This is a loose rendition of a speech given by the late Dr. Bussard of EMC2 at Google on November 9, 2006. The talk covered the history of the IEC fusor at EMC2, why Tokamak is not the answer and a brief history of previous fusion ideas and experiments and how they contributed to the IEC concept.
We are bombarded with information about alternative energy sources. Why don't we hear more about fusion? I don't have the answer but I do know why fusion is important. If we burn hydrogen and oxygen we obtain water and energy. The energy released for each water molecule is about 10 eV. The energy obtained fusing a deuterium and tritium atom is 17.6 MeV. Almost 2 million times more. That's because in chemical reactions the heat generated or absorbed is due to removed or added chemical bonds while in a nuclear reaction some mass converts to energy conform Einstein's E=mc2 equation.
Mass to energy conversion is efficient but hard to control and potentially dangerous. All stars, our sun included, as nuclear reactors and nuclear and thermonuclear weapons use mass to energy conversion. Mass to energy conversions happen in two types of nuclear interactions: fission and fusion.
Fission generates radioactive materials and radiation besides energy (Remember Three miles island, Chernobyl and any detonated nuclear or thermonuclear bomb).
Fusion can be "hot", "cold" or "generally cold, locally hot". "Hot" fusion examples are the stars (such as our sun) or thermonuclear explosions (not to be mistaken with nuclear explosions, Hiroshima and Nagasaki were nuclear explosions, the Bikini atoll experiment was a thermonuclear explosion).
"Cold" fusion examples: muon-catalyzed fusion and electrodes (usually palladium) in heavy water. The first one works but is not efficient, the second one has been tried but up to date there are no clear results. Most experiments have reported intermittent production of excess energy that cannot be reproduced on demand.
"Generally cold, locally hot fusion" is the type of fusion we will concentrate from now on. I will use the term "fusor" for all devices designed to achieve this kind of fusion and GCLH for "Generally cold, locally hot" fusion.
To achieve fusion, 2 nuclei have to be brought close enough so that the strong nuclear force is larger than the electrostatic repulsion allowing the 2 nuclei to fuse into one and release energy. Overcoming the electrostatic repulsion requires an amount of energy higher than the energy barrier also named the Coulomb barrier which is specific for each fuel mixture. Passing the Coulomb barrier requires kinetic energy. It can happen three ways: One moving nucleus hits a nucleus at rest, Beam-Target fusion. Two moving nuclei collide, Beam-Beam fusion. Both nuclei are moving as part of a plasma near thermal equilibrium, Thermonuclear fusion.
The Lawson criterion. It was first derived by John D. Lawson and it evaluates the product of density, confinement time and plasma temperature vs. a minimum required to achieve sustained fusion (ignition).
Confinement. To achieve and sustain fusion, we need to keep the fuel nuclei in a limited region of space at a sufficiently "hot" temperature and for enough time for them to fuse. Confinement can be gravitational (the gravitational field of a star would be needed), magnetic (charged ions follow helical path along field lines, Tokamak, Stellarator and mirror confinement) and inertial (H-bomb, laser, ion, beam, electron beam and Z-pinch).
The best confinement, found in stars, is gravitational confinement because the action of the field upon nuclei always points towards the center of the sphere of matter.
Magnetic confinement. From the Stellarator to Tokamak and then to the Spherical Tokamak (ST) huge advances were made in controlling the D-T fusion. The same problems remain: plasma losses due to holes in the containing magnetic field, size (it takes hundreds or thousands of nuclei rotations along the field lines to achieve fusion and their orbit grows with each revolution so the reactor has to be big), tritium is made by absorbing the generated neutrons into a melted lithium pool (sitting next to super cooled electromagnets) and no sun is toroidal
.
Inertial confinement. Rapidly raise the temperature and pressure of a small fuel pellet to induce partial fusion. To rapidly raise the temperature and pressure of the pellet several methods are used: x-rays (thermonuclear bomb), laser (shiva and nova at Lawrence-Livermore), ions, electron beam, z-pinch, and controlled explosions.
Stars generate a lot of energy because they're big but if we look at the rate of energy generated per volume, the human body generates 4 times more energy. Obviously stellar fusion reactions are not useful to us. The table below lists some of the fusion reactions that are worth considering and offers an easy way to judge their advantages and disadvantages:
Examples of fusion reactions, their energy and neutron output
Neutrons output Reaction formula Total fusion energy Non neutron energy (W/m3/kPa2) Neutronicity Power density Relative to D/T
High neutron output 21D + 31T -> 42He + n0 17.6 MeV 3.5 MeV .80 34 1
Low neutron output 21D + 21D -> 31T 12.5 MeV 4.2 MeV .66 .43 .013
Neutron free output p+ + 115B -> 342He 8.7 MeV 8.7 MeV .001 .014 .00041
To be continued and revised...09/24/2008
Copyright © Constantin Popa 2008 All rights reserved
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