Fusion of four protons into helium is sufficiently probable in the Sun's interior - taking the timescales into account which are available there.

In technical plants on Earth the process has to be, and may be, drastically shortened.

Helium, the result of hydrogen fusion (with two protons and two neutrons in the atomic nucleus), results from encounters as well of the heavier hydrogen sorts deuterium (D) and tritium (T).

The D-T reaction is more yielding than the D-D reaction and will be technically reached first. Its shortage: tritium is radioactive. On a long-term basis, one should therefore aspire to the D-D reaction.