Fusion ignition is the point at which a nuclear fusion reaction becomes self-sustaining. This occurs when the energy being given off by the fusion reactions heats the fuel mass more rapidly than various loss mechanisms cool it. At this point, the external energy needed to heat the fuel to fusion temperatures is no longer needed. As the rate of fusion varies with temperature, the point of ignition for any given machine is typically expressed as a temperature. Ignition should not be confused with breakeven, a similar concept that compares the total energy being given off to the energy being used to heat the fuel. The key difference is that breakeven ignores losses to the surroundings, which do not contribute to heating the fuel, and thus are not able to make the reaction self-sustaining. Breakeven is an important goal in the fusion energy field, but ignition is required for a practical energy producing design. In nature, stars reach ignition at temperatures similar to that of the Sun, around 15 million Kelvin. Stars are so large that the fusion products will almost always interact with the plasma before it can be lost to the environment at the outside of the star. In comparison, man-made reactors are far less dense and much smaller, allowing the fusion products to easily escape the fuel. To offset this, much higher rates of fusion are required, and thus much higher temperatures; most man-made fusion reactors are designed to work at temperatures around 100 million degrees, or higher. To date, no man-made reactor has reached breakeven, let alone ignition. Ignition has however been achieved in the cores of detonating thermonuclear weapons.
Current research
Lawrence Livermore National Laboratory has its 1.8 MJ laser system running at full power. This laser system is designed to compress and heat a mixture of deuterium and tritium, which are both isotopes of hydrogen, in order to compress the isotopes to a fraction of their original size, and fuse them into helium atoms. In January 2012, National Ignition Facility Director Mike Dunne predicted in a Photonics West 2012 plenary talk that ignition would be achieved at NIF by October 2012. However,, NIF is operating at conditions about 1/10 to 1/3 of breakeven. Confusingly, by LLNL definitions, ignition and breakeven occur at the same point, due to specifics of their experiment. The world's first fusion reactor predicted to be capable of 'breakeven' is currently underway. Based on the Tokamak reactor design, the ITER is intended to achieve fusion for a prolonged period of time before structural integrity is affected. Construction is expected to be completed in 2025. Experts believe that achieving fusion ignition is the first step towards the potentially limitless energy source that is nuclear fusion.
