Fusion: An Alternative Energy Source

What is Fusion?

Fusion is a process that combines the atomic nuclei of light elements, like the isotopes of hydrogen called deuterium and tritium, to form heavier elements. It is the same process by which the sun creates energy. In the process of fusing these elements, energy is released which can be captured and used to generate heat and eventually electricity.

The fusion equation, converting deuterium and tritium to helium and energy.

Fusion fuel is abundant and inexpensive; it is essentially sea water. Each gallon of water has an eighth of a gram of deuterium – an isotope of hydrogen with one neutron – which if burned in a fusion reactor would produce as much energy as 300 gallons of gasoline. In addition to using abundant water as it's fuel, fusion is a clean energy source. Because fusion does not "burn" fuel as normal electric generation plants, there is no pollution. Fusion is much safer than nuclear fission, which splits heavy elements like uranium into smaller atoms. Although fusion does produce a small amount of radioactivity, the radioactive byproducts are short lived, and decay into harmless substances in a few months. Moreover, a fusion reactor has very little fuel in the reactor at any one time. There is not way for a fusion generation plant to go supercritical or "meltdown" like a fission plant.

Fusion Research

To achieve nuclear fusion, a plasma must be heated to extremely high temperatures Ð at least one hundred million (100,000,000) degrees Celsius. The nuclei of atoms have electric charge and naturally repel one another. Thus, they can be made to collide and fuse only when moving very rapidly. In addition to heating the plasma, one must confine the hot plasma. One way to do this is with a magnetic field. The purpose of the magnetic field is not so much to prevent the plasma from melting the walls of the container; the heat capacity of fuel is too small for this. Rather, the plasma has to be prevented from touching the walls, lest it cool down and stop reacting. Several methods of confining a plasma are discussed below.
Unfortunately, although the sun and stars are fusion reactors, the production of useful controlled fusion on earth has not yet been achieved, despite forty years of world-wide effort. The problem is that we are not able to confine the plasma long enough for appreciable fusion to occur. Recently an experiment in Europe, the Joint European Tokamak (JET), produced a record 1 megawatt of fusion power. There are many such fusion experiments around the world. Future experiments will need to produce more than 200 megawatts of fusion power in order to make fusion a viable alternative to present energy sources.
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Fusion Confinement Schemes

The most common way to confine the hot plasma used to make fusion energy is to use strong magnetic fields. The plasma can't be confined by the material walls, because the plasma is millions of degrees Celsius. (Actually, the problem is the reverse: the vessel walls are so cold, that the cool the plasma and prevent fusion.) Physicists have been working since the 1950s on making better "magnetic bottles," a problem which has been compared to holding jello (the plasma) with rubber bands (the magnetic field). The magnetic field acts like a net to the charged particles of the plasma, preventing the plasma from leaking out. For esoteric reasons, one can't make the magnetic field into a ball Ð which would be the best way to trap the plasma Ð but you can make the magnetic field into a donut shape called a torus. The magnetic "net" traps the plasma better if the magnetic field spirals around the torus, rather than being straight.

The helical magnetic field of a fusion reactor traps the plasma and prevents it from contacting the walls of the container.

Stellarator

Drawing of the LHD stellarator showing the two twisted magnetic field coils.

Traditional stellarators consist of a set of continuous helical, or "slinky-like" coils that create the twisted magnetic field needed to confine a plasma. The Compact Toroidal Hybrid (CTH), currently under construction at Auburn University, is an example of a traditional, helically wound stellarator. The Large Helical Device (LHD) is the largest such stellarator, and is located in Japan. One can also make modular magnetic coils the mimic the helical field, but without the problem of making the continuous winding. TheW7-AS stellarator is an example of this type, as is HSX at the University of Wisconsin.

The advantage of this type of confinement scheme is that the magnetic field can be created independently of the plasma. Thus, scientists can study the confinement properties of the magnetic field with and without plasma. Moreover, because the plasma does not influence the magnetic field (much) this method of trapping the plasma is much more stable, compared to the tokamak. However, the disadvantage is that despite its stability, it does not confine the plasma as well. The problem is the "bumps" - called helical ripple - created by the helical field coils. The Quatos experiment was a unique stellarator design that "fools" the plasma into thinking there is no helical ripple.

Tokamak

Outside and Inside views of the JET tokamak.

A tokamak is a toroidal plasma confinement device invented in the 1950s by the Russians Tamm and Sakharov (yes the famous one). The word "tokamak" is a contraction of the Russian words: "toroidalnaya", "kamera", and "magnitnaya", meaning "toroidal chamber-magnetic." In contrast to the stellarator, the magnetic fields in a tokamak are produced by a combination of currents flowing in external coils and currents flowing within the plasma itself. The tokamak concept is the most successful fusion confinement scheme to date. There are several major tokamak facilities throughout the world, the largest being the Joint European Tokamak (JET) in England. Others are DIIID in San Diego, Alcator C-Mod (with which the University of New Mexico has a strong collaboration), and JT-60 in Japan. Although the tokamak has excellent confinement properties, it has the disadvantage that the current in the plasma tends to make it unstable. This can lead to a "disruption", where the plasma dramatically crashes against the inside of the container.

Alternate Concepts

There are several "alternative" magnetic confinement schemes - meaning it's not a stellarator of tokamak. One, the Reversed Field Pinch is similar to a tokamak with a lot more current in the plasma, which creates a stronger poloidal magnetic field. The Madison Symmetric Torus is one such device. The Spheromak is another idea, this one again somewhat similar to the reversed field pinch, but in a spherical containment vessel, rather than a torus. There is also the Field Reversed Configuration (FRC), like FRX-L at Los Alamos National Lab. An different take on the "pinch" idea is the z-pinch, like Z at Sandia National Lab, where a strong current through a wire creates its own magnetic field to compress and heat the plasma. At UNM P&FS Center we have an both FRX-L collaboration and Z collaboration.

Inertial Confinement

Inertial Confinement is a fancy name for smashing a hydrogen pellet with a big hammer. Actually, rather than using a hammer, many very strong lasers are shot at a frozen hydrogen pellet which causes it to smash together, creating densities higher that at the core of the sun. No magnetic fields are needed in this method. The big problem is getting the lasers to hit the pellet just right so that it compresses symmetrically, rather that squirting out one side.
Most of this research, until very recently, was highly classified. It was thought to have relevance for nuclear weapons. However, now much of it is open to the public. The largest facility is the National Ignition Facility (NIF) at Lawrence Livermore Nation Lab, and another at the University of Rochester.

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