Stability of a closed-line Scyllac, revisited

Abstract

An earlier δW-calculation of MHD stability of a Scyllac configuration with equal amplitude ℓ = ±1 helical fields is corrected. In the early calculation, the MHD equilibrium was consistent to first order in the amplitude of the helical field, but not to second order. Unlike other high-β stellarator equilibria, secondorder force balance could not be achieved by the addition of new second-order fields, which would have no effect on stability. In fact, equilibrium to second order could only be constructed with the addition of a Z-independent field of first order. The corrected stability calculation shows that both vertical and horizontal displacements are unstable, with a growth rate scale smaller by ha ( 1) than the earlier prediction. The ℓ = ±1 configuration shows some improvement over the ℓ = 1 Scyllac because, although the horizontal growth rate is essentially unchanged, the vertical one proves to be smaller by the factor [(1/2)(1-β)/(4-3β)]^1/2. On the other hand, the field line topology of the corrected ℓ = ±1 system is much less satisfactory in that all the magnetic lines outside the plasma leave the system.