Why are extremely high temperatures effective at causing fusion? What is used to produce such temperatures in a thermonuclear warhead?

The Coulomb barrier is a term that describes the electrostatic force of repulsion that two positively charged nuclei experience due to their similar charge. This force must be overcome for nuclear fusion to occur, as like charges repel each other. The Sun, for example, utilizes its immense gravitational pressure to overcome this barrier. But how do we achieve fusion here on Earth, where we can’t replicate the Sun’s gravitational force?

Nuclear physicists have learned that by raising the temperature to the extreme, atoms move with such velocity and energy that they can collide and overcome this Coulomb barrier. The temperatures required are usually in the range of millions of degrees, which translates to the kinetic energy needed for the protons to come sufficiently close for the strong nuclear force to take over and fuse them together. Therefore, producing temperatures this high—on the order of millions of degrees Celsius—is critical in experiments and applications involving controlled nuclear fusion.

A thermonuclear warhead, often colloquially known as the hydrogen bomb, is an advanced type of nuclear weapon that generates explosive energy through the fusion of hydrogen isotopes. Unlike a fission bomb, which relies solely on the splitting of heavy atomic nuclei, a thermonuclear warhead includes a second stage that fuses lightweight nuclei together after they've been heated and pressurized by the initial fission explosion.

In the context of thermonuclear warheads, the necessary high temperatures and pressures to overcome the Coulomb barrier are achieved artificially through an initial fission reaction, typically triggered by conventional explosives. The fission reaction produces high-energy X-rays which then compress and heat fusion fuel, such as mixtures of the hydrogen isotopes deuterium and tritium, to the point where nuclear fusion reactions can take place, releasing a tremendous amount of additional energy.

The strong nuclear force is one of the four fundamental forces of nature and is essential in the process of nuclear fusion. It acts between the protons and neutrons of atoms, overcoming the repulsive electromagnetic force when the atomic particles are extremely close to each other. This force is incredibly powerful but only works over very short ranges—less than the diameter of a proton.

During fusion, once the Coulomb barrier is penetrated - thanks to the high kinetic energy delivered by the temperatures of millions of degrees - the strong nuclear force takes over, pulling the protons and neutrons together to form a more massive nucleus. This fusion releases an enormous amount of energy because the strong nuclear force itself is extremely energy-dense. Understanding and harnessing this energy is at the heart of controlled fusion research and also, regrettably, in the construction of more powerful weapons like the thermonuclear warhead.

Hydrogen isotopes play a starring role in both natural and human-made nuclear fusion reactions. Hydrogen has three isotopes, protium, deuterium, and tritium, while deuterium and tritium are the ones commonly used in fusion reactions. Deuterium, also known as heavy hydrogen, contains one neutron, while tritium contains two neutrons, in addition to the one proton that all hydrogen atoms have.

These isotopes are valuable in fusion processes because they can more readily overcome the Coulomb barrier and achieve fusion at lower temperatures than other elements. This makes them ideal candidates for experiments and practical applications in producing fusion energy. In a thermonuclear warhead, the blend of deuterium and tritium provides the fuel for the fusion reaction that follows the initial fission blast, resulting in the vastly increased power of the explosion compared to a pure fission bomb.