For a demonstration reactor (DEMO) we seek an experiment that combines the best features of ITER, with a system of QAS coils providing external rotational transform, which is a measure of the poloidal field.
In calculations of this kind the rotational transform i can change sign so that a sizeable island appears where it vanishes.
If there is no net current and l stands for the rotational transform, the variables [theta] + l[empty set] and [empty set] can be renormalized to become invariant poloidal and toroidal angles on each flux surface s = const., where s is the radial coordinate.
A QAS stellarator is a good alternate configuration to study, since some rotational transform should come from the external magnetic field.
Therefore NSTAB computations are needed to see whether the effect of islands is tolerable provided the rotational transform avoids dangerous resonances.
Success depends on the fortunate circumstance that when the quotient of the aspect ratio divided by the number of field periods is near 2, then the value of the Fourier coefficient [B.sub.21] producing the bulk of the rotational transform becomes remarkably small.
Neilson said "toroidal plasmas have many configuration variables," citing things like aspect ratio, shape, magnetic symmetry, rotational transform
, divertor configuration and startup/control strategy.