Green energy sources such as wind turbines, hydroelectric power and solar power have proved they can provide us with some of the energy we need, but they will never be able to fully replace fossil fuels. Nuclear fission power plants are an option but the waste from these is harmful and hard to dispose of and there is always the possibility of another Chernobyl.
This leaves us with one other possible option, nuclear fusion, an energy source that has long been touted as the potential saviour of mankind but until recently has shown little experimental progress. This year however the National Ignition Facility (NIF), a laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in California has presented some very promising results that could finally pave the way to a commercial fusion reactor. The Inspec Database covers many aspects of nuclear fusion.
- a2588 Fusion reactions
Nuclear fusion is the process which powers the sun and other stars.
- a9530C Elementary particle and nuclear physics processes in astrophysics.
In fusion reactions, two light atomic nuclei fuse together to form a heavier nucleus whose mass is lower than the sum of the masses of the light nuclei. A large amount of energy is released as the missing mass is converted to energy according to the famous E=mc2 equation. It is this energy which we can use as a source of power.
In the sun the primary fusion process is protons forming alpha particles, huge pressure allows this reaction to occur at a relatively low temperature of 10 million Celsius. At the much lower pressure that is possible on Earth, temperatures to produce fusion need to be much higher - above 100 million Celsius. However we are not constrained to fusing protons together; instead on earth, the most promising fusion process is that between deuterium and tritium (which are heavy isotopes of hydrogen).
There are two main branches of fusion energy research currently being tested, magnetic confinement fusion using doughnut-shaped Tokamaks and inertial confinement fusion which is the basis of NIF.
- a5255G Plasma in the torus (stellarator, Tokamak, etc)
- a5255M Nonmagnetic plasma confinement systems (e.g. electrostatic, inertial, high frequency and laser confinement, etc.)
- a5255P Confinement in fusion experiments
The magnetic approach is more highly developed and is usually considered more promising for energy production as the plasma can be confined for longer and it operates in a steady state as opposed to the inherently pulsed operation of inertial confinement. As the name suggests the magnetic approach uses the electrical conductivity of the plasma to contain it within magnetic fields. The plasma is heated by running a current through the plasma (ohmic heating), injecting it with high energy beams of neutral atoms (neutral beam injection) or using high-frequency electromagnetic waves (radio frequency heating).
- a5250G Plasma Heating
Currently, no magnetic confinement fusion experiment has produced more energy than has been put into it. The Joint European Torus (JET) holds the world record for peak fusion power of 16 MW which equates 0.7 of the input power. An experiment called the International Thermonuclear Experimental Reactor (ITER) currently being built in the south of France will aim to produce ten times as much power as is put into it. The magnetic approach may, however, find itself overtaken by developments at the NIF.
In inertial confinement fusion at the NIF the deuterium and tritium form a pellet which is placed inside a centimetre scale cylinder of gold called a hohlraum. Lasers are used to deliver energy onto the target hohlraum walls; the hohlraum then re-emits the energy as intense X-rays, which are more evenly distributed and symmetrical than the original laser beams. The X-rays strip material from the outer shell of the fuel pellet collapsing it, heating it up to millions of degrees and producing a plasma in the process.
- a5250J Plasma production and heating by laser beams
During the collapse of the fuel, shock waves form and travel into the centre of the fuel at high speed. If these shock waves are powerful enough they can compress and heat the fuel at the centre so much that fusion reactions occur.
- a5235T Shock waves in plasma
These reactions release further energy which heats the surrounding fuel which will also the undergo fusion. Ignition occurs when this heating process causes a chain reaction that burns a significant portion of the fuel. This ignition point is crucial if fusion power is to ever become practical as it is the point at which more energy can be produced from the machine than we initially have to put into it.
- a2852C Fusion reactor ignition
A possible problem with the NIF method has been that the plasma would interrupt the hohlraum target's ability to absorb the lasers' energy causing problems with funnelling it uniformly into the fuel; this would impact negatively on the chances of ignition. Recent experiments performed at NIF have shown that this problem does not exist as the hohlraum absorbed 95% of the incident laser light. Indeed during the experiments, it was shown that the laser plasma interactions could be used to manipulate the plasma to increase the uniformity of the compression helping with the ignition. In performing the experiments the record for the highest energy produced by a laser was broken with a value of 1 megajoule being delivered to the target.
Calculations show that an energy of 1.2 megajoules is required for ignition, this is well within the capacity of the NIF which can run up to 1.8 megajoules. Experiments so far have used a target without fuel present, researchers are hopeful that within this year experiments using slightly larger hohlraums with fusion-ready fuel pellets will take place and ignition will be achieved. If these experiments are successful the NIF will have taken a major step forward in the race to produce a working nuclear fusion power plant.
- a2852J Fusion reactor theory and design
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