In my previous post, I mentioned the various problems facing fusion scientists and engineers today. If you have not read it yet, I suggest doing so. It is an introductory to nuclear fusion, and will clarify some of the topics discussed in this post.
In this post, I will present to you, the various aspects of modern fusion reactors, as well as how these designs solve some of the aforementioned problems in my previous post. There are two common methods of creating controlled fusion reactions that I will discuss: Magnetic confinement fusion, and Inertial confinement fusion.
Magnetic confinement rests upon the property that charged particles, like those in a plasma, will travel along the lines of a magnetic field because objects with a charge are subject to electromagnetism. By arranging magnetic fields in the right way, scientists have been able to trap the plasma within the fields. While the plasma is held, it can be heated with a combination of multiple methods. The plasma can be heated by passing a current through the plasma. It is called ohmic or resistive heating; the heat generated depends on the resistance between the plasma and current. However, as temperature rises, resistance drops, making this form of heating less and less effective. Other methods are neccessary in addition in order to heat the plasma to required temperatures. Another method of heating is neutral-beam injection where high energy, neutral atoms are shot into the plasma, and are immediately ionized. These ions then get trapped by the magnetic fields, and transfer some of their energy to the surrounding plasma particles through collisions, thus raising the overall temperature. The plasma can also be heated through a rapid compression. This is called the magnetic compression method, which is possible by increasing the magnetic field. In the tokamak, this compression occurs by moving the plasma to an area of a higher magnetic field. Radiofrequency heating is another option, where high-frequency waves are launched into the plasma through the use of oscillators. If the waves have the right wavelength, their energy can be transferred into certain particles, which then transfer the energy through collisions with others.
Currently there are two types of magnetic confinement systems: the mirror (open) and the toroidal (closed). The mirror and the toroidal method are both combinations of what you see in the above picture. The mirror method, is where the superconducting field magnets are arranged in a line with both ends open, and the plasma can be reflected back and forth by magnets on both sides (above). Something similar to this, is shown in the picture below. It is a linear accelerator, and is more suited to future spacecraft where the plasma can be ejected out one end as propellant.
The other type of magnetic confinement device is called the tokamak, a word formed from the Russian words for Toroidal Chamber and Magnetic Coil. Tokamaks were originally designed and used in Russia. In this design, the chamber is toroidal, or doughnut-shaped, thus having no open ends. The magnetic field is generated through the current running in the solenoid coils that are wrapped around the reactor. The field is stronger towards the center, causing the plasma to tend towards the outer wall. However, another magnetic field generated by a current going through the plasma itself combines with the coils’ magnetic field to create magnetic lines that spiral around the torus. This spiralling counteracts the drifting effect on the plasma because of the strong inner field, and effectively traps the plasma.
In this method, increasing the magnetic feild also increases the density of the plasma, and thus increases the amount of collisions and fusions that occur. All fusion reactors attempt to meet the Lawson criterion, which varies for different types of fusion, and states the overall conditions which must be met for a yield of more energy than is required for the heating of the plasma. These conditions are usually stated in terms of the product of ion density and confinement time.
Inertial confinement is another method of plasma confinement. This technique involves imploding a small fuel pellet. If it is compressed quickly and hard enough, temperature and density rise, allowing the reaction to reach or exceed the Lawson criterion. It is the inertia of the imploding pellet that keeps it confined momentarily, and because it is confined only by its own inertia, the plasma lasts for about one nanosecond. Therefore, to achieve breakeven point, a very large density is needed, usually around 1024 particles/cm3.
The fuel pellet, or target, is compressed and heated with what are called energy drivers. These high-powered sources of energy are usually either high-powered laser or ion beams, which bombard the target from all sides symmetrically. The outer layer of the pellet vaporizes and moves away from the pellet like a rocket. This projection creates shock waves which go on to compress and heat the core. The compressed fuel then burns, releasing much energy, and expands. This is partially offset by the shock waves, which tend to continue compressing the material. This behavior is known as inertia. The result is an inertal confinement fusion reaction.
There are two types of targets: a direct-drive inertial fusion energy target, and an indirect one. The direct-drive targets are just the spherical pellets containing the fuel which will be pounded directly by a laser or ion beam.
The indirect-drive targets have the fuel pellet placed inside a hohlraum, which is a small and thin cylindrical container composed of a high atomic number material, like gold or lead. The container will convert the driver beams into x-rays, which subsequently compresses the fuel.
A variation of the standard inertial confinement methods is the fast ignitor. The difference is that an extremely short and intense laser creates a hotspot in the center of the fuel which ignites the core. First, the pellet is compressed the standard way as explained above. Then, an ultra short and intense laser pulse punches a hole through the atmosphere left over from the compression after which an even smaller, intense laser pulse is shot down the newly formed channel, creating a hotspot on the dense fuel for ignition. The burn then spreads throughout the rest of the fuel, releasing large amounts of energy. The size and complexity of the primary compression laser system is reduced, and the amount of energy released to energy absorbed could also increase.
There are other, less conventional methods of achieving controlled plasma fusion, such as sonofusion, but so far, these two methods are the most widely used, as well as the most widely accepted possibilities for future plasma fusion reactors.