Physics 464 Optics

Physics 464 – Optics

Laser Fusion – An Overview

Grant Bowen

Abstract

This report presents an overview of laser fusion. This topic is of interest on many levels, from theory to application. Much research is going on currently in material science, in laser technology, in deuterium-tritium fuel pellet production, and in fusion as a reliable energy source; all of which are pixels of the total image for laser fusion. Each of the sub-topics in the larger topic of laser fusion can stand alone as a very technical topic. My purpose in researching this topic has been to gain a basic, top level, understanding of the mechanics of laser fusion. As a result, the information that is presented is lacking of much technical detail, except where detail is required for clarity. For the most part, the reader will find a description of the processes involved in achieving laser fusion. It may be thought of as a “view from 50000 feet”. Basically, I have pondered the following: What is fusion? How does it occur? Why lasers in fusion research? How much energy is released in a fusion reaction? These, and other laser fusion related questions, will be discussed.

It is important to note that laser fusion has not yet been harnessed as a reliable energy source. Much of the research and development is very expensive to conduct. In the United States, the Naval Research Laboratory is managing a research and development program that has a number of participants in both the government and the private sector, and from a few universities. The Lawrence Livermore National Laboratory, one of the participants, is building a 1 billion dollar facility, known as the National Ignition Facility, where 192 lasers will be trained on a pea-sized target, in hopes of achieving laser fusion.

Introduction

There ought to be an inexpensive way to produce electrical power. After all, not all parts of the globe were created equal. Some areas abound in natural resources, necessary in producing power, while others wallow in abject resource-poverty. For example, the Columbia River basin in the Pacific Northwest is populated by a number of hydroelectric dams. The energy produced collectively by these dams provides power to half of North America. Not all areas of the world benefit from such a river system.

In contrast, deuterium, one of the components of the fuel of fusion, can be extracted from water, which is abundant in one form or another over the earth. A relatively small amount of deuterium, when appropriately reacted with tritium, the other component in the fuel, will yield a very, very large amount of energy. The Naval Research Laboratory likens the available energy supply of fusion to filling the Atlantic and Pacific oceans 300 times over with gasoline. This results from the fact that fully burning the deuterium available in one gallon of water (about 1/8 gram) in a fusion reaction is equivalent to burning 300 gallons of gasoline.[i]

Content

So what is fusion anyway?

Fusion is a reaction that takes place at the nuclear level, when two protons collide, and “fuse” together, forming a different element. When one talks about a single proton, he is talking about the hydrogen element. The two protons that are needed for the collision, and subsequent fusion, are isotopes of hydrogen – deuterium and tritium. These are called isotopes of hydrogen, because while the nucleus of each has a single proton[1] in it, the number of neutrons (the neutral part of the nucleus) differs in the two nuclei, and thus the two nuclei differ in mass. Deuterium has a single neutron attached to its proton, while tritium has a pair of neutrons in its nucleus. Incidentally, the names of these two isotopes serve as a pneumonic device for remembering the structure of their nucleus. The deu- prefix implies two, meaning two particles (a proton and a neutron), and the tri- prefix indicates that three particles are present in the nucleus. The fusion of the two protons yields an enormous amount of energy, and produces helium plus a “fast” neutron, as shown in the following figure.

Figure 1 – The fusion reaction of deuterium, and tritium

More on the Fuel of Fusion

As mentioned earlier, deuterium exists in water in small amounts. Tritium, on the other hand, does not exist in nature, as it has a half-life of 14 years. It is an unstable isotope of hydrogen, and so it has to be manufactured. This is accomplished by combining a lone neutron, called a fast neutron, with a nucleus of lithium-6. This reaction yields tritium and helium.

Overcoming Strong Protonic Repulsion

When two protons come into close proximity, their natural tendency is to push away from each other, because they both have a positive charge. In order for fusion to occur, this strong repulsive force must be overcome. To do this, two conditions must exist. First, the two nuclei must be moving very rapidly, so that the force associated with their momentum will exceed the force exerted due to repulsion. Second, the two protons must be confined in space, so that the likelihood of collision will increase. As one might imagine, protons will move very rapidly when very high temperatures are present. Hence, very high temperatures, on the order of tens of millions of degrees, are required. Lasers are useful in achieving both conditions, as will be described later.

At these extreme temperatures, (50 million degrees, according to the Naval Research Laboratory) atoms dissociate into a mixture of unbounded protons and electrons. This is known as the plasma phase of matter. In this state, protons are moving quickly enough to potentially collide.

Inertial Confinement Fusion (ICF)

The fuel pellets, or targets, to be used in laser fusion are made up of a small, hollow sphere, about the size of a pea (4 mm in diameter).

