The requirement to obtain a gain in energy output, from a burning plasma, is often expressed in terms of the "Triple Product" defined as below.

The figure below shows the requirements on the triple product with the Deuterium-Tritium fusion reaction compared to other possible fusion reactions: Deuterium-Deuterium and Deuterium-Helium3. It shows that D-T fusion can be achieved with the lowest values of the triple product and temperature, hence this is the reaction which is pursued in the majority of fusion projects.


Progress toward Fusion Reactor Conditions

To achieve reactor conditions, the progress has been to construction bigger and bigger devices as indicated in the figure below.

The plot below shows how these increasingly large machines have progressed in increasing the triple product plasma values. The goals for ITER and the requirements for actual power plants are indicated.