Constrained Ripple Optimization of Tokamak Bundle Divertors

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Abstract
Magnetic field ripple from a tokamak bundle divertor is localized to a small toroidal sector and must be treated differently from the usual (distributed) toroidal field (TF) coil ripple. Generally, in a tokamak with an unoptimized divertor design, all of the banana-trapped fast ions are quickly lost due to banana drift diffusion or to trapping between the 1/R variation in |B| Ξ B and local field maxima due to the divertor.Studying the full three-dimensional shape of the B-surfaces allows quick evaluation of many inferior designs. For designs that eliminate any large maxima in B, low on-axis ripple is a good indicator of avoiding deleterious effects due to the divertor. However, this must be achieved while satisfying many engineering constraints.A computer code has been written to optimize automatically on-axis ripple subject to these constraints, while varying up to nine design parameters. Optimum configurations have low on-axis ripple (<0.2%) so that, now, most banana-trapped fast ions are confined. Only those ions with banana tips near the outside region f|0|≤45 deg) are lost. However, because finite-sized TF coils have not been used in this study, the flux bundle is not expanded.