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When the nuclei of light elements such as Hydrogen fuse together to produce heavier elements such as Helium, large amounts of energy are released. It is this process of nuclear fusion which powers the stars. An internationally coordinated research effort is seeking to harness fusion as a universal source of energy on Earth. When successful, we will have an inexhaustible and environmentally friendly source of energy. By producing electricity, hydrogen and other transportable forms of energy, fusion can provide all sectors of our society with clean fuel. |
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| When nuclei fuse together and form heavier elements it is the nuclear strong force which pull them together. This force is only effective at very short distances. So for fusion to take place the nuclei must be brought very close together. To do this, the electrostatic force which repels the nuclei from each other, must be overcome. |
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This is achieved if the two nuclei approach each other with sufficient speed and they are heading straight for each other. One might visualize this as a head on collision. Head on collisions take place all the time in any gas due to the random thermal motion. As we increase the temperature of the gas the average speed of the gas molecules increases and the collisions become harder. To make the collisions sufficiently hard that the nuclei come so close that the nuclear strong force becomes effective and fusion takes place, we need temperatures in the range of 100-200 million degrees C. At these temperatures the electrons have been knocked off the atoms by collisions, so we have a gas of ions and electrons. This is a plasma, the fourth state of matter. Plasmas at these temperatures are currently reached in fusion plasma experimental devices such as JET.
| When the plasma density increases, the frequency of useful head on collisions also increase and hence the rate of fusion reactions. To have useful reaction rates for a power-plant the plasma density should be in the range of 2-3´1020 ions per cubic metre (much lower than the density of molecules in the atmosphere). Plasma densities in this range have also regularly been achieved in experimental fusion plasmas. |
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To maintain the high temperature of the plasma with the energy deposited in the plasma by the fusion reactions and a modest level of re-injection of power from outside the plasma, quanta of energy put into the plasma should stay there for 2-3 seconds in a commercial reactor. We call this time the confinement time. In current fusion plasmas we reach confinement times of approximately 1 second. We do not simultaneously reach the record values for temperature, density and confinement time. A useful measure of performance is the Lawson triple product of temperature, density and confinement time, for which we currently reach 1 atmosphere ´ seconds. (Product of density and temperature can be measured in units of pressure.) For a power-plant we must reach 6 atmosphere ´ seconds. So to put things simply, we are a factor of 6 away from the target. To put this in perspective, the progress over the past 30 years of fusion research has seen the Lawson triple product increase by more than ten-thousand. The final step towards reaching the plasma conditions required for fusion power production is expected to be reached with the ITER facility, which has been designed and currently awaits the political decision to go ahead. |