Effect of shaping on the equilibrium and stability of high current, high beta tokamaks

The effect of cross-sectional shaping on the equilibrium and stability of high current, high beta tokamaks is examined for geometry relevant to both ITER and CIT. Two different approaches to stability analysis are taken. In the first, optimum pressure and current profiles are found that maximize the achievable beta in a given geometry. In the second, a survey of the stability of a large set of equilibria covering the relevant operating space is analyzed. It is found that, as elongation is increased above 2, g=βcrit(%)a(m)BT(T)/I(MA) decreases. However, higher values of beta can still be attained for 2 ≤ κ ≤ 2.5, owing to the increased current carrying capacity. Triangularity is found to have only a modest effect on g. Higher triangularities, while more ballooning stable, tend to be more kink unstable. Finally, the problem of designing the poloidal field shaping coils necessary to obtain these highly shaped equilibria is addressed. Results show that, with increasing poloidal field coil distance and with more strongly shaped plasma cross-section, plasma equilibria become much less robust. Consequently, increasingly severe control problems may arise. (author). 8 refs, 3 figs, 2 tabs