Fusion power

Fusion power is the energy released when hydrogen nuclei (single proton or hydrogen isotopes) collide and fuse to form heavier hydrogen nuclei or helium. The process of nuclear fusion, the opposite of nuclear fission, is what powers the sun and other stars in the universe. Using fusion power to generate electricity on Earth has the potential to provide an almost inexhaustible energy source without carbon emissions.

However, creating sustainable net energy positive (i.e., the rate of heat emitted by the fusion process exceeds the rate of energy injected into it) is a significant engineering challenge. Generating the dense, high-energy environment needed for nuclear fusion reactions requires overcoming the repellant forces of hydrogen nuclei with the same charge. To cause collisions, a gas must be raised to high energies, exceeding 2 x 10^{8 o}C. At these temperatures, the gas breaks down into a plasma made up of separate electrons and ions. Researchers are attempting to find methods of controlling and sustaining the superheated plasma produced in order to make fusion power a viable source of energy.

There are multiple possible nuclear fusion reactions with the potential for generating electricity. With current technology, the deuterium-tritium (D-T) reaction is the most readily feasible. It involves two isotopes of hydrogen, deuterium with a nucleus containing 1 proton and 1 neutron, and tritium with a nucleus containing 1 proton and 2 neutrons. Each D-T fusion reaction releases 17.6 MeV (2.8 x 10^-12 J) of energy.

Research into fusion power began in the USA and USSR and was linked to atomic weapon development. Today, many countries participate in fusion research to varying extents, led by the European Union, the USA, Russia, and Japan, with programs also underway in China, Brazil, Canada, and Korea. Most major research efforts can be separated into two experimental approaches to confine hydrogen isotopes and generate the conditions needed for fusion reactions:

• Magnetic confinement—using strong magnets to contain the superheated plasma
• Intertial confinement—compressing small fusion fuel pellets to extremely high densities using lasers or particle beams

Potential advantages of fusion power include the following:

• No carbon emissions—the by-products of fusion reactions are small amounts of helium, an inert gas that can be safely released without harming the environment.
• Abundant fuels—deuterium can be extracted from water, and tritium can be produced inside power stations from lithium, an element abundant in the Earth’s crust and seawater. Estimates suggest Earth possesses thousands of years of deuterium/tritium supply, even with widespread fusion adoption.
• Efficiency—one kilogram of fusion fuel has the potential to provide the same energy as 10 million kilograms of fossil fuel.
• Reduced radioactive waste—while fusion reactions do not produce radioactive waste, fusion can produce radioactive reactor components depending on the structural materials used. Research is underway to identify suitable materials that minimize decay times as much as possible.
• Safety—with only a small amount of fuel required, fusion reactors cannot cause large-scale nuclear accidents.

These advantages and the potential for a long-term solution to transitioning from fossil fuels have led governments around the world to invest in fusion power research initiatives.

The following are significant challenges to implementing fusion energy:

• Confining fusion fuel as a superhot plasma
• Designing exhaust systems that can handle the intense heat produced by fusion power
• Developing materials to withstand the conditions inside fusion reactors
• Handling the tritium fuel required for widespread fusion use
• Reactor maintenance

The most common approach, being implemented by multiple private sector organizations and major international research projects like ITER (International Thermonuclear Experimental Reactor), is magnetic confinement fusion. This approach uses magnetic fields to confine cubic meters of deuterium-tritium (D-T) plasma at a few atmospheres of pressure in a vacuum chamber. The magnetic field induces an electrical current that increases its temperature. Additional heating systems are also applied to achieve fusion temperatures, including microwaves, radiowaves, or accelerated particles.

The charged particles (ions and electrons) in the plasma follow magnetic field lines, helping scientists prevent them from coming into contact with reactor walls, where they would dissipate their heat, destroying the reaction. The best magnetic configuration to isolate the plasma is a toroidal shape (donut), where the magnetic field curves around to form a closed loop. Several toroidal confinement systems have been developed, including tokamaks, stellarators, and reversed field pinch (RFP) devices.

A newer approach is internal confinement fusion, using laser or ion beams focused on a small pellet of D-T fuel, compressing it intensely to generate fusion reactions. The method follows the same principles as the fusion reactions that take place when thermonuclear weapons are detonated, but at a much smaller scale.

As the outer layer of the pellet heats from the laser beam, it will explode outwards, generating an inward-moving force that compresses and heats the inner layers of the fuel. This can compress the core to up to a thousand times its liquid density, creating the conditions where fusion can occur. Energy from the core's fusion reactions can heat the surrounding fuel, producing further fusion across the pellet in an outward chain reaction. The time needed for these reactions is limited by the inertia of the fuel (<1ms), giving the technique its name.

A number of other methods are under investigation. Often, they draw from one or both magnetic confinement and inertial confinement. Other methods include:

• Magnetized target fusion (MTF)—a pulsed approach combining compressional heating of inertial confinement fusion with reduced thermal transport and enhanced heating of magnetic confinement fusion.
• Hybrid fusion—a combination of fusion and fission surrounding the fusion fuel core with a subcritical fission reactor.
• Z-pinch—using an electrical current in the plasma to generate the magnetic field that compresses the plasma. The name comes from the direction of the current.
• Fusor—confining the fuel in electrostatic potential wells with a cathode surrounded by an anode held at ground. Ions are injected into the chamber, falling towards the center at high speeds.
• Polywell—augmentation of the fusor design, adding strong electromagnets in grids to further compress the electron cloud.

Both fusor and polywell fusion reactor designs utilize inertial electrostatic confinement.