<P> To evaluate the usefulness of these reactions, in addition to the reactants, the products, and the energy released, one needs to know something about the cross section . Any given fusion device has a maximum plasma pressure it can sustain, and an economical device would always operate near this maximum . Given this pressure, the largest fusion output is obtained when the temperature is chosen so that <σv> / T is a maximum . This is also the temperature at which the value of the triple product nTτ required for ignition is a minimum, since that required value is inversely proportional to <σv> / T (see Lawson criterion). (A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating .) This optimum temperature and the value of <σv> / T at that temperature is given for a few of these reactions in the following table . </P> <Table> <Tr> <Th> fuel </Th> <Th> T (keV) </Th> <Th> <σv> / T (m / s / keV) </Th> </Tr> <Tr> <Td> - </Td> <Td> 13.6 </Td> <Td> 1.24 × 10 </Td> </Tr> <Tr> <Td> - </Td> <Td> 15 </Td> <Td> 1.28 × 10 </Td> </Tr> <Tr> <Td> - He </Td> <Td> 58 </Td> <Td> 2.24 × 10 </Td> </Tr> <Tr> <Td> p - Li </Td> <Td> 66 </Td> <Td> 1.46 × 10 </Td> </Tr> <Tr> <Td> p - </Td> <Td> 123 </Td> <Td> 3.01 × 10 </Td> </Tr> </Table> <Tr> <Th> fuel </Th> <Th> T (keV) </Th> <Th> <σv> / T (m / s / keV) </Th> </Tr> <Tr> <Td> - </Td> <Td> 13.6 </Td> <Td> 1.24 × 10 </Td> </Tr>

When does nuclear fusion of hydrogen and helium occur