The output energy is also proportional to t , the time that we are able to maintain the plasma at the ignition temperature. There are various ways that physicists try to satisfy the lawson criteria and achieve ignition temperature. This is a torus, which, using complicated magnetic fields confines the plasma. There is a toroidal current outside the torus that has a dual role of heating the plasma and of creating the magnetic field confining the plasma. The biggest problems with this design is the existence of various instabilities in the plasma that must be overcome.
Current designs are on the verge of achieving breakeven condition and ignition temperature.
The other very promising design is the inertial confinement machine. This plans to maintain the plasma for short periods of time only, but at very high densities. The high densities are achieved by concenrated strike of a large number of high powered laser beams on a pellet made of solid d-t mixture. Again, the current designs are at the verge of satisfying the Lawson criterion and reaching ignition temperature. It is expected that by the second decade of the new millenium we will be close to build commercial fusion reactors.