Inertial Electrostatic Confinement Fusion
Nuclear fusion is the process of two lighter atoms combining to form a heavier atom. In this process, large amounts of energy are released in accordance with Einstein’s famous equation: \(E = mc^2\). Unlike nuclear fission, fusion requires a large quantity of energy to initiate. The sun uses its gravity to fuse atoms together in its core, but on Earth we lack the technology required to recreate such extreme conditions. Instead, scientists must find other ways to initiate nuclear fusion. For amateur scientists, the easiest is a method called inertial electrostatic confinement or IEC for short. IEC fusion reactors use strong electric fields to heat atoms to fusion conditions. Temperatures in these reactors usually exceed several hundred million degrees Kelvin, which makes the reactor core the hottest place in our known universe according to scientists’ current understanding. You may have some questions such as “how is that possible?” or “is that even safe?” or “can this be done in my bedroom?” Don’t worry, all of these questions will soon be answered.
A standard IEC fusion reactor consists of two vacuum pumps, a mechanical vacuum pump and an oil diffusion pump, a gate valve, a main chamber, an inner conductor also called the grid, a power supply, a pressure gauge, and a lecture bottle of deuterium gas with a regulator and needle valve. First, the mechanical vacuum pump is turned on, which evacuates the entire system to a pressure of about one 100 thousandth of an atmosphere. For reference, one atmosphere is the air pressure at sea level. Then the oil diffusion pump is turned on, which takes almost all of the remaining air out of the vacuum system. Interestingly, diffusion pumps have no moving parts. They remove air by shooting scalding streams of oil at the air molecules. The oil then falls to the base of the diffusion pump where the mechanical pump can easily remove the trapped air from the system. After the diffusion pump is finished pumping down the chamber, the pressure is equivalent to around six trillionths of an atmosphere.
Then deuterium gas is leaked into the chamber until the pressure rises to about six millionths of an atmosphere. This ensures that the contents of the chamber is only deuterium. Now you may be wondering why deuterium gas is so important. Deuterium is the special name for hydrogen atoms that have one extra neutron in their nucleus. This makes deuterium atoms less stable than regular hydrogen atoms, which means that deuterium atoms can fuse with less energy. Maintaining a constant deuterium pressure is difficult, as both the deuterium leak rate into the chamber and the rate of gas flow out of the chamber must be the same. The needle valve controls the flow of deuterium into the chamber and the gate valve is able to throttle the diffusion pump, limiting its ability to remove gas from the system. After the main vacuum chamber contains a constant pressure of deuterium gas, the power supply can be turned on.
When the power supply is active, the grid becomes charged to a very high negative electric potential. The charged grid generates a strong electric field. This electric field is so strong, it is able to knock electrons off some of the deuterium atoms. The now free electrons go on to knock more electrons off other deuterium atoms. This process is called ionization and results in the generation of a deuterium plasma. Plasma is the fourth state of matter that consists of ions, charged particles such as atomic nuclei and electrons. A deuterium plasma is basically a mess of positively charged deuterium nuclei, also called deuterons, and free electrons. The positively charged deuterons are attracted to the negatively charged grid. The specific geometry of the grid causes the generated electric field to have weak spots. This can cause beams of plasma to shoot out from the grid’s interior. The deuterons accelerate to very high velocities following a Maxwell-Boltzmann velocity distribution. This means that only some of the deuterons have enough energy to undergo fusion. The grid is bombarded with deuterons, which causes it to glow white hot as the kinetic energy of the particles is transferred into heat energy. Some of the deuterons do not collide with the grid and have a small chance to fuse with each other.
However, there is something that is not adding up. If two deuterons are both positively charged, don’t they repel each other? The answer is yes. The coulomb repulsion force between them is actually so powerful, they would require an infinite amount of energy to meet. So then why does the sun shine? The answer lies in the strong nuclear force and quantum tunneling.
The specifics of strong force interactions between subatomic particles can get a little confusing, so here is a brief explanation of why the strong force is so important. Consider a helium atom. Its nucleus is composed of two protons and two neutrons. As I mentioned before, positively charged particles repel each other. The two protons in the helium nucleus want to get as far away from each other as possible, but for some reason they are attracted. This can be explained by the strong force. The strong force is incredibly powerful, but its range is very limited. For two protons to experience an attraction due to the strong force, they must be within just several protons' width of each other. However, once they are within that range, it becomes nearly impossible to split them apart. The distance at which the attractive strong force overpowers the repulsive Coulomb force is called the coulomb barrier. In order to overcome that energy barrier, two positively charged particles must approach each other with very high kinetic energies. If they are not moving fast enough, they will simply be repelled without experiencing any attraction due to the strong force.
