Lawson's criterion is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition
. the heating of the plasma by the products of the fusion reactions is sufficient to maintain the temperature of the plasma against all losses without external power input. As originally formulated the Lawson criterion gives a minimum required value for the product of the plasma (electron) density ne
and the "energy confinement time" ?E
. Later analyses suggested that a more useful figure of merit
is the "triple product" of density, confinement time, and plasma temperature T
. The triple product also has a minimum required value, and the name "Lawson criterion" often refers to this inequality.