The Difficulty of Fusion
There are many reasons that the development of fusion has been slow… IT'S HARD! According to the plasma physicist, Stewart Prager, in the preface of Fusion: The Energy of the Universe, "A reader of this book will be rewarded by an understanding of why fusion is one of the most challenging scientific endeavors undertaken, how plasmas are rich with fascinating phenomena, how scientists across the world have met the challenges of fusion, and the road that lies ahead."(14) Like many of the scientists who work on fusion, I want to believe that we can solve the issues to make it viable. I think that if it were to become a national and international priority, along with the commitment of the requisite resources, that like the Apollo missions were in the 50's and 60's we would undoubtedly be able to harness fusion's potential. Although fusion was much more challenging than expected, I am hopeful that, as the great Russian physicist Lev Andreevich Artsimovich wrote in the 1970's, "nevertheless, 'thermonuclear [fusion] energy will be ready when mankind needs it.'"(15) I sure hope that he is right!
Problems and Successes with Fusion Reaction
One of the questions asked is why it is so difficult to achieve fusion. After all, CERN is able to accelerate protons and antiprotons to energies of 7 TeV. Isn't this enough energy? It turns out that it is not just an issue of energy there is also the issue of what enables a fusion event. Only 1 collision in 100 million results in fusion. So why is this? It has to do with the very small cross-section of the 'fusion hole' and the very steep 'hill.' Most of the accelerated nuclei bounce off the 'hill' and never get close enough to the target nucleus to fuse. The energy that has been invested in accelerating them is lost… the problem is not so much the number of lost balls but the amount of energy that is lost with them."(16) The analogy is made with the golf course with the hole at the top of a very steep hill. The chances of making it are 1 in 100,000,000… not good… so we have to consider another method. Another method is to put the "balls" on the analogy of a billiard table. Now once confined, they can continue to bounce around without losing energy. The likelihood of a successful collision is still 1 in 100,000,000 but energy is not lost. This is an analogy for magnetic confinement.
As indicated, fusion requires incredibly high temperatures that even exceed those of the sun. There are no materials that can contain this hot fuel. However, there are at least two distinct approaches to the problem. The first is called Magnetic Confinement Fusion (MCF), which we have already discussed, and the second is Inertial Confinement Fusion (ICF).
Inertial Confinement Fusion
Inertial Confinement Fusion solves the problem in a completely different way. Although the triple point conditions, T ixnxΤ E, must still be met, the method is to attempt to compress and heat the fusion fuel so quickly that there is no need to have a containment vessel. The fusion reaction will occur before the fuel even has a chance to expand. The amount of energy is tremendous, and the energy is supplied by hundreds of powerful lasers on a small, hollow, millimeter sized ball of fusion fuel, made of deuterium and tritium. The lasers must be incredibly precisely aimed and symmetrical as well as pulsed for optimal effect. Currently the approach is to attempt a single event. In order to provide the energy for a reactor, the process would have to occur approximately 10 times per second or 100,000 times per day. Significant progress is being made on ICF, but the lasers have to be amazingly powerful and precise.
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