace{7px}lasers)}$$
$$Engineering\hspace{7px}Q\hspace{7px}(Q_{eng}) = {electricity\hspace{7px}to\hspace{7px}the\hspace{7px}grid \over electricity\hspace{7px}needed\hspace{7px}to\hspace{7px}run\hspace{7px}the\hspace{7px}entire\hspace{7px}power\hspace{7px}plant}$$
Significant breakthrough!
NIF made history on December 5, 2022, by being the first and only facility in the world to achieve Qsci > 1. They used inertial confinement, with 192 of the world's highest energy lasers converging on a peppercorn-sized capsule filled with DT fuel. Since then, NIF has achieved Qsci > 1 multiple times with increasing energy yields. Even so, NIF is still far from demonstrating Qeng > 1.
Step 3: Achieve continuous fusion to generate electricity in a thermal power plant
How would a commercial fusion power plant work? A commercial fusion power plant would require Qeng > 1 to deliver net electricity to the grid. Like coal and nuclear fission, fusion would use a thermal power plant to generate electricity. Heat generated from continuous fusion reactions would boil water, make steam, turn a turbine and generator, and produce electricity. Helium, an inert gas, would be the main product of the fusion reactions. Tritium could be continuously generated in fusion power plants using a lithium blanket wall. The vast majority of the tritium would be consumed as fuel in the power plant.
Source: Entler, Approximation of the economy of fusion energy
Unlike with nuclear fission, runaway reactions in a fusion plant are not possible. If something were to go wrong, the temperature of the plasma would decrease, the plasma would extinguish, and fusion reactions would cease.
Current and future trends
Innovation: The United States, China, and the EU lead the world in fusion research. The U.S. and China each invested over $1 billion in public and private fusion research in 2024. Institutions conducting notable fusion research include ITER (a collaboration of 33 nations that is based in Southern France), China’s EAST, NIF, Princeton, MIT, and 50+ private fusion companies.
Buying future fusion power: Google recently purchased future fusion power in a deal with Commonwealth Fusion Systems, hoping fusion energy can be an option for 24/7 clean power. Similarly, Helion just began construction on a site for a planned fusion power plant to supply power to Microsoft data centers by 2028.
In the news
News: ITER has completed all components for the world’s largest, most powerful pulsed superconducting electromagnet system, which they will use in their tokamak fusion experiment. Major components of the new electromagnet system were built by the U.S., Russia, Europe, and China. Once assembled, the pulsed magnet system will weigh almost 3,000 tons.
Context: It's taken ITER decades to get to this point. Construction of their massive tokamak experiment site began in 2010, and initial operations are planned to start in 2035. Private fusion companies tend to iterate on faster timelines but at smaller scales than ITER.
Once their tokamak experiment is fully operational, ITER plans to produce 500 megawatts of fusion power from 50 megawatts of input heating power. If attained, this tenfold energy gain would likely be enough to result in the fusion reaction being largely self heating, another milestone on the path to viable commercial fusion. ITER is also a tremendous geopolitical achievement, with thousands of scientists and engineers on three continents contributing components to build a single machine.
Fun Fact
Lightning is one of the few naturally occurring plasmas found on Earth!
The formation of plasma in lightning begins with the build-up of electrical charges in a storm cloud. Air turbulence causes particles of ice and water in the cloud to collide, creating friction that separates charges. Positively charged particles accumulate at the cloud's top, while negatively charged particles collect at the bottom, eventually creating an electric field between the cloud and the Earth. When that electric field becomes strong enough, gas molecules in the atmosphere shed electrons and convert into plasma. This forms a conductive path for electricity to flow, resulting in the brilliant flash of lightning.
Lightning plasma reaches temperatures as high as 30,000 degrees Celsius. The heat creates the white light of a lightning bolt. The super-fast heating of the air around the lightning makes the air expand very quickly, causing the accompanying thunder.
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Guest contributors: Dr. Scott Hsu and Dr. Clea Kolster, Lowercarbon Capital
Understand Energy team contributors: Dr. Diana Gragg, Racheal Moore, Sharon Poore, and Shirley Chang
The data in this issue are current as of September 2025. For the most current data, visit our Fusion Fast Facts.