In order to solidify this concept, consider a marble on a frictionless track. The track has two very steep hills and a valley between them. Your goal is to find a way to get the marble trapped in the valley. The only tool you have to do this is a spring launcher that can launch the marble at a specified speed. However, after several attempts you will realize that this task is impossible. If you give the marble enough energy so that it can go over the first hill, it will also have enough energy to continue along the path and then go over the second hill. The marble will have to lose energy while in the valley in order to become trapped. You can probably guess that I am not really talking about marbles. Think of the marble as a speeding deuteron and the bottom of the valley as deuteron that is fixed in place. The hills represent both the repulsive Coulomb force and the attractive strong force. As one deuteron approaches the other, the Coulomb force increases just like the slope of the hill. The peak of the hill represents the coulomb barrier. If a deuteron has enough energy to overcome the coulomb barrier, it accelerates towards the other deuteron due to strong force interactions. This is represented by the valley between the two hills. However, just like in the marble example, a deuteron that enters the attractive realm of the strong force can actually escape and pass the coulomb barrier on the other side. This means that in order to accomplish fusion, the deuterons have to perform a nearly perfect collision.
Now you may think that this solves the deuteron fusion problem, but for our specific case involving an IEC fusion reactor built in my bedroom, it does not. The kinetic energies of the deuterons are simply not high enough to breach the coulomb barrier. Now you may be wondering why I went through that long explanation of the coulomb barrier if it is not even the mechanism that allows fusion to occur. Well, we’ll get back to that in a moment, but right now I need to explain quantum tunneling.
I will start out by saying that quantum tunneling counters our classical physics based intuition. Subatomic particles such as electrons and atomic nuclei don’t have a defined position until they are observed. These particles’ probabilistic nature can be described by wave functions. The probability of a particle being observed at a specific location is dependent on the wave function of that particle. For example, let’s allow the wave function of the speeding deuteron in our previous example to be interpreted as a probability cloud. In this example, the probability cloud is essentially a mass of points, where each point represents a possible location of the deuteron after it has been observed. The points are denser near the center of this cloud as there is a high probability of finding the particle near that location. Going back to our marble on a track example, let’s apply the phenomenon of quantum tunneling.
Let’s say that the spring launcher gave the marble just enough energy to reach three quarters of the height of the first hill. At the three quarter mark, the marble changes direction and rolls back down the hill as is does not have enough energy to reach the peak. This is consistent with classical physics and does not account for quantum tunneling. However, if quantum tunneling is considered, this seemingly simple test can yield a different and surprising result. Let’s freeze the moment that the marble reaches the maximum height on the hill. If we replace the marble with a probability cloud, you will notice that there are points in the cloud that exist on the other side of the hill. This means that there is a chance that the marble may be found inside the valley if observed. When the marble tunnels through the barrier it does not gain or lose energy, so it cannot escape the valley as it lacks the required energy. The marble then oscillates between the two hills. It is important to point out that if the marble was given more initial energy and was able to climb higher on the hill, its probability cloud would have more of its points in the valley, which means that its probability of tunneling would be higher. But, marbles do not tunnel through barriers. I am really discussing the behavior of two deuterons. The marble tunneling through the hill is an analogy of a deuteron tunneling though the coulomb barrier. When a deuteron tunnels past the coulomb barrier, it becomes trapped by the energy well created by the attractive strong force. The two deuterons then oscillate around each other until one of them releases energy and they fuse together.
Consider two deuterons that are about to fuse. In order for the fusion to occur, energy must be released. In deuterium-deuterium fusion, there are two possible results that can occur. Either a neutron is ejected leaving a helium-3 atom, or a proton is ejected leaving a tritium atom. This ejection of mass allows one of the oscillating particles to lose energy, which leads to the synthesis of new atoms and the generation of energy.
IEC fusion is just one of the many methods that may lead to a fusion-powered future. One of the largest ongoing fusion projects, ITER, is located in France and has gained international support. ITER is a tokamak fusion reactor that uses strong magnetic fields to confine a fusion plasma in a torus-shaped vessel. ITER is being designed to produce around 500 megawatts of output power while only needing 50 megawatts to operate. While ITER will not provide continuous power to the surrounding area when it is turned on in the late 2020s, the project hopes to demonstrate that nuclear fusion is a viable source of energy and provide a platform for continued research and development of fusion systems. Once fusion power is realized, human society's increasing demand for energy may be satisfied. Fusion power produces no dangerous waste and does not pose environmental threats to surrounding areas. Fusion is the energy of the future and has the capacity to push humanity to new breakthroughs in science and technology leading to prosperity in a society without suffering. I am optimistic that this future may be realized, but it will take a global initiative to become a reality.