Fusion
Fusion reactions power the Sun. At high temperatures and under extreme pressure of gravity hydrogen is converted into helium. Experimental fusion reactors and the first fusion power plants will be powered by fusion of deuterium and tritium at temperature around a hundred million degrees.
The nucleus of hydrogen is a proton.
The nucleus of deuterium consists of a proton and a neutron.
The nucleus of tritium consists of a proton and two neutrons.
Tritium will be produced by nuclear reaction with lithium in the blanket of the fusion reactor.