Stable bootstrap-current driven equilibria for low aspect ratio tokamaks

Low aspect ratio tokamaks can potentially provide a high ratio of plasma pressure to magnetic pressure beta and high plasma current I at a modest size, ultimately leading to a high power density compact fusion power plant. For the concept to be economically feasible, bootstrap current must be a major component of the plasma current. A high value of the Troyon factor beta(N) and strong shaping are required to allow simultaneous operation at high beta and high bootstrap current fraction. Ideal magnetohydrodynamic stability of a range of equilibria at aspect ratio 1.4 is systematically explored by varying the pressure profile and shape. The pressure and current profiles are constrained in such a way as to assure complete bootstrap current alignment. Both beta(N) and beta are defined in terms of the vacuum toroidal field. Equilibria with beta(N) greater than or equal to 8 and beta similar to 35% to 55% exist which are stable to n = infinity ballooning modes, and stable to n = 0,1,2,3 kink modes with a conducting wall. The dependence of beta and beta(N) with respect to aspect ratio is also considered.