351Energy from Nuclear Fusion
Within the past two decades, innovations in tokamaks have dramatically
increased their ability to conne plasma and to make them more desirable as
potential power plants, as summarized below.
1. Until the 1980s, tokamaks were inherently pulsed devices. Now,
potentially steady-state operation has been demonstrated with the
current driven by neutral beams or rf. Current drive efciencies are
adequate when most (perhaps 80%–90%) of the current is supplied
by a bootstrap effect in which the outward diffusion of plasma den-
sity, peaked on the axis, drives the required currents.
2. Higher-β operation has been achieved by using vertically elongated
plasmas, with sharper bends at upper and lower corners.
3. Connement ...