PLASMA STABILITY IN HELIAC-LIKE QUASI-HELICALLY SYMMETRICAL STELLARATORS

An attempt is made to find an optimal quasi-helically symmetric (QHS) four period magnetic configuration with a Heliac-like structure of the magnetic surface cross-sections. This means that the cross-sections r otate in phase with the magnetic axis principal normal in contrast to the Helias-like systems where the magnetic surface cross-sections rota te half as fast as the principal normal. The equilibrium and local sta bility are studied with the VMEC and TERPSICHORE codes. It is shown th at, in the configuration found, the equilibrium beta limit is about 9% , the Mercier beta limit-is about 4% and the ballooning modes restrict the beta value to 1%. The accuracy of the fulfilment of the QHS condi tion is given by X similar to 3 (i.e. the largest amplitude of the mag netic held strength B harmonic that violates the quasi-symmetry is a t hird that of of the main helical harmonic of B in Boozer cc-ordinates) .