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Topic: A fusion reactor, a doughnut, and a hairy ball (Read 688 times) |
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rhinoceros
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A fusion reactor, a doughnut, and a hairy ball
« on: 2005-07-08 12:27:59 » |
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After almost 20 years of haggling, it was decided that the International Thermonuclear Experimental Reactor (ITER) will be built in France, while Japan will be supported in the development of a subsidiary fusion research facility.
International Fusion Reactor
http://www.technologyreview.com/articles/05/07/wo/wo_070705hutchinson.asp
<quote>
The nuclear reactions that release energy by combining light nuclei, like hydrogen, to form heavier nuclei, such as helium, are called fusion. They are, in a sense, the opposite of the nuclear fission reactions that power present-day nuclear plants; fission breaks up the nuclei of heavy elements such as uranium. Fusion has the potential to provide practically inexhaustible energy with greatly reduced levels of radioactive waste compared with fission.
To make fusion reactions take place requires the fuel to be heated to tremendously high temperatures (over 100 million degrees), so that it enters an electrically-conducting state beyond that of a gas. This state of matter is called plasma. The plasma must also be maintained long enough for the reactions to occur.
Fusion is the energy source that powers the sun and stars. In these natural fusion reactors, it is gravity that confines the plasma in a wonderfully stable and long-lived configuration. A human-scale fusion reactor must also use a non-material container, but to make the reactor small enough, it must use a much stronger force than gravity: the force of a magnetic field. ITER is to be a magnetic confinement device of the type called a tokamak, which has a toroidal (donut-shaped) configuration and a strong, confining magnetic field. The tokamak configuration has been under study by fusion plasma scientists since the 1960s, and has proven to have the best confinement of all the configurations so far envisioned.
Even so, the achievement of sufficiently good confinement of the plasma to permit useful release of energy has turned out to be far more difficult than the first fusion researchers hoped. Many important optimizations have been discovered and developed. One unavoidable way to obtain sufficient confinement is to make the plasma large. The existing large tokamak experiments typically have plasma radii of three meters. Fueled with the most reactive isotopes of hydrogen, those tokamaks demonstrated substantial release of fusion energy. For example, the world's largest tokamak, JET (Joint European Torus), obtained up to 16 megawatts of fusion reactions for just under a second. But, to sustain the plasma in these devices required additional heating that was larger.
The next big step in fusion development is to create a plasma that keeps itself hot by the energy released in its own fusion reactions. The ITER international collaboration has developed a design to sustain such a so-called "burning plasma," generating about 500 megawatts of fusion reactions for approximately 1,000 seconds. To achieve this requires a plasma about twice as large, and also requires the use of superconducting magnets that consume negligible electric power for their operation.
Because of its size and technological complexity, ITER will cost about $5 billion to construct. It is not a commercial power plant, nor even an engineering demonstration plant; it is an experiment that is designed to establish the scientific feasibility of controlled fusion. Sharing its significant cost is one motivation for pursuing it as an international collaboration. But other motivations include the long-term nature of fusion research. ITER will take about 10 years to build. A road map recently developed in the United States for a relatively fast-track to fusion envisages over 30 years before fusion would be sufficiently developed for commercial deployment. Therefore, fusion is a technological grand challenge that is not dominated by short-term economic competition, and is ideal for international cooperation. Indeed, ITER will be one of the largest joint-scientific projects ever undertaken by an international consortium. The present ITER partners are the European Community, Japan, the United States, Russia, China, and South Korea.
<snip>
Tokamak
http://en.wikipedia.org/wiki/Tokamak
A tokamak is a toroidal (doughnut-shaped) magnetic plasma confinement device, the leading candidate for producing magnetic fusion energy.
<snip>
Why doughnut shaped?
The distinctive shape of the fusion reactor is necessary because of a particular property of a doughnut that a sphere (for example) does not have. Essentially the problem is the hairy ball theorem: if a sphere has hair growing out of it then it is impossible to comb it so that no hair sticks up. However a hairy doughnut can be so combed. This is important because a fusion reactor is a hairy doughnut with the hair being the magnetic field lines. A strand of hair that is standing on end would be an instability in the reactor.
<snip>
Hairy Ball Theorem
http://en.wikipedia.org/wiki/Hairy_ball_theorem
The hairy ball theorem of algebraic topology states that, in layman's terms, "one cannot comb the hair on a ball in a smooth manner".
This fact is immediately convincing to most people, even though they might not recognize the more formal statement of the theorem, that there is no nonvanishing continuous tangent vector field on the sphere. Less briefly, if f is a continuous function that assigns a vector in R3 to every point p on a sphere, and for all p the vector f(p) is a tangent direction to the sphere at p, then there is at least one p such that f(p) = 0.
<snip>
One surprising consequence of the hairy ball theorem: The Earth is approximately a ball, and at each point on the surface, wind has a direction. It follows from the theorem that there is always a place where the air is perfectly still.
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