Nuclear Fusion Explained

The nuclear fusion of atoms has been a process well developed by the Universe. Indeed, it is the process by which stars burn and release their vast amounts of energy. It is the reason that you and I are able to exist. We owe a lot to this fundamental process. Perhaps one could even think of it as a testament to the greatness of our Universe that, in this day and age of technology, it is a problem scientists are yet to crack on Earth. 

On paper, it seems simple. Two atomic nuclei must fuse together to become one and in the process release a vast amount of energy. In practice however, there are an endless amount of problems to overcome. For example, the problem can be likened to pushing the same pole of two magnets together. In atomic physics, this is known as the Lorentz force. It is a force which pushed two like charges away from each other. Now, the stronger the charge of each atom involved in the process, the stronger the Lorentz force. This is the reason scientists like to use smaller atoms. Each nucleus is made of protons and neutrons with electrons orbiting this nucleus. Therefore, choosing a smaller atom will mean fewer protons (with each proton carrying one unit of charge).

However, it is not just a simple case of choosing the smallest particle available, hydrogen. One must also consider the probability that these particles are going to collide. The smaller the particle, the less likely a collision. Therefore, a compromise is made. In fact, a rather good compromise is made. Scientists choose what is known as an isotope of hydrogen. An isotope is a particle which contains the same number of protons, with a different number of neutrons. Therefore, the charge stays the same, but the size of the particle may increase. In practice, deuterium and tritium atoms are used. These have a nucleus consisting of one proton with one and two neutrons respectively. 

Indeed, the electrons also carry a negative charge, opposite to that of the proton. These will also cause the particles to repel each other. Scientists overcome this problem by heating up the particles to a certain temperature whereby the electrons are energetic enough to leave their constraining orbits. This state of matter is known as plasma.  The bare nuclei must also have enough energy left to overcome the repulsive Lorentz force and fuse with another bare nuclei. To enable this process, the particles must be heated to a temperature ten times hotter than the temperature at the core of the sun. Not an easy feat. 

Currently, these processes are carried out in devices known as Tokamaks. Typically, these have reaction chambers in the shape of a torus (the shape of a doughnut). However, there is a considerable amount of effort being put into looking at spherical Tokamaks which have reaction chambers in the shape of a sphere. They work by heating the particles to form a plasma using electrical currents, microwaves and neutral beams. Neutral beams are very advantageous as the light produced from the neutrals colliding with the plasma can be used to diagnose many crucial parameters.

To keep the particles at this temperature, they must be stopped from colliding with the vessel walls.  Huge magnetic fields are put in place which confine the plasma. However, unfortunately, these are not perfectly efficient. Anomolous transport of particles is still not fully understood, and moves particles from areas of good confinement onto the plasma walls. This serves to reduce the temperature of the plasma, and also knocks heavy particles off of the wall and into the plasma. This can be thought of like pouring soot onto a coal fire. It suffocates the fire. 

The ultimate goal of fusion is to obtain ignition. This means that the plasma becomes a self-sustaining process and more energy is extracted than put in. To obtain ignition, one must abide by the Lawson criterion. This states how long a certain amount of density and temperature the particles must be confined for. There are two new machines, known as ITER and DEMO, currently in the process of being built which will ultimately hope to show this ignition occurring.  

However, science is never complete without competition. These magnetically confined Tokamaks are now not the only way scientists are looking at producing fusion power. A huge amount of effort is being put into Inertial Confinement fusion devices. This involves focusing a large number of high power lasers onto one fuel source. However, this topic, is for another article.