FAQs

Q—also called "fusion gain"—measures the ratio between the power produced by the fusion reactions, and the external heating power that must be injected in a tokamak to sustain the reactions.

Let's review how ITER will create the conditions for fusion inside its vacuum chamber.

• Fuel is injected in gaseous form into the vacuum vessel (the gas weighs only a few grams and fills the entire volume of the tokamak);
• Electricity flowing through the electromagnets, particularly the central solenoid, produces a voltage across the gas;
• This voltage rips electrons from the fuel atoms, turning them into charged particles (ions). This new state of matter is called a plasma;
• The changing magnetic fields that are used to control the plasma produce a heating effect ("ohmic heating"). But in order to obtain the temperatures approaching 150,000,000 °C that are needed for deuterium-tritium fusion, three sources of external heating must be applied from outside of the tokamak;
• The megawatts of heating power injected by these external systems is part of the ratio measured by Q, which compares input heating power to output fusion power.

In ITER, the programmatic goal, Q≥10, signifies delivering ten times more thermal power (500 MW) than that which is delivered by the heating systems (50 MW).

Very. Breakeven, which corresponds to Q=1, is the moment when the total fusion power produced during a plasma pulse equals the power injected into the systems that heat the plasma. This has never been achieved in a magnetic confinement fusion device; the current record worldwide is held by the European tokamak JET (UK), which succeeded in generating a Q of 0.67 in the 1990s. ITER is the machine that has been designed to do it, which explains the participation of so many nations—who run domestic fusion programs at home—in the international collaboration surrounding the project.

ITER's Q value of ≥10 makes it a first-of-kind machine and a unique scientific device. 

The fusion between the nuclei of the hydrogen isotopes deuterium (D) and tritium (T) produces one helium nucleus, also called an "alpha particle," and one neutron.

The helium nucleus, which carries 20 percent of the energy produced by the fusion reaction, is electrically charged and remains confined by the magnetic fields of the tokamak (whereas the neutron escapes). The heating provided by these alpha particles contributes to maintaining the temperature of the plasma and decreases the need for external heating. When heating by the helium nuclei ("alpha heating") is dominant (over 50 percent) the plasma is said to be a "burning plasma."

This is a state of matter that has never been produced in a controlled manner on Earth. Read more about it in this article.

Yes, it will, and there is a large worldwide consensus around the necessity of building such a device. Achieving a burning plasma in which at least 50 percent of the energy to drive the fusion reaction is generated internally through the alpha particles is an essential last step in the 70-year quest to control fusion reactions in a magnetic fusion device.

At Q = 5, approximately 50 percent of the plasma heating is contributed by the alpha particles. At Q = 10 (ITER), this percentage rises to 66 percent. At Q=20 alpha heating represents 80 percent.

The primary motivation behind the design of ITER is to provide Members' scientists with the opportunity to study, and better understand, a burning plasma. The knowledge acquired in ITER will help scientists and engineers design the commercial fusion-generated electricity plants of the future. As a research device, ITER will be equipped with far more diagnostics and other research components than the commercial facilities that will follow.

Accounting for the size of ITER's vacuum vessel and the strength of the confining magnetic field (5.3 Tesla), the ITER plasma (830 cubic metres) can carry a current of up to 15 megaamperes.

Under these conditions, an input thermal power of 50 megawatts is needed to bring the hydrogen plasma in the vessel to about 150 million degrees Celsius. This temperature in turn translates to a high enough velocity, among a sufficient population of hydrogen nuclei, to induce fusion at a rate that will produce at least 500 megawatts of thermal power output.

Why not design ITER for a Q of 30, or 50? The answer is clear: expense. For tokamaks, size matters: if all other parameters are equal, larger size means greater Q. In simple terms, increasing Q would require an increase in the major radius or in the magnetic field strength. Either approach would have increased the cost of the device unnecessarily, whereas the achievement of Q ≥ 10 is sufficient to allow the primary scientific and technology goals of the project to be satisfied.

This would also have required an increase in cost with no great benefit to the goals of the project. ITER is an experimental device designed to operate with a wide range of plasma conditions in order to develop a deeper understanding of the physics of burning plasmas, and to allow the exploration of optimum parameters for plasma operation in a power plant. The addition of the systems required to convert fusion power to high temperature steam to drive an electricity generator would not have been cost-effective, since the pattern of experimental operation of a tokamak such as ITER will allow for very limited generation of electricity.

Plasma energy breakeven is the moment when the efficiency of the fusion reaction reaches Q = 1 (please see explanations on "Q" in the preceding paragraphs); that is, when the total fusion power produced during a plasma pulse equals the power injected into the systems that heat the plasma. This important scientific goal—never before achieved—is the "raison d'être" for the scale of ITER and the design of many of its key technological systems (superconducting magnets, external heating, blanket, divertor, etc.).

Engineering breakeven would take into consideration all of the plants systems—and not just external heating systems—in the evaluation of the input/output power balance of an electricity-producing fusion power plant. Commercial fusion plants will be designed based on a power balance that accounts for the entire facility: the electricity output, sent to the industrial grid, compared to the electricity consumed by the facility itself—not only in tokamak heating, but also in secondary systems such as the electricity used to power the electromagnets, cool the cryogenics plant, and run diagnostics and control systems.