From How Nuclear Fusion Reactors Work:
In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It’s the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.
Conceptually, harnessing nuclear fusion in a reactor is a no-brainer. But it has been extremely difficult for scientists to come up with a controllable, non-destructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.
When hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). If you’ve ever tried to place two magnets together and felt them push apart from each other, you’ve experienced this principle first-hand.
To achieve fusion, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:
High temperature - The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons.
- Fusion requires temperatures about 100 million Kelvin (approximately six times hotter than the sun’s core).
- At these temperatures, hydrogen is a plasma, not a gas. Plasma is a high-energy state of matter in which all the electrons are stripped from atoms and move freely about.
- The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core. We must use energy from microwaves, lasers and ion particles to achieve these temperatures.
High pressure - Pressure squeezes the hydrogen atoms together. They must be within 1x10-15 meters of each other to fuse.
- The sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core.
- We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams.
There are two ways to achieve the temperatures and pressures necessary for hydrogen fusion to take place:
- Magnetic confinement uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The International Thermonuclear Experimental Reactor project in France is using this method.
- Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility of Lawrence Livermore Laboratory in the United States.
Let’s look at magnetic confinement first. Here’s how it would work:
Microwaves, electricity and neutral particle beams from accelerators heat a stream of hydrogen gas. This heating turns the gas into plasma. This plasma gets squeezed by super-conducting magnets, thereby allowing fusion to occur. The most efficient shape for the magnetically confined plasma is a donut shape (toroid).