Figure 2 – Fuel Pellet: Implosion stage, compression stage, and burn stage respectively

Possibly the most significant fact about the fuel pellet is that is maintained at a very cold temperature initially (18 Kelvin, according to NRL). This is because the deuterium-tritium layer of the shell (DT hereafter) needs to be frozen solid, so that the protons of the two isotopes will be in close proximity before the process starts. The DT layer surrounds a cavity containing DT gas, and an ablator surrounds it. An ablator is a material that, when heated, accelerates away from the heat. In this case, the ablator is heated during a very short pulse by about 60 laser beams. For this configuration, that means that the ablator accelerates inward, and thus the shell implodes. When the shell has reached approximately 1/16 of its original diameter, and about 2000 times its original density, the conditions are just about right for the fuel to “burn”. Compared to the DT shell, the inner region of the compressed fuel pellet is much less dense, but also much hotter. This inner region is where the ignition originates, is called the “spark plug”. If the conditions in the spark plug are right, nuclear reactions there will cause the spark plug to continue heating itself, and the burn will propagate outward, reacting the DT layer, and yielding a lot of energy. The time it takes a sound wave to go from the center to the edge of the compressed fuel, approximately 10-11 seconds, is known as the inertial confinement time. The energy gain of the fuel pellet is 100. This means that after the energy requirements of operating the lasers, and manufacturing the pellets are considered, 100 units of energy will still be given off by the reaction.

It seems like a contradiction to say that the DT layer needs to be very cold, and yet a very high temperature is needed to ignite the reaction. UCLA researcher Chand Joshi explains that when the laser light contacts the fuel pellet, the resulting plasma layer, which begins the implosion process, shields the fuel pellet from receiving any more light.[ii] This impedes the lights ability to directly contribute its energy to the implosion momentum. The light that is reflected by the plasma cloud creates waves in the plasma, causing electrons to be shot into the core of the fuel pellet. The fuel pellet then heats, and expands, before it has had a chance to fully compress. As a result, the correct conditions for fusion are not achieved. Joshi believes, “The trick is to keep the target cold, gently pushing on it until it reaches hundreds of times solid density. Then . . . boom!” The key point to remember is that the central region of the fuel pellet becomes very hot, having been compressed by the implosion process. One of the problems researchers are addressing is how to compress the pellet enough, so that the spark plug becomes dense enough, and hot enough to cause a fusion reaction.

From Fusion to Power

and major renovation is not required. Another important aspect of the design of the reactor is that the target factory, and the laser house, are physically separate from the reactor chamber. This means that work there can go on with out interrupting the operation of the reactor.

Some Notes About the Laser

The ICF approach to fusion could be achieved with lasers, ion beams, or x-rays. NRL uses a krypton-fluoride laser, which has a wavelength of 0.25 microns, and a bandwidth of 3 Terahertz. The shorter wavelength is needed for absorption into the pellet at higher densities. In addition, the beam must be very smooth, so that the beam itself does not cause any reflections. A uniform beam is a smooth beam. Uniformity is achieved as shown in the next figure.

Figure 4 – Optically Smoothed Laser Beams

Laser light is shone into the cavity of a diffuser, where the light scatters, and becomes uniform. The light is then emitted from the diffuser, bends through the pinhole aperture, and is bent by the optics, through the laser amplifier, and it focuses on the target. If the beam is not very smooth, if the intensity is very non-uniform, then the symmetry of the implosion will be lost, and fusion will fail.

The energy of the KrF laser is about 1.5 megajoules. This is about what is required to heat a gallon of water from 0 degrees C to 100 degrees C. The light duration is only 4 ns. Due to the very short duration of the laser pulse, the power is extremely high, about 4 x 1014 Watts. Keep in mind that this is the peak power, and that the duration is very short. Still, it is 500 times higher than the output of the entire U.S. electric grid.

A Promising Resource

All energy resources have finite availability. For the United States, coal produces more than half of all electricity that is produced. The availability of other resources, such as hydro, wind, and solar, may be subject to conditions in the climate. The resources that depend on deposits in the earth, like coal, and oil, may last for hundreds, or thousands of years. In contrast, fusion energy resources are used up very slowly, and are very abundant. Fusion energy resources may last millions of years.

Conclusion

Before undertaking the research for this report, I knew only that laser fusion had something to do with lasers causing a nuclear reaction. Through the course of my investigation, I have learned that laser fusion may become a reliable energy source in the not too distant future. If achieved, it would prove to be an invaluable resource, as it would be capable of providing power for the remotest of regions on the earth. Much still needs to be accomplished, in the areas of research and development, before this resource can be realized. Fortunately, this particular area of research is one that is a priority in the scientific world, and is receiving ample funding. The key areas of research for this technology are in the lasers used in the process, and in the manufacture of the fuel pellets, and, of course, in the process itself. The development of this topic will be interesting to follow for years to come.

References

[1] All chemical elements are identified by the number of protons present in the nucleus. Hydrogen has one proton; helium has two protons, etc.

[i] “Laser Fusion Energy”. Naval Research Laboratory, US Navy.

[ii] “Laser Fusion”. UCLA.