Fusion energy gain factor facts for kids

The explosion of the Ivy Mike hydrogen bomb. This was the first device to create a lot of fusion energy.

The fusion energy gain factor, often called Q, is a way to measure how well a nuclear fusion reactor is working. It compares the amount of fusion power a reactor makes to the power it needs to keep the super-hot gas, called plasma, going.

When Q equals 1, it means the fusion reactions are making as much power as is needed to heat the plasma. This special point is called breakeven or scientific breakeven.

Some of the energy from fusion reactions can heat the fuel itself. This is called self-heating. But most fusion reactions also release energy that can't be captured by the plasma. So, a reactor at Q = 1 would still cool down without extra heating. For a fusion reaction to keep itself going without outside help, it usually needs to reach a Q of about 5.

If Q goes even higher, the self-heating becomes so strong that no more outside heating is needed. At this point, the reaction becomes self-sustaining. This important condition is called ignition. Ignition is like having an infinite Q because you don't need to add any more heating power. It's a big goal for building practical fusion reactors.

Over time, other terms have been created to describe different levels of fusion success. Energy not used for self-heating can be turned into electricity. This electricity can then be used to power the reactor's heating systems. If a reactor can power itself this way, it has reached engineering breakeven. A machine operating above engineering breakeven would make more electricity than it uses. It could then sell the extra power. If it sells enough electricity to cover its running costs, it's sometimes called economic breakeven.

Also, fusion fuels like tritium are very expensive. So, many experiments use cheaper test gases like hydrogen or deuterium. If a reactor running on these test fuels would reach breakeven if tritium were added, it's said to be at extrapolated breakeven.

For many years, the JET machine held the record for Q at 0.67. The JT-60 machine held the record for extrapolated Q at 1.25. In December 2022, the National Ignition Facility (NIF) made history. It reached a Q of 1.54, producing 3.15 million joules of energy from 2.05 million joules of laser heating. This is the current record as of 2023^[update].

What is Fusion Energy Gain?

The fusion energy gain factor, Q, simply compares the power released by fusion reactions (P_fus) to the heating power supplied (P_heat). This is for normal, steady operation. For reactors that run in short bursts, Q is calculated by adding up all the fusion energy produced and all the energy used to create the burst.

What is Breakeven?

In 1957, a scientist named John Lawson studied how energy flows in a fusion reactor. He looked at how much power would be lost and how much heating was needed to keep the reaction going. This balance is now known as the Lawson criterion.

In a successful fusion reactor, the fusion reactions create power, P_fus. Some of this energy, P_loss, is lost. This mostly happens when the fuel moves to the reactor walls or through different types of radiation. To keep the reaction going, the system must provide heating to make up for these losses. So, P_loss must equal P_heat to keep the temperature stable.

The most basic idea of breakeven is when Q = 1. This means the power from fusion (P_fus) is equal to the heating power supplied (P_heat).

Scientific Breakeven

Over time, new types of fusion devices were developed. One important type is inertial confinement fusion, or ICF. Magnetic confinement fusion (MCF) devices usually run continuously. They keep the plasma in fusion conditions for seconds or even minutes. The goal is for most of the fuel to undergo fusion during this long time.

In contrast, ICF reactions last only a very short time. They try to make sure a lot of fuel fuses in that brief moment. To do this, ICF devices squeeze the fuel to extreme conditions. This makes the self-heating reactions happen very quickly.

In an MCF device, powerful magnets create and hold the plasma. Modern superconducting magnets need very little energy to run. Once the plasma is set up, it's kept hot by injecting heat into it. These heating systems use most of the energy needed to keep the system running. They are quite efficient, turning about half the electricity into plasma energy. So, for MCF, P_heat is close to the total energy put into the reactor.

However, in ICF devices, the energy needed to create the extreme conditions is huge. The devices that do this, usually lasers, are very inefficient, sometimes only 1% efficient. If P_heat included all the energy put into the system, ICF devices would seem very inefficient. For example, the NIF uses over 400 million joules of electrical power to produce 3.15 million joules of output. This energy is needed for every single reaction, not just to start the system.

ICF scientists explain that other "drivers" could be used to improve this. To understand how well an ICF system performs, it's more important to look at the fusion process itself, not how efficient the drivers are. So, for ICF devices, P_heat is usually defined as the amount of driver energy that actually hits the fuel. For NIF, this was about 2 million joules. Using this definition, NIF reached a Q of 1.5.

To make this clear, this definition is often called scientific breakeven, or sometimes Q_sci or Q_plasma.

Extrapolated Breakeven

Since the 1950s, most fusion reactor designs use a mix of deuterium and tritium as their main fuel. This is because this fuel mix is the easiest to ignite. However, tritium is radioactive and expensive. It also raises safety concerns.

To save money, many experimental machines run on test fuels like hydrogen or deuterium alone. They leave out the tritium. In these cases, the term extrapolated breakeven, or Q_ext, is used. It describes how well the machine would perform with D-T fuel, based on its performance with hydrogen or deuterium.

The records for extrapolated breakeven are a bit higher than for scientific breakeven. Both JET and JT-60 have reached values around 1.25 when running on D-D fuel. When JET ran on D-T fuel, its maximum performance was about half of its extrapolated value.

Engineering Breakeven

Another term is engineering breakeven, often called Q_E or Q_eng. This considers the full process of making electricity. It includes taking energy from the reactor, turning it into electrical energy, and then sending some of that electricity back to power the heating system. This closed loop is called recirculation.

D-T fusion reactions release most of their energy as neutrons. A smaller amount comes out as charged particles, like alpha particles. Neutrons have no electric charge, so they fly out of the plasma before they can heat it. Only the charged particles can be captured within the fuel and cause self-heating. If f_ch is the fraction of energy released as charged particles, then P_ch = f_chP_fus. If all of P_ch is captured, the power available for making electricity is (1 - f_ch)P_fus.

For D-T fuel, neutrons carry most of the useful energy. This neutron energy is usually captured in a "blanket" made of lithium. This blanket produces more tritium, which is used to fuel the reactor. The blanket can also add a small amount of energy. The total energy released to the environment, available for power production, is called P_R.

The blanket is cooled, and the cooling fluid drives traditional steam turbines and generators. This creates electricity, which is then sent back to the heating system. Each step in this process has an efficiency. Plasma heating systems, for example, are about 60-70% efficient. Modern generators are about 35-40% efficient. Overall, the power conversion loop has a net efficiency of about 20-25%. This means only about 20-25% of P_R can be sent back to the heating system.

For a magnetic confinement device to reach engineering breakeven (Q_E = 1), it typically needs a Q value between 5 and 8. Inertial devices need much higher Q values, around 50 to 100, because their heating systems are less efficient.

What is Ignition?

As the plasma gets hotter, fusion reactions happen much faster. This also increases the self-heating. Other energy losses, like x-rays, don't grow as fast. So, as the temperature rises, self-heating becomes more efficient. Less outside energy is needed to keep the plasma hot.

Eventually, the external heating power (P_heat) drops to zero. This means all the energy needed to keep the plasma hot comes from self-heating. This point is called ignition. For D-T fuel, where only 20% of the energy helps with self-heating, ignition can only happen when the plasma produces at least five times the power needed to stay hot.

Ignition means Q is infinite. However, it doesn't mean that no power is recirculated. Other parts of the system, like magnets and cooling systems, still need power. But these usually need much less energy than the heaters. This means that once ignition is reached, any further improvements in plasma performance can directly lead to more electricity for sale.

What is Commercial Breakeven?

The final definition of breakeven is commercial breakeven. This happens when the money earned from selling the extra electricity (after recirculation) is enough to pay for the reactor. This depends on the reactor's building cost, its running costs (like fuel and maintenance), and the price of electricity.

Commercial breakeven depends on things outside the reactor's technology. It's possible that even a reactor with a fully ignited plasma, working well beyond engineering breakeven, might not make enough money fast enough to pay for itself. Whether future reactors like ITER can reach this goal is still being discussed.

Practical Example

Most fusion reactor designs being studied as of 2017^[update] use the D-T reaction. This is because it's the easiest to ignite and produces a lot of energy. This reaction releases most of its energy as a fast neutron. Only 20% of the energy comes out as an alpha particle. So, for the D-T reaction, f_ch = 0.2. This means self-heating won't equal external heating until Q is at least 5.

Efficiency values change with different designs. But typically, heating efficiency (η_heat) might be 70%, and electrical efficiency (η_elec) might be 40%. A practical fusion reactor needs to produce power, not just recirculate it. So, about 20% of the power (f_recirc = 0.2) might be recirculated. Using these numbers, a practical reactor would need a Q of about 22.

Consider ITER, which is designed to produce 500 million watts of fusion power from 50 million watts of input. If 20% of the output is self-heating, then 400 million watts escape. With the same efficiencies (η_heat = 0.7 and η_elec = 0.4), ITER could theoretically produce up to 112 million watts of heating. This means ITER would operate at engineering breakeven. However, ITER is not built to extract power. So, this remains theoretical until follow-up machines like DEMO are built.

Fusion Records and Achievements

For over two decades since 1997, the JET machine held the record for Q at 0.67. The JT-60 machine held the record for extrapolated Q at 1.25. This slightly beat JET's earlier extrapolated Q of 1.14.

National Ignition Facility (NIF) Success

The Lawrence Livermore National Laboratory (LLNL), a leader in ICF research, uses a slightly different Q definition. They define P_heat as the energy the laser delivers to the fuel capsule, not the total energy put into the laser. This definition gives much higher Q values.

On October 7, 2013, LLNL announced that the National Ignition Facility (NIF) had achieved scientific breakeven. In that experiment, P_fus was about 14 kilojoules (kJ), while the laser output was 1.8 million joules (MJ). By their older definition, this would be a Q of 0.0077. For this announcement, they changed Q again. They said P_heat was only the energy that reached "the hottest part of the fuel," which they calculated as only 10 kJ of the original laser energy. This new definition caused some debate.

On August 17, 2021, NIF announced a major step forward. In early August 2021, an experiment achieved a Q value of 0.7. It produced 1.35 MJ of energy from a fuel capsule by focusing 1.9 MJ of laser energy on it. This was eight times more energy than any previous experiment.

Then, on December 13, 2022, the United States Department of Energy announced a historic achievement. On December 5, 2022, NIF had finally passed the long-sought Q ≥ 1 milestone. It produced 3.15 MJ of energy after delivering 2.05 MJ to the target. This resulted in an equivalent Q of 1.54.