Taylor Wilson

Fusion Basics

Nuclear Fusion is one of the most fundamentally important physical processes in the Cosmos. These reactions are not only our most important energy source, they created the building blocks of life from the most basic constituents of the early universe. While the energy we humans use today has been transformed into various other forms, locked up in chemical bonds of the carbohydrates in our food and hydrocarbons that power our cars, it originally fell to earth as solar energy from our closest neighboring fusion reactor: the Sun.

The prospect of controlled fusion as a future terrestrial power source has captivated generations, but unlike the swords of atomic weapons, those of the hydrogen bomb have proved infinitely more difficult to forge into plowshares. The promise of fusion is one of truly unlimited energy production, free of environmental concerns, powerful and dense, and without legacy radioactivity for future generations. When we are able to thoroughly control a fusion reaction here on earth, we will have tamed a small piece of cosmic fire and paved the way for the human race to thrive long into the future. 

To begin let's take a look at what exactly fusion is and how it works...

Both of the images above, one from my earliest fusion experiment, the other of our sun erupting a massive solar flare, show a glowing, ionized state of matter known as plasma. 

As you heat ordinary matter, it tends to transition in state from the ordered to the disordered, as the atoms and molecules that compose it gain kinetic energy. Take a cube of ice: at low temperatures the water forms an ordered solid, but as you heat the ice it transitions to a liquid and then into a gas as the water molecules of bonded Hydrogen and Oxygen atoms begin to gain energy. But what happens if you keep adding energy? You'll need a powerful source of heat, but continue this process and eventually you will have provided so much energy that the atoms composing the water transition into a fourth state of matter called plasma. Here there is so much energy available that the electrons bound to the atoms by the electromagnetic force give way forming a chaotic soup of charged ions and electrons. If you look out into the stars it is plasma like this that composes the vast majority of the visible universe.

Then for a moment imagine if you took that water you had heated and heated until it disassociated into a glowing plasma. Imagine if you kept heating it until the atoms that composed it had a temperature of millions of Kelvin. At this point those Hydrogen atoms will begin to have enough energy to overcome the electromagnetic repulsion that the nuclei of those ionized atoms feel for each other. At these speeds (just another way to think of an atom's kinetic energy or temperature) the nuclei can slam into each other so hard that they squeeze very close together. At these distances the electromagnetic force is no match, and the strongest force in the known universe can take over, the Strong Nuclear Force. This force squeezes together the particles composing these atoms even closer, fusing them together and a new atom is born. Looking closely at the products of these reactions, however, one would notice a problem. There appears to be some mass missing! This difference is accounted for by Albert Einstein's famous equation equating energy with matter. The two are interchangeable, and nuclear reactions such as these are proof of this. It is this missing mass that has been converted to energy that makes fusion reactions of light elements exothermic and ultimately provide the energy source that fuels you, me, and the universe around us.

The Reactions

Fusion reactions date back to some of the earliest moments of creation. As proposed by the Big Bang theory, matter began to congeal out of the incredible heat and density following the early inflationary period. At the point which we think that a Quark-Gluon Plasma (a hot, degenerate form of matter containing the basic elementary particles that compose nuclear matter) began to cool into the earliest nuclear particles (protons and neutrons), fusion reactions could begin to take place. For up to 20 minutes after the initial moment of the Big Bang, the universe was hot enough for these nucleons to fuse together, producing not only basic Hydrogen but Deuterium, and isotopes of Helium, Lithium, and Beryllium. This initial nuclear process known as Big Bang nucleosynthesis, produced the primordial building blocks for the universe we know today. Everything heavier would be built in a later process once the first generation of stars had formed: Stellar nucleosynthesis.

In our Sun, Hydrogen is fused into Helium in a process known as the Proton Chain. Stars larger than the Sun can fuse hydrogen in the CNO cycle, and in heavy stars stars like these, elements such as Neon, Oxygen, and Silicon, can be burned- all the way up to radioactive Nickel which decays into Iron-56 lying at the crest of the curve of Binding Energy. At this point fusion reactions cease to become exothermic (to the left of iron on the curve fusion reactions can liberate energy, to the right this is only true for fission reactions). Once a high mass star begins producing Iron, it has only a fleeting life left, without being able to produce energy from fusion it will lose the outward pressure keeping it intact. It will soon implode and then explode enriching the landscape of space with many of the periodic table's most diverse elements, including the  heaviest ones like Uranium. These heavy elements were produced in nuclear reactions similar to the neutron capture experiments I perform with my fusion devices, where neutrons were added in rapid succession utilizing the large excess of them in these vast thermonuclear explosions known as a Supernova. These reactions known as the r and s  processes built very heavy elements, including ones not found naturally on Earth today because of their nuclides short half-lives. Similar reactions actually occurred in the hydrogen bomb tests referenced below, and elements 99 and 100 (Einsteinium and Fermium) were first identified in debris from the Ivy Mike test.

We have built devices on earth that can replicate any known nuclear process, but in order to create fusion reactions on Earth that would yield fusion in quantities plentiful enough to be useful, we must choose reactions that occur at comparatively low temperatures (millions of degrees). These include, from coldest to hottest: Deuterium and Tritium, Deuterium and Deuterium, Deuterium and Helium-3, and Proton and Boron-11. 

Fusion on Earth

George, a nuclear test in 1951, was the first thermonuclear burn here on earth, consisting of a "boosted" nuclear weapon design. Ivy Mike would take the distinction of being the first true hydrogen bomb. While these devices were large cryogenically fueled experiments, they shepherd in a new age. While many scientists working in the field harbored reservations about the quest for larger and larger weapons, they also saw an opportunity in thermonuclear fusion. One for truly limitless, clean power without a military dimension and without the concerns associated with energy from fission. To learn more about these early tests of thermonuclear weapons, follow the count down into the thermonuclear age of Ivy Mike with Joint Task Force 132 with this Department of Defense film produced at Lookout Mountain. (for more information on the hydrogen bomb see article on Mark-17 Broken Arrow on the history page of this site)

The Instabilities

Fusion power turned out to be a much harder nut to crack than the hydrogen bomb. The difficulty of taming fusion lead many scientists and observers to stating it was '20-30 years away, and always will be.' While an exploding primary from a nuclear weapon provides plenty of energy to ignite a fusion secondary, when we tried to do the same with more traditional means, we started to run into plasma instabilities. With Inertial Confinement (similar to a hydrogen bomb in theory, without the nuclear explosive driver), we primarily encountered the Rayleigh-Taylor instability (imagine the behavior a drop of ink into water), and with Magnetic confinement, many instabilities were found. Because of these, to date, no fusion reactor on earth has ever produced more power out than is input into a reactor. However many approaches exist and the Q (a measure of efficiency, Power Out/Power In) of our controlled fusion reactors has followed an exponential growth curve much like Moore's Law. Fusion power isn't easy to master, but with the right approach its potential will likely one day soon be unlocked.

The Approaches

The two leading schools of thought for producing power from a fusion reaction on earth are focused on Magnetic and Inertial Fusion. Magnetic confinement fusion (MCF), best represented by a series of experiments involving Magnetic Mirrors, Stellerators, and Tokamaks, attempts to confine a relatively dilute plasma in a magnetic bottle, providing heating and confining the fuel and reaction products (typically Alpha particles in the case of a DT reaction) to produce large amounts of energy over long time scales.

Inertial confinement fusion (ICF) seeks to do essentially the opposite, by using large pulsed power devices such as lasers or imploding wire arrays to compress a small quantity of fusion fuel (typically about as big as a peppercorn) to incredibly high densities as well as high temperatures in a small fraction of a second. In a facility such as NIF, laser energy is upconverted in wavelength and then patterned onto an object known as a Hohlraum. This Hohlraum in a way recreates the conditions of an exploding nuclear weapon, by converting the laser energy into x-rays. These x-rays provide the energy necessary to implode a coated fuel capsule to the densities and temperatures required for thermonuclear fusion. For this approach to be used commercially the facility would have to be repeatedly pulsed over long time scales to integrate significant amounts of energy. There do exist alternatives for standard inertial schemes such as Magnetized Liner Inertial Fusion (MagLIF) (for z-pinch confinement) and Fast Ignition (for laser confinement), that by providing a pulse of energy (laser, fast ions, etc.) into the ignition target prior to the main compression shot, the driver energy may be significantly reduced. Experiments such as these may represent the most accessible route to ignition with preexisting driver facilities.

An alternative approach to the two traditional schools of thought is Magnetized Target Fusion (MTF) which reduces the overall demand on the driver by compressing a plasma target instead of cold fuel. This approach is being investigated by many private groups including British Columbia's General Fusion. General Fusion hopes to ignite plasma using a combination of a pneumatically driven liquid metal liner and a plasma target formed by injectors producing plasma structures known as Compact Toroids. This approach, if it works, may just be the best bet we have of realizing the dream of fusion power on earth, in a way that is feasibly scalable and economically accessible.

Additional Reading

For more on Laser Inertial Confinement Fusion: Lawrence Livermore National Laboratory Laser and Photon Science

Experimental results from the MagLIF project at Sandia National Laboratories

Home page of the ITER international collaboration  

Dark Sun: The Making of the Hydrogen Bomb
by Richard Rhodes