 So while 100 keV represents a temperature of over one billion degrees, in order to produce fusion events the fuel does not have to be heated to this temperature as a whole; some reactions will occur even at lower temperatures due to the small number of high-energy particles in the mix.As the fusion reactions give off large amounts of energy, and some of that energy will be deposited back in the fuel, these reactions heat the fuel.There is a critical temperature at which the rate of reactions, and thus the energy deposited, balances losses to the environment.