A minimum criterion for a fusion reactor is that the thermonuclear power P_{Th} generated in the plasma should replace the power losses P_{L}.

One way of achieving this is to recycle a fraction η of the total power leaving the plasma. This will then be used to reheat the plasma and balance the power losses-

η(P_{Th}+P_{L})=P_{L}

The

*Lawson criterion* states this in terms of the product of the plasma

density n and the

energy confinement time τ

_{E}. This criterion is a function of

temperature. At 10

KeV (optimal for the

fusion of

deuterium and

tritium) it is given by-

nτ_{E}>10^{19}m^{-3} s

See also the stricter criterion required for plasma ignition